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<div class="section" id="chapter-5-extending-the-language-control-flow">
<h1>5. Chapter 5: Extending the Language: Control Flow<a class="headerlink" href="#chapter-5-extending-the-language-control-flow" title="Permalink to this headline">¶</a></h1>
<p>Written by <a class="reference external" href="mailto:sabre%40nondot.org">Chris Lattner</a> and <a class="reference external" href="http://max99x.com">Max
Shawabkeh</a></p>
<div class="section" id="introduction">
<h2>5.1. Introduction<a class="headerlink" href="#introduction" title="Permalink to this headline">¶</a></h2>
<p>Welcome to Chapter 5 of the <a class="reference external" href="http://www.llvm.org/docs/tutorial/index.html">Implementing a language with
LLVM</a> tutorial. Parts
1-4 described the implementation of the simple Kaleidoscope language and
included support for generating LLVM IR, followed by optimizations and a
JIT compiler. Unfortunately, as presented, Kaleidoscope is mostly
useless: it has no control flow other than call and return. This means
that you can’t have conditional branches in the code, significantly
limiting its power. In this episode of “build that compiler”, we’ll
extend Kaleidoscope to have an if/then/else expression plus a simple
‘for’ loop.</p>
</div>
<hr class="docutils" />
<div class="section" id="if-then-else">
<h2>5.2. If/Then/Else<a class="headerlink" href="#if-then-else" title="Permalink to this headline">¶</a></h2>
<p>Extending Kaleidoscope to support if/then/else is quite straightforward.
It basically requires adding lexer support for this “new” concept to the
lexer, parser, AST, and LLVM code emitter. This example is nice, because
it shows how easy it is to “grow” a language over time, incrementally
extending it as new ideas are discovered.</p>
<p>Before we get going on “how” we add this extension, lets talk about
“what” we want. The basic idea is that we want to be able to write this
sort of thing:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="k">def</span> <span class="nf">fib</span><span class="p">(</span><span class="n">x</span><span class="p">)</span>
<span class="k">if</span> <span class="n">x</span> <span class="o"><</span> <span class="mi">3</span> <span class="n">then</span>
<span class="mi">1</span>
<span class="k">else</span>
<span class="n">fib</span><span class="p">(</span><span class="n">x</span><span class="o">-</span><span class="mi">1</span><span class="p">)</span> <span class="o">+</span> <span class="n">fib</span><span class="p">(</span><span class="n">x</span><span class="o">-</span><span class="mi">2</span><span class="p">)</span>
</pre></div>
</div>
<p>In Kaleidoscope, every construct is an expression: there are no
statements. As such, the if/then/else expression needs to return a value
like any other. Since we’re using a mostly functional form, we’ll have
it evaluate its conditional, then return the ‘then’ or ‘else’ value
based on how the condition was resolved. This is very similar to the C
”?:” expression.</p>
<p>The semantics of the if/then/else expression is that it evaluates the
condition to a boolean equality value: 0.0 is considered to be false and
everything else is considered to be true. If the condition is true, the
first subexpression is evaluated and returned, if the condition is
false, the second subexpression is evaluated and returned. Since
Kaleidoscope allows side-effects, this behavior is important to nail
down.</p>
<p>Now that we know what we “want”, let’s break this down into its
constituent pieces.</p>
<div class="section" id="lexer-extensions-for-if-then-else">
<h3>5.2.1. Lexer Extensions for If/Then/Else<a class="headerlink" href="#lexer-extensions-for-if-then-else" title="Permalink to this headline">¶</a></h3>
<p>The lexer extensions are straightforward. First we add new token classes
for the relevant tokens:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="k">class</span> <span class="nc">IfToken</span><span class="p">(</span><span class="nb">object</span><span class="p">):</span>
<span class="k">pass</span>
<span class="k">class</span> <span class="nc">ThenToken</span><span class="p">(</span><span class="nb">object</span><span class="p">):</span>
<span class="k">pass</span>
<span class="k">class</span> <span class="nc">ElseToken</span><span class="p">(</span><span class="nb">object</span><span class="p">):</span>
<span class="k">pass</span>
</pre></div>
</div>
<p>Once we have that, we recognize the new keywords in the lexer. This is
pretty simple stuff:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="o">...</span>
<span class="k">if</span> <span class="n">identifier</span> <span class="o">==</span> <span class="s">'def'</span><span class="p">:</span>
<span class="k">yield</span> <span class="n">DefToken</span><span class="p">()</span>
<span class="k">elif</span> <span class="n">identifier</span> <span class="o">==</span> <span class="s">'extern'</span><span class="p">:</span>
<span class="k">yield</span> <span class="n">ExternToken</span><span class="p">()</span>
<span class="k">elif</span> <span class="n">identifier</span> <span class="o">==</span> <span class="s">'if'</span><span class="p">:</span>
<span class="k">yield</span> <span class="n">IfToken</span><span class="p">()</span>
<span class="k">elif</span> <span class="n">identifier</span> <span class="o">==</span> <span class="s">'then'</span><span class="p">:</span>
<span class="k">yield</span> <span class="n">ThenToken</span><span class="p">()</span>
<span class="k">elif</span> <span class="n">identifier</span> <span class="o">==</span> <span class="s">'else'</span><span class="p">:</span>
<span class="k">yield</span> <span class="n">ElseToken</span><span class="p">()</span>
<span class="k">else</span><span class="p">:</span>
<span class="k">yield</span> <span class="n">IdentifierToken</span><span class="p">(</span><span class="n">identifier</span><span class="p">)</span>
</pre></div>
</div>
</div>
<div class="section" id="ast-extensions-for-if-then-else">
<h3>5.2.2. AST Extensions for If/Then/Else<a class="headerlink" href="#ast-extensions-for-if-then-else" title="Permalink to this headline">¶</a></h3>
<p>To represent the new expression we add a new AST node for it:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="c"># Expression class for if/then/else.</span>
<span class="k">class</span> <span class="nc">IfExpressionNode</span><span class="p">(</span><span class="n">ExpressionNode</span><span class="p">):</span>
<span class="k">def</span> <span class="nf">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">condition</span><span class="p">,</span> <span class="n">then_branch</span><span class="p">,</span> <span class="n">else_branch</span><span class="p">):</span>
<span class="bp">self</span><span class="o">.</span><span class="n">condition</span> <span class="o">=</span> <span class="n">condition</span>
<span class="bp">self</span><span class="o">.</span><span class="n">then_branch</span> <span class="o">=</span> <span class="n">then_branch</span>
<span class="bp">self</span><span class="o">.</span><span class="n">else_branch</span> <span class="o">=</span> <span class="n">else_branch</span>
<span class="k">def</span> <span class="nf">CodeGen</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="o">...</span>
</pre></div>
</div>
<p>The AST node just has pointers to the various subexpressions.</p>
</div>
<div class="section" id="parser-extensions-for-if-then-else">
<h3>5.2.3. Parser Extensions for If/Then/Else<a class="headerlink" href="#parser-extensions-for-if-then-else" title="Permalink to this headline">¶</a></h3>
<p>Now that we have the relevant tokens coming from the lexer and we have
the AST node to build, our parsing logic is relatively straightforward.
First we define a new parsing function:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="c"># ifexpr ::= 'if' expression 'then' expression 'else' expression</span>
<span class="k">def</span> <span class="nf">ParseIfExpr</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="bp">self</span><span class="o">.</span><span class="n">Next</span><span class="p">()</span> <span class="c"># eat the if.</span>
<span class="c"># condition.</span>
<span class="n">condition</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">ParseExpression</span><span class="p">()</span>
<span class="k">if</span> <span class="ow">not</span> <span class="nb">isinstance</span><span class="p">(</span><span class="bp">self</span><span class="o">.</span><span class="n">current</span><span class="p">,</span> <span class="n">ThenToken</span><span class="p">):</span>
<span class="k">raise</span> <span class="ne">RuntimeError</span><span class="p">(</span><span class="s">'Expected "then".'</span><span class="p">)</span>
<span class="bp">self</span><span class="o">.</span><span class="n">Next</span><span class="p">()</span> <span class="c"># eat the then.</span>
<span class="n">then_branch</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">ParseExpression</span><span class="p">()</span>
<span class="k">if</span> <span class="ow">not</span> <span class="nb">isinstance</span><span class="p">(</span><span class="bp">self</span><span class="o">.</span><span class="n">current</span><span class="p">,</span> <span class="n">ElseToken</span><span class="p">):</span>
<span class="k">raise</span> <span class="ne">RuntimeError</span><span class="p">(</span><span class="s">'Expected "else".'</span><span class="p">)</span>
<span class="bp">self</span><span class="o">.</span><span class="n">Next</span><span class="p">()</span> <span class="c"># eat the else.</span>
<span class="n">else_branch</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">ParseExpression</span><span class="p">()</span>
<span class="k">return</span> <span class="n">IfExpressionNode</span><span class="p">(</span><span class="n">condition</span><span class="p">,</span> <span class="n">then_branch</span><span class="p">,</span> <span class="n">else_branch</span><span class="p">)</span>
</pre></div>
</div>
<p>Next we hook it up as a primary expression:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="k">def</span> <span class="nf">ParsePrimary</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="k">if</span> <span class="nb">isinstance</span><span class="p">(</span><span class="bp">self</span><span class="o">.</span><span class="n">current</span><span class="p">,</span> <span class="n">IdentifierToken</span><span class="p">):</span>
<span class="k">return</span> <span class="bp">self</span><span class="o">.</span><span class="n">ParseIdentifierExpr</span><span class="p">()</span>
<span class="k">elif</span> <span class="nb">isinstance</span><span class="p">(</span><span class="bp">self</span><span class="o">.</span><span class="n">current</span><span class="p">,</span> <span class="n">NumberToken</span><span class="p">):</span>
<span class="k">return</span> <span class="bp">self</span><span class="o">.</span><span class="n">ParseNumberExpr</span><span class="p">();</span>
<span class="k">elif</span> <span class="nb">isinstance</span><span class="p">(</span><span class="bp">self</span><span class="o">.</span><span class="n">current</span><span class="p">,</span> <span class="n">IfToken</span><span class="p">):</span>
<span class="k">return</span> <span class="bp">self</span><span class="o">.</span><span class="n">ParseIfExpr</span><span class="p">()</span>
<span class="k">elif</span> <span class="bp">self</span><span class="o">.</span><span class="n">current</span> <span class="o">==</span> <span class="n">CharacterToken</span><span class="p">(</span><span class="s">'('</span><span class="p">):</span>
<span class="k">return</span> <span class="bp">self</span><span class="o">.</span><span class="n">ParseParenExpr</span><span class="p">()</span>
<span class="k">else</span><span class="p">:</span>
<span class="k">raise</span> <span class="ne">RuntimeError</span><span class="p">(</span><span class="s">'Unknown token when expecting an expression.'</span><span class="p">)</span>
</pre></div>
</div>
</div>
<div class="section" id="llvm-ir-for-if-then-else">
<h3>5.2.4. LLVM IR for If/Then/Else<a class="headerlink" href="#llvm-ir-for-if-then-else" title="Permalink to this headline">¶</a></h3>
<p>Now that we have it parsing and building the AST, the final piece is
adding LLVM code generation support. This is the most interesting part
of the if/then/else example, because this is where it starts to
introduce new concepts. All of the code above has been thoroughly
described in previous chapters.</p>
<p>To motivate the code we want to produce, lets take a look at a simple
example. Consider:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="n">extern</span> <span class="n">foo</span><span class="p">();</span>
<span class="n">extern</span> <span class="n">bar</span><span class="p">();</span>
<span class="k">def</span> <span class="nf">baz</span><span class="p">(</span><span class="n">x</span><span class="p">)</span> <span class="k">if</span> <span class="n">x</span> <span class="n">then</span> <span class="n">foo</span><span class="p">()</span> <span class="k">else</span> <span class="n">bar</span><span class="p">();</span>
</pre></div>
</div>
<p>If you disable optimizations, the code you’ll (soon) get from
Kaleidoscope looks something like this:</p>
<div class="highlight-llvm"><div class="highlight"><pre><span class="k">declare</span> <span class="kt">double</span> <span class="vg">@foo</span><span class="p">()</span>
<span class="k">declare</span> <span class="kt">double</span> <span class="vg">@bar</span><span class="p">()</span>
<span class="k">define</span> <span class="kt">double</span> <span class="vg">@baz</span><span class="p">(</span><span class="kt">double</span> <span class="nv">%x</span><span class="p">)</span> <span class="p">{</span>
<span class="nl">entry:</span>
<span class="nv">%ifcond</span> <span class="p">=</span> <span class="k">fcmp</span> <span class="k">one</span> <span class="kt">double</span> <span class="nv">%x</span><span class="p">,</span> <span class="m">0.000000e+00</span>
<span class="k">br</span> <span class="k">i1</span> <span class="nv">%ifcond</span><span class="p">,</span> <span class="kt">label</span> <span class="nv">%then</span><span class="p">,</span> <span class="kt">label</span> <span class="nv">%else</span>
<span class="nl">then:</span> <span class="c">; preds = %entry</span>
<span class="nv">%calltmp</span> <span class="p">=</span> <span class="k">call</span> <span class="kt">double</span> <span class="vg">@foo</span><span class="p">()</span>
<span class="k">br</span> <span class="kt">label</span> <span class="nv">%ifcont</span>
<span class="nl">else:</span> <span class="c">; preds = %entry</span>
<span class="nv">%calltmp1</span> <span class="p">=</span> <span class="k">call</span> <span class="kt">double</span> <span class="vg">@bar</span><span class="p">()</span>
<span class="k">br</span> <span class="kt">label</span> <span class="nv">%ifcont</span>
<span class="nl">ifcont:</span> <span class="c">; preds = %else, %then</span>
<span class="nv">%iftmp</span> <span class="p">=</span> <span class="k">phi</span> <span class="kt">double</span> <span class="p">[</span> <span class="nv">%calltmp</span><span class="p">,</span> <span class="nv">%then</span> <span class="p">],</span> <span class="p">[</span> <span class="nv">%calltmp1</span><span class="p">,</span> <span class="nv">%else</span> <span class="p">]</span>
<span class="k">ret</span> <span class="kt">double</span> <span class="nv">%iftmp</span>
<span class="p">}</span>
</pre></div>
</div>
<p>To visualize the control flow graph, you can use a nifty feature of the
LLVM <a class="reference external" href="http://llvm.org/cmds/opt.html">opt</a> tool. If you put this LLVM
IR into “t.ll” and run <tt class="docutils literal"><span class="pre">llvm-as</span> <span class="pre"><</span> <span class="pre">t.ll</span> <span class="pre">|</span> <span class="pre">opt</span> <span class="pre">-analyze</span> <span class="pre">-view-cfg</span></tt>, a
<a class="reference external" href="http://www.llvm.org/docs/ProgrammersManual.html#ViewGraph">window will pop
up</a> and
you’ll see this graph:</p>
<p>Another way to get this is to call “<tt class="docutils literal"><span class="pre">function.viewCFG()</span></tt>” or
“<tt class="docutils literal"><span class="pre">function.viewCFGOnly()</span></tt>” (where F is a “<tt class="docutils literal"><span class="pre">llvm.core.Function</span></tt>”)
either by inserting actual calls into the code and recompiling or by
calling these in the debugger. LLVM has many nice features for
visualizing various graphs, but note that these are available only if
your LLVM was built with Graphviz support (accomplished by having
Graphviz and Ghostview installed when building LLVM).</p>
<p>Getting back to the generated code, it is fairly simple: the entry block
evaluates the conditional expression (“x” in our case here) and compares
the result to 0.0 with the
<a class="reference external" href="http://www.llvm.org/docs/LangRef.html#i_fcmp">fcmp</a> one instruction
(‘one’ is “Ordered and Not Equal”). Based on the result of this
expression, the code jumps to either the “then” or “else” blocks, which
contain the expressions for the true/false cases.</p>
<p>Once the then/else blocks are finished executing, they both branch back
to the ‘ifcont’ block to execute the code that happens after the
if/then/else. In this case the only thing left to do is to return to the
caller of the function. The question then becomes: how does the code
know which expression to return?</p>
<p>The answer to this question involves an important SSA operation: the
<a class="reference external" href="http://en.wikipedia.org/wiki/Static_single_assignment_form">Phi
operation</a>.
If you’re not familiar with SSA, <a class="reference external" href="http://en.wikipedia.org/wiki/Static_single_assignment_form">the wikipedia
article</a>
is a good introduction and there are various other introductions to it
available on your favorite search engine. The short version is that
“execution” of the Phi operation requires “remembering” which block
control came from. The Phi operation takes on the value corresponding to
the input control block. In this case, if control comes in from the
“then” block, it gets the value of “calltmp”. If control comes from the
“else” block, it gets the value of “calltmp1”.</p>
<p>At this point, you are probably starting to think “Oh no! This means my
simple and elegant front-end will have to start generating SSA form in
order to use LLVM!”. Fortunately, this is not the case, and we strongly
advise <em>not</em> implementing an SSA construction algorithm in your
front-end unless there is an amazingly good reason to do so. In
practice, there are two sorts of values that float around in code
written for your average imperative programming language that might need
Phi nodes:</p>
<blockquote>
<div><ul class="simple">
<li><ol class="first arabic">
<li>Code that involves user variables: <tt class="docutils literal"><span class="pre">x</span> <span class="pre">=</span> <span class="pre">1;</span> <span class="pre">x</span> <span class="pre">=</span> <span class="pre">x</span> <span class="pre">+</span> <span class="pre">1;</span></tt></li>
</ol>
</li>
<li><ol class="first arabic" start="2">
<li>Values that are implicit in the structure of your AST, such as the
Phi node in this case.</li>
</ol>
</li>
</ul>
</div></blockquote>
<p>In <a class="reference external" href="PythonLangImpl7.html">Chapter 7</a> of this tutorial (“mutable
variables”), we’ll talk about #1 in depth. For now, just believe me that
you don’t need SSA construction to handle this case. For #2, you have
the choice of using the techniques that we will describe for #1, or you
can insert Phi nodes directly, if convenient. In this case, it is really
really easy to generate the Phi node, so we choose to do it directly.</p>
<p>Okay, enough of the motivation and overview, lets generate code!</p>
</div>
<div class="section" id="code-generation-for-if-then-else">
<h3>5.2.5. Code Generation for If/Then/Else<a class="headerlink" href="#code-generation-for-if-then-else" title="Permalink to this headline">¶</a></h3>
<p>In order to generate code for this, we implement the <tt class="docutils literal"><span class="pre">Codegen</span></tt> method
for <tt class="docutils literal"><span class="pre">IfExpressionNode</span></tt>:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="k">def</span> <span class="nf">CodeGen</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="n">condition</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">condition</span><span class="o">.</span><span class="n">CodeGen</span><span class="p">()</span>
<span class="c"># Convert condition to a bool by comparing equal to 0.0.</span>
<span class="n">condition_bool</span> <span class="o">=</span> <span class="n">g_llvm_builder</span><span class="o">.</span><span class="n">fcmp</span><span class="p">(</span>
<span class="n">FCMP_ONE</span><span class="p">,</span> <span class="n">condition</span><span class="p">,</span> <span class="n">Constant</span><span class="o">.</span><span class="n">real</span><span class="p">(</span><span class="n">Type</span><span class="o">.</span><span class="n">double</span><span class="p">(),</span> <span class="mi">0</span><span class="p">),</span> <span class="s">'ifcond'</span><span class="p">)</span>
</pre></div>
</div>
<p>This code is straightforward and similar to what we saw before. We emit
the expression for the condition, then compare that value to zero to get
a truth value as a 1-bit (bool) value.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="n">function</span> <span class="o">=</span> <span class="n">g_llvm_builder</span><span class="o">.</span><span class="n">basic_block</span><span class="o">.</span><span class="n">function</span>
<span class="c"># Create blocks for the then and else cases. Insert the 'then' block at the</span>
<span class="c"># end of the function.</span>
<span class="n">then_block</span> <span class="o">=</span> <span class="n">function</span><span class="o">.</span><span class="n">append_basic_block</span><span class="p">(</span><span class="s">'then'</span><span class="p">)</span>
<span class="n">else_block</span> <span class="o">=</span> <span class="n">function</span><span class="o">.</span><span class="n">append_basic_block</span><span class="p">(</span><span class="s">'else'</span><span class="p">)</span>
<span class="n">merge_block</span> <span class="o">=</span> <span class="n">function</span><span class="o">.</span><span class="n">append_basic_block</span><span class="p">(</span><span class="s">'ifcond'</span><span class="p">)</span>
<span class="n">g_llvm_builder</span><span class="o">.</span><span class="n">cbranch</span><span class="p">(</span><span class="n">condition_bool</span><span class="p">,</span> <span class="n">then_block</span><span class="p">,</span> <span class="n">else_block</span><span class="p">)</span>
</pre></div>
</div>
<p>This code creates the basic blocks that are related to the if/then/else
statement, and correspond directly to the blocks in the example above.
The first line gets the current Function object that is being built. It
gets this by asking the builder for the current BasicBlock, and asking
that block for its “parent” (the function it is currently embedded
into).</p>
<p>Once it has that, it creates three block which are automatically
inserted into the end of the function. Once the blocks are created, we
can emit the conditional branch that chooses between them. Note that
creating new blocks does not implicitly affect the Builder, so it is
still inserting into the block that the condition went into.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="c"># Emit then value.</span>
<span class="n">g_llvm_builder</span><span class="o">.</span><span class="n">position_at_end</span><span class="p">(</span><span class="n">then_block</span><span class="p">)</span>
<span class="n">then_value</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">then_branch</span><span class="o">.</span><span class="n">CodeGen</span><span class="p">()</span>
<span class="n">g_llvm_builder</span><span class="o">.</span><span class="n">branch</span><span class="p">(</span><span class="n">merge_block</span><span class="p">)</span>
<span class="c"># Codegen of 'Then' can change the current block; update then_block for the</span>
<span class="c"># PHI node.</span>
<span class="n">then_block</span> <span class="o">=</span> <span class="n">g_llvm_builder</span><span class="o">.</span><span class="n">basic_block</span>
</pre></div>
</div>
<p>After the conditional branch is inserted, we move the builder to start
inserting into the “then” block. Strictly speaking, this call moves the
insertion point to be at the end of the specified block. However, since
the “then” block is empty, it also starts out by inserting at the
beginning of the block. :)</p>
<p>Once the insertion point is set, we recursively codegen the “then”
expression from the AST. To finish off the “then” block, we create an
unconditional branch to the merge block. One interesting (and very
important) aspect of the LLVM IR is that it <a class="reference external" href="http://www.llvm.org/docs/LangRef.html#functionstructure">requires all basic blocks
to be
“terminated”</a>
with a <a class="reference external" href="http://www.llvm.org/docs/LangRef.html#terminators">control flow
instruction</a> such
as return or branch. This means that all control flow, <em>including
fallthroughs</em> must be made explicit in the LLVM IR. If you violate this
rule, the verifier will emit an error.</p>
<p>The final line here is quite subtle, but is very important. The basic
issue is that when we create the Phi node in the merge block, we need to
set up the block/value pairs that indicate how the Phi will work.
Importantly, the Phi node expects to have an entry for each predecessor
of the block in the CFG. Why then, are we getting the current block when
we just set it to then_block 5 lines above? The problem is that the
“Then” expression may actually itself change the block that the Builder
is emitting into if, for example, it contains a nested “if/then/else”
expression. Because calling Codegen recursively could arbitrarily change
the notion of the current block, we are required to get an up-to-date
value for code that will set up the Phi node.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="c"># Emit else block.</span>
<span class="n">g_llvm_builder</span><span class="o">.</span><span class="n">position_at_end</span><span class="p">(</span><span class="n">else_block</span><span class="p">)</span>
<span class="n">else_value</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">else_branch</span><span class="o">.</span><span class="n">CodeGen</span><span class="p">()</span>
<span class="n">g_llvm_builder</span><span class="o">.</span><span class="n">branch</span><span class="p">(</span><span class="n">merge_block</span><span class="p">)</span>
<span class="c"># Codegen of 'Else' can change the current block, update else_block for the</span>
<span class="c"># PHI node.</span>
<span class="n">else_block</span> <span class="o">=</span> <span class="n">g_llvm_builder</span><span class="o">.</span><span class="n">basic_block</span>
</pre></div>
</div>
<p>Code generation for the ‘else’ block is basically identical to codegen
for the ‘then’ block. The only significant difference is the first line,
which adds the ‘else’ block to the function. Recall previously that the
‘else’ block was created, but not added to the function. Now that the
‘then’ and ‘else’ blocks are emitted, we can finish up with the merge
code:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="c"># Emit merge block.</span>
<span class="n">g_llvm_builder</span><span class="o">.</span><span class="n">position_at_end</span><span class="p">(</span><span class="n">merge_block</span><span class="p">)</span>
<span class="n">phi</span> <span class="o">=</span> <span class="n">g_llvm_builder</span><span class="o">.</span><span class="n">phi</span><span class="p">(</span><span class="n">Type</span><span class="o">.</span><span class="n">double</span><span class="p">(),</span> <span class="s">'iftmp'</span><span class="p">)</span>
<span class="n">phi</span><span class="o">.</span><span class="n">add_incoming</span><span class="p">(</span><span class="n">then_value</span><span class="p">,</span> <span class="n">then_block</span><span class="p">)</span>
<span class="n">phi</span><span class="o">.</span><span class="n">add_incoming</span><span class="p">(</span><span class="n">else_value</span><span class="p">,</span> <span class="n">else_block</span><span class="p">)</span>
<span class="k">return</span> <span class="n">phi</span>
</pre></div>
</div>
<p>The first line changes the insertion point so that newly created code
will go into the “merge” block. Once that is done, we need to create the
PHI node and set up the block/value pairs for the PHI.</p>
<p>Finally, the CodeGen function returns the phi node as the value computed
by the if/then/else expression. In our example above, this returned
value will feed into the code for the top-level function, which will
create the return instruction.</p>
<p>Overall, we now have the ability to execute conditional code in
Kaleidoscope. With this extension, Kaleidoscope is a fairly complete
language that can calculate a wide variety of numeric functions. Next up
we’ll add another useful expression that is familiar from non-functional
languages...</p>
</div>
</div>
<hr class="docutils" />
<div class="section" id="for-loop-expression">
<h2>5.3. ‘for’ Loop Expression<a class="headerlink" href="#for-loop-expression" title="Permalink to this headline">¶</a></h2>
<p>Now that we know how to add basic control flow constructs to the
language, we have the tools to add more powerful things. Lets add
something more aggressive, a ‘for’ expression:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="n">extern</span> <span class="n">putchard</span><span class="p">(</span><span class="n">char</span><span class="p">)</span>
<span class="k">def</span> <span class="nf">printstar</span><span class="p">(</span><span class="n">n</span><span class="p">)</span>
<span class="k">for</span> <span class="n">i</span> <span class="o">=</span> <span class="mi">1</span><span class="p">,</span> <span class="n">i</span> <span class="o"><</span> <span class="n">n</span><span class="p">,</span> <span class="mf">1.0</span> <span class="ow">in</span>
<span class="n">putchard</span><span class="p">(</span><span class="mi">42</span><span class="p">)</span> <span class="c"># ascii 42 = '*'</span>
<span class="c"># print 100 '*' characters</span>
<span class="n">printstar</span><span class="p">(</span><span class="mi">100</span><span class="p">)</span>
</pre></div>
</div>
<p>This expression defines a new variable (<tt class="docutils literal"><span class="pre">i</span></tt> in this case) which
iterates from a starting value, while the condition (<tt class="docutils literal"><span class="pre">i</span> <span class="pre"><</span> <span class="pre">n</span></tt> in this
case) is true, incrementing by an optional step value (“1.0” in this
case). If the step value is omitted, it defaults to 1.0. While the loop
is true, it executes its body expression. Because we don’t have anything
better to return, we’ll just define the loop as always returning 0.0. In
the future when we have mutable variables, it will get more useful.</p>
<p>As before, lets talk about the changes that we need to Kaleidoscope to
support this.</p>
<div class="section" id="lexer-extensions-for-the-for-loop">
<h3>5.3.1. Lexer Extensions for the ‘for’ Loop<a class="headerlink" href="#lexer-extensions-for-the-for-loop" title="Permalink to this headline">¶</a></h3>
<p>The lexer extensions are the same sort of thing as for if/then/else:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="o">...</span>
<span class="k">class</span> <span class="nc">ThenToken</span><span class="p">(</span><span class="nb">object</span><span class="p">):</span>
<span class="k">pass</span>
<span class="k">class</span> <span class="nc">ElseToken</span><span class="p">(</span><span class="nb">object</span><span class="p">):</span>
<span class="k">pass</span>
<span class="k">class</span> <span class="nc">ForToken</span><span class="p">(</span><span class="nb">object</span><span class="p">):</span>
<span class="k">pass</span>
<span class="k">class</span> <span class="nc">InToken</span><span class="p">(</span><span class="nb">object</span><span class="p">):</span>
<span class="k">pass</span>
<span class="o">...</span>
<span class="k">def</span> <span class="nf">Tokenize</span><span class="p">(</span><span class="n">string</span><span class="p">):</span>
<span class="o">...</span>
<span class="k">elif</span> <span class="n">identifier</span> <span class="o">==</span> <span class="s">'else'</span><span class="p">:</span>
<span class="k">yield</span> <span class="n">ElseToken</span><span class="p">()</span>
<span class="k">elif</span> <span class="n">identifier</span> <span class="o">==</span> <span class="s">'for'</span><span class="p">:</span>
<span class="k">yield</span> <span class="n">ForToken</span><span class="p">()</span>
<span class="k">elif</span> <span class="n">identifier</span> <span class="o">==</span> <span class="s">'in'</span><span class="p">:</span>
<span class="k">yield</span> <span class="n">InToken</span><span class="p">()</span><span class="o"></</span><span class="n">b</span><span class="o">></span>
<span class="k">else</span><span class="p">:</span>
<span class="k">yield</span> <span class="n">IdentifierToken</span><span class="p">(</span><span class="n">identifier</span><span class="p">)</span>
</pre></div>
</div>
</div>
<div class="section" id="ast-extensions-for-the-for-loop">
<h3>5.3.2. AST Extensions for the ‘for’ Loop<a class="headerlink" href="#ast-extensions-for-the-for-loop" title="Permalink to this headline">¶</a></h3>
<p>The AST node is just as simple. It basically boils down to capturing the
variable name and the constituent expressions in the node.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="c"># Expression class for for/in.</span>
<span class="k">class</span> <span class="nc">ForExpressionNode</span><span class="p">(</span><span class="n">ExpressionNode</span><span class="p">):</span>
<span class="k">def</span> <span class="nf">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">loop_variable</span><span class="p">,</span> <span class="n">start</span><span class="p">,</span> <span class="n">end</span><span class="p">,</span> <span class="n">step</span><span class="p">,</span> <span class="n">body</span><span class="p">):</span>
<span class="bp">self</span><span class="o">.</span><span class="n">loop_variable</span> <span class="o">=</span> <span class="n">loop_variable</span>
<span class="bp">self</span><span class="o">.</span><span class="n">start</span> <span class="o">=</span> <span class="n">start</span>
<span class="bp">self</span><span class="o">.</span><span class="n">end</span> <span class="o">=</span> <span class="n">end</span>
<span class="bp">self</span><span class="o">.</span><span class="n">step</span> <span class="o">=</span> <span class="n">step</span>
<span class="bp">self</span><span class="o">.</span><span class="n">body</span> <span class="o">=</span> <span class="n">body</span>
<span class="k">def</span> <span class="nf">CodeGen</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="o">...</span>
</pre></div>
</div>
</div>
<div class="section" id="parser-extensions-for-the-for-loop">
<h3>5.3.3. Parser Extensions for the ‘for’ Loop<a class="headerlink" href="#parser-extensions-for-the-for-loop" title="Permalink to this headline">¶</a></h3>
<p>The parser code is also fairly standard. The only interesting thing here
is handling of the optional step value. The parser code handles it by
checking to see if the second comma is present. If not, it sets the step
value to null in the AST node:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="c"># forexpr ::= 'for' identifier '=' expr ',' expr (',' expr)? 'in' expression</span>
<span class="k">def</span> <span class="nf">ParseForExpr</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="bp">self</span><span class="o">.</span><span class="n">Next</span><span class="p">()</span> <span class="c"># eat the for.</span>
<span class="k">if</span> <span class="ow">not</span> <span class="nb">isinstance</span><span class="p">(</span><span class="bp">self</span><span class="o">.</span><span class="n">current</span><span class="p">,</span> <span class="n">IdentifierToken</span><span class="p">):</span>
<span class="k">raise</span> <span class="ne">RuntimeError</span><span class="p">(</span><span class="s">'Expected identifier after for.'</span><span class="p">)</span>
<span class="n">loop_variable</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">current</span><span class="o">.</span><span class="n">name</span>
<span class="bp">self</span><span class="o">.</span><span class="n">Next</span><span class="p">()</span> <span class="c"># eat the identifier.</span>
<span class="k">if</span> <span class="bp">self</span><span class="o">.</span><span class="n">current</span> <span class="o">!=</span> <span class="n">CharacterToken</span><span class="p">(</span><span class="s">'='</span><span class="p">):</span>
<span class="k">raise</span> <span class="ne">RuntimeError</span><span class="p">(</span><span class="s">'Expected "=" after for variable.'</span><span class="p">)</span>
<span class="bp">self</span><span class="o">.</span><span class="n">Next</span><span class="p">()</span> <span class="c"># eat the '='.</span>
<span class="n">start</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">ParseExpression</span><span class="p">()</span>
<span class="k">if</span> <span class="bp">self</span><span class="o">.</span><span class="n">current</span> <span class="o">!=</span> <span class="n">CharacterToken</span><span class="p">(</span><span class="s">','</span><span class="p">):</span>
<span class="k">raise</span> <span class="ne">RuntimeError</span><span class="p">(</span><span class="s">'Expected "," after for start value.'</span><span class="p">)</span>
<span class="bp">self</span><span class="o">.</span><span class="n">Next</span><span class="p">()</span> <span class="c"># eat the ','.</span>
<span class="n">end</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">ParseExpression</span><span class="p">()</span>
<span class="c"># The step value is optional.</span>
<span class="k">if</span> <span class="bp">self</span><span class="o">.</span><span class="n">current</span> <span class="o">==</span> <span class="n">CharacterToken</span><span class="p">(</span><span class="s">','</span><span class="p">):</span>
<span class="bp">self</span><span class="o">.</span><span class="n">Next</span><span class="p">()</span> <span class="c"># eat the ','.</span>
<span class="n">step</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">ParseExpression</span><span class="p">()</span>
<span class="k">else</span><span class="p">:</span>
<span class="n">step</span> <span class="o">=</span> <span class="bp">None</span>
<span class="k">if</span> <span class="ow">not</span> <span class="nb">isinstance</span><span class="p">(</span><span class="bp">self</span><span class="o">.</span><span class="n">current</span><span class="p">,</span> <span class="n">InToken</span><span class="p">):</span>
<span class="k">raise</span> <span class="ne">RuntimeError</span><span class="p">(</span><span class="s">'Expected "in" after for variable specification.'</span><span class="p">)</span>
<span class="bp">self</span><span class="o">.</span><span class="n">Next</span><span class="p">()</span> <span class="c"># eat 'in'.</span>
<span class="n">body</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">ParseExpression</span><span class="p">()</span>
<span class="k">return</span> <span class="n">ForExpressionNode</span><span class="p">(</span><span class="n">loop_variable</span><span class="p">,</span> <span class="n">start</span><span class="p">,</span> <span class="n">end</span><span class="p">,</span> <span class="n">step</span><span class="p">,</span> <span class="n">body</span><span class="p">)</span>
</pre></div>
</div>
</div>
<div class="section" id="llvm-ir-for-the-for-loop">
<h3>5.3.4. LLVM IR for the ‘for’ Loop<a class="headerlink" href="#llvm-ir-for-the-for-loop" title="Permalink to this headline">¶</a></h3>
<p>Now we get to the good part: the LLVM IR we want to generate for this
thing. With the simple example above, we get this LLVM IR (note that
this dump is generated with optimizations disabled for clarity):</p>
<div class="highlight-llvm"><div class="highlight"><pre><span class="k">declare</span> <span class="kt">double</span> <span class="vg">@putchard</span><span class="p">(</span><span class="kt">double</span><span class="p">)</span>
<span class="k">define</span> <span class="kt">double</span> <span class="vg">@printstar</span><span class="p">(</span><span class="kt">double</span> <span class="nv">%n</span><span class="p">)</span> <span class="p">{</span>
<span class="nl">entry:</span>
<span class="c">; initial value = 1.0 (inlined into phi)</span>
<span class="k">br</span> <span class="kt">label</span> <span class="nv">%loop</span>
<span class="nl">loop:</span> <span class="c">; preds = %loop, %entry</span>
<span class="nv">%i</span> <span class="p">=</span> <span class="k">phi</span> <span class="kt">double</span> <span class="p">[</span>
<span class="m">1.000000e+00</span><span class="p">,</span> <span class="nv">%entry</span> <span class="p">],</span> <span class="p">[</span> <span class="nv">%nextvar</span><span class="p">,</span> <span class="nv">%loop</span> <span class="p">]</span>
<span class="c">; body</span>
<span class="nv">%calltmp</span> <span class="p">=</span> <span class="k">call</span> <span class="kt">double</span> <span class="vg">@putchard</span><span class="p">(</span><span class="kt">double</span> <span class="m">4.200000e+01</span><span class="p">)</span>
<span class="c">; increment</span>
<span class="nv">%nextvar</span> <span class="p">=</span> <span class="k">fadd</span> <span class="kt">double</span> <span class="nv">%i</span><span class="p">,</span> <span class="m">1.000000e+00</span>
<span class="c">; termination test</span>
<span class="nv">%cmptmp</span> <span class="p">=</span> <span class="k">fcmp</span> <span class="k">ult</span> <span class="kt">double</span> <span class="nv">%i</span><span class="p">,</span> <span class="nv">%n</span>
<span class="nv">%booltmp</span> <span class="p">=</span> <span class="k">uitofp</span> <span class="k">i1</span> <span class="nv">%cmptmp</span> <span class="k">to</span> <span class="kt">double</span>
<span class="nv">%loopcond</span> <span class="p">=</span> <span class="k">fcmp</span> <span class="k">one</span> <span class="kt">double</span> <span class="nv">%booltmp</span><span class="p">,</span> <span class="m">0.000000e+00</span>
<span class="k">br</span> <span class="k">i1</span> <span class="nv">%loopcond</span><span class="p">,</span> <span class="kt">label</span> <span class="nv">%loop</span><span class="p">,</span> <span class="kt">label</span> <span class="nv">%afterloop</span>
<span class="nl">afterloop:</span> <span class="c">; preds = %loop</span>
<span class="c">; loop always returns 0.0</span>
<span class="k">ret</span> <span class="kt">double</span> <span class="m">0.000000e+00</span> <span class="p">}</span>
</pre></div>
</div>
<p>This loop contains all the same constructs we saw before: a phi node,
several expressions, and some basic blocks. Lets see how this fits
together.</p>
</div>
<div class="section" id="code-generation-for-the-for-loop">
<h3>5.3.5. Code Generation for the ‘for’ Loop<a class="headerlink" href="#code-generation-for-the-for-loop" title="Permalink to this headline">¶</a></h3>
<p>The first part of Codegen is very simple: we just output the start
expression for the loop value:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="k">def</span> <span class="nf">CodeGen</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="c"># Emit the start code first, without 'variable' in scope.</span>
<span class="n">start_value</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">start</span><span class="o">.</span><span class="n">CodeGen</span><span class="p">()</span>
</pre></div>
</div>
<p>With this out of the way, the next step is to set up the LLVM basic
block for the start of the loop body. In the case above, the whole loop
body is one block, but remember that the body code itself could consist
of multiple blocks (e.g. if it contains an if/then/else or a for/in
expression).</p>
<div class="highlight-python"><div class="highlight"><pre><span class="c"># Make the new basic block for the loop header, inserting after current</span>
<span class="c"># block.</span>
<span class="n">function</span> <span class="o">=</span> <span class="n">g_llvm_builder</span><span class="o">.</span><span class="n">basic_block</span><span class="o">.</span><span class="n">function</span>
<span class="n">pre_header_block</span> <span class="o">=</span> <span class="n">g_llvm_builder</span><span class="o">.</span><span class="n">basic_block</span>
<span class="n">loop_block</span> <span class="o">=</span> <span class="n">function</span><span class="o">.</span><span class="n">append_basic_block</span><span class="p">(</span><span class="s">'loop'</span><span class="p">)</span>
<span class="c"># Insert an explicit fallthrough from the current block to the loop_block.</span>
<span class="n">g_llvm_builder</span><span class="o">.</span><span class="n">branch</span><span class="p">(</span><span class="n">loop_block</span><span class="p">)</span>
</pre></div>
</div>
<p>This code is similar to what we saw for if/then/else. Because we will
need it to create the Phi node, we remember the block that falls through
into the loop. Once we have that, we create the actual block that starts
the loop and create an unconditional branch for the fall-through between
the two blocks.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="c"># Start insertion in loop_block.</span>
<span class="n">g_llvm_builder</span><span class="o">.</span><span class="n">position_at_end</span><span class="p">(</span><span class="n">loop_block</span><span class="p">);</span>
<span class="c"># Start the PHI node with an entry for start.</span>
<span class="n">variable_phi</span> <span class="o">=</span> <span class="n">g_llvm_builder</span><span class="o">.</span><span class="n">phi</span><span class="p">(</span><span class="n">Type</span><span class="o">.</span><span class="n">double</span><span class="p">(),</span> <span class="bp">self</span><span class="o">.</span><span class="n">loop_variable</span><span class="p">)</span>
<span class="n">variable_phi</span><span class="o">.</span><span class="n">add_incoming</span><span class="p">(</span><span class="n">start_value</span><span class="p">,</span> <span class="n">pre_header_block</span><span class="p">)</span>
</pre></div>
</div>
<p>Now that the “pre_header_block” for the loop is set up, we switch to
emitting code for the loop body. To begin with, we move the insertion
point and create the PHI node for the loop induction variable. Since we
already know the incoming value for the starting value, we add it to the
Phi node. Note that the Phi will eventually get a second value for the
backedge, but we can’t set it up yet (because it doesn’t exist!).</p>
<div class="highlight-python"><div class="highlight"><pre><span class="c"># Within the loop, the variable is defined equal to the PHI node. If it</span>
<span class="c"># shadows an existing variable, we have to restore it, so save it now.</span>
<span class="n">old_value</span> <span class="o">=</span> <span class="n">g_named_values</span><span class="o">.</span><span class="n">get</span><span class="p">(</span><span class="bp">self</span><span class="o">.</span><span class="n">loop_variable</span><span class="p">,</span> <span class="bp">None</span><span class="p">)</span>
<span class="n">g_named_values</span><span class="p">[</span><span class="bp">self</span><span class="o">.</span><span class="n">loop_variable</span><span class="p">]</span> <span class="o">=</span> <span class="n">variable_phi</span>
<span class="c"># Emit the body of the loop. This, like any other expr, can change the</span>
<span class="c"># current BB. Note that we ignore the value computed by the body.</span>
<span class="bp">self</span><span class="o">.</span><span class="n">body</span><span class="o">.</span><span class="n">CodeGen</span><span class="p">()</span>
</pre></div>
</div>
<p>Now the code starts to get more interesting. Our ‘for’ loop introduces a
new variable to the symbol table. This means that our symbol table can
now contain either function arguments or loop variables. To handle this,
before we codegen the body of the loop, we add the loop variable as the
current value for its name. Note that it is possible that there is a
variable of the same name in the outer scope. It would be easy to make
this an error (emit an error and return null if there is already an
entry for VarName) but we choose to allow shadowing of variables. In
order to handle this correctly, we remember the Value that we are
potentially shadowing in <tt class="docutils literal"><span class="pre">old_value</span></tt> (which will be None if there is
no shadowed variable).</p>
<p>Once the loop variable is set into the symbol table, the code
recursively codegen’s the body. This allows the body to use the loop
variable: any references to it will naturally find it in the symbol
table.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="c"># Emit the step value.</span>
<span class="k">if</span> <span class="bp">self</span><span class="o">.</span><span class="n">step</span><span class="p">:</span>
<span class="n">step_value</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">step</span><span class="o">.</span><span class="n">CodeGen</span><span class="p">()</span>
<span class="k">else</span><span class="p">:</span>
<span class="c"># If not specified, use 1.0.</span>
<span class="n">step_value</span> <span class="o">=</span> <span class="n">Constant</span><span class="o">.</span><span class="n">real</span><span class="p">(</span><span class="n">Type</span><span class="o">.</span><span class="n">double</span><span class="p">(),</span> <span class="mi">1</span><span class="p">)</span>
<span class="n">next_value</span> <span class="o">=</span> <span class="n">g_llvm_builder</span><span class="o">.</span><span class="n">fadd</span><span class="p">(</span><span class="n">variable_phi</span><span class="p">,</span> <span class="n">step_value</span><span class="p">,</span> <span class="s">'next'</span><span class="p">)</span>
</pre></div>
</div>
<p>Now that the body is emitted, we compute the next value of the iteration
variable by adding the step value, or 1.0 if it isn’t present.
<tt class="docutils literal"><span class="pre">next_value</span></tt> will be the value of the loop variable on the next
iteration of the loop.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="c"># Compute the end condition and convert it to a bool by comparing to 0.0.</span>
<span class="n">end_condition</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">end</span><span class="o">.</span><span class="n">CodeGen</span><span class="p">()</span>
<span class="n">end_condition_bool</span> <span class="o">=</span> <span class="n">g_llvm_builder</span><span class="o">.</span><span class="n">fcmp</span><span class="p">(</span>
<span class="n">FCMP_ONE</span><span class="p">,</span> <span class="n">end_condition</span><span class="p">,</span> <span class="n">Constant</span><span class="o">.</span><span class="n">real</span><span class="p">(</span><span class="n">Type</span><span class="o">.</span><span class="n">double</span><span class="p">(),</span> <span class="mi">0</span><span class="p">),</span> <span class="s">'loopcond'</span><span class="p">)</span>
</pre></div>
</div>
<p>Finally, we evaluate the exit value of the loop, to determine whether
the loop should exit. This mirrors the condition evaluation for the
if/then/else statement.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="c"># Create the "after loop" block and insert it.</span>
<span class="n">loop_end_block</span> <span class="o">=</span> <span class="n">g_llvm_builder</span><span class="o">.</span><span class="n">basic_block</span>
<span class="n">after_block</span> <span class="o">=</span> <span class="n">function</span><span class="o">.</span><span class="n">append_basic_block</span><span class="p">(</span><span class="s">'afterloop'</span><span class="p">)</span>
<span class="c"># Insert the conditional branch into the end of loop_end_block.</span>
<span class="n">g_llvm_builder</span><span class="o">.</span><span class="n">cbranch</span><span class="p">(</span><span class="n">end_condition_bool</span><span class="p">,</span> <span class="n">loop_block</span><span class="p">,</span> <span class="n">after_block</span><span class="p">)</span>
<span class="c"># Any new code will be inserted in after_block.</span>
<span class="n">g_llvm_builder</span><span class="o">.</span><span class="n">position_at_end</span><span class="p">(</span><span class="n">after_block</span><span class="p">)</span>
</pre></div>
</div>
<p>With the code for the body of the loop complete, we just need to finish
up the control flow for it. This code remembers the end block (for the
phi node), then creates the block for the loop exit (“afterloop”). Based
on the value of the exit condition, it creates a conditional branch that
chooses between executing the loop again and exiting the loop. Any
future code is emitted in the “afterloop” block, so it sets the
insertion position to it.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="c"># Add a new entry to the PHI node for the backedge.</span>
<span class="n">variable_phi</span><span class="o">.</span><span class="n">add_incoming</span><span class="p">(</span><span class="n">next_value</span><span class="p">,</span> <span class="n">loop_end_block</span><span class="p">)</span>
<span class="c"># Restore the unshadowed variable.</span>
<span class="k">if</span> <span class="n">old_value</span><span class="p">:</span>
<span class="n">g_named_values</span><span class="p">[</span><span class="bp">self</span><span class="o">.</span><span class="n">loop_variable</span><span class="p">]</span> <span class="o">=</span> <span class="n">old_value</span>
<span class="k">else</span><span class="p">:</span>
<span class="k">del</span> <span class="n">g_named_values</span><span class="p">[</span><span class="bp">self</span><span class="o">.</span><span class="n">loop_variable</span><span class="p">]</span>
<span class="c"># for expr always returns 0.0.</span>
<span class="k">return</span> <span class="n">Constant</span><span class="o">.</span><span class="n">real</span><span class="p">(</span><span class="n">Type</span><span class="o">.</span><span class="n">double</span><span class="p">(),</span> <span class="mi">0</span><span class="p">)</span>
</pre></div>
</div>
<p>The final code handles various cleanups: now that we have the
“next_value”, we can add the incoming value to the loop PHI node. After
that, we remove the loop variable from the symbol table, so that it
isn’t in scope after the for loop. Finally, code generation of the for
loop always returns 0.0, so that is what we return from
<tt class="docutils literal"><span class="pre">ForExpressionNode::CodeGen</span></tt>.</p>
<p>With this, we conclude the “adding control flow to Kaleidoscope” chapter
of the tutorial. In this chapter we added two control flow constructs,
and used them to motivate a couple of aspects of the LLVM IR that are
important for front-end implementors to know. In the next chapter of our
saga, we will get a bit crazier and add <a class="reference external" href="PythonLangImpl6.html">user-defined
operators</a> to our poor innocent language.</p>
</div>
</div>
<hr class="docutils" />
<div class="section" id="full-code-listing">
<h2>5.4. Full Code Listing<a class="headerlink" href="#full-code-listing" title="Permalink to this headline">¶</a></h2>
<p>Here is the complete code listing for our running example, enhanced with
the if/then/else and for expressions:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="c">#!/usr/bin/env python</span>
<span class="kn">import</span> <span class="nn">re</span>
<span class="kn">from</span> <span class="nn">llvm.core</span> <span class="kn">import</span> <span class="n">Module</span><span class="p">,</span> <span class="n">Constant</span><span class="p">,</span> <span class="n">Type</span><span class="p">,</span> <span class="n">Function</span><span class="p">,</span> <span class="n">Builder</span>
<span class="kn">from</span> <span class="nn">llvm.ee</span> <span class="kn">import</span> <span class="n">ExecutionEngine</span><span class="p">,</span> <span class="n">TargetData</span>
<span class="kn">from</span> <span class="nn">llvm.passes</span> <span class="kn">import</span> <span class="n">FunctionPassManager</span>
<span class="kn">from</span> <span class="nn">llvm.core</span> <span class="kn">import</span> <span class="n">FCMP_ULT</span><span class="p">,</span> <span class="n">FCMP_ONE</span>
<span class="kn">from</span> <span class="nn">llvm.passes</span> <span class="kn">import</span> <span class="p">(</span><span class="n">PASS_INSTRUCTION_COMBINING</span><span class="p">,</span>
<span class="n">PASS_REASSOCIATE</span><span class="p">,</span>
<span class="n">PASS_GVN</span><span class="p">,</span>
<span class="n">PASS_CFG_SIMPLIFICATION</span><span class="p">)</span>
</pre></div>
</div>
<div class="section" id="globals">
<h3>5.4.1. Globals<a class="headerlink" href="#globals" title="Permalink to this headline">¶</a></h3>
<div class="highlight-python"><div class="highlight"><pre><span class="c"># The LLVM module, which holds all the IR code.</span>
<span class="n">g_llvm_module</span> <span class="o">=</span> <span class="n">Module</span><span class="o">.</span><span class="n">new</span><span class="p">(</span><span class="s">'my cool jit'</span><span class="p">)</span>
<span class="c"># The LLVM instruction builder. Created whenever a new function is entered.</span>
<span class="n">g_llvm_builder</span> <span class="o">=</span> <span class="bp">None</span>
<span class="c"># A dictionary that keeps track of which values are defined in the current scope</span>
<span class="c"># and what their LLVM representation is.</span>
<span class="n">g_named_values</span> <span class="o">=</span> <span class="p">{}</span>
<span class="c"># The function optimization passes manager.</span>
<span class="n">g_llvm_pass_manager</span> <span class="o">=</span> <span class="n">FunctionPassManager</span><span class="o">.</span><span class="n">new</span><span class="p">(</span><span class="n">g_llvm_module</span><span class="p">)</span>
<span class="c"># The LLVM execution engine.</span>
<span class="n">g_llvm_executor</span> <span class="o">=</span> <span class="n">ExecutionEngine</span><span class="o">.</span><span class="n">new</span><span class="p">(</span><span class="n">g_llvm_module</span><span class="p">)</span>
</pre></div>
</div>
</div>
<div class="section" id="lexer">
<h3>5.4.2. Lexer<a class="headerlink" href="#lexer" title="Permalink to this headline">¶</a></h3>
<div class="highlight-python"><div class="highlight"><pre><span class="c"># The lexer yields one of these types for each token.</span>
<span class="k">class</span> <span class="nc">EOFToken</span><span class="p">(</span><span class="nb">object</span><span class="p">):</span>
<span class="k">pass</span>
<span class="k">class</span> <span class="nc">DefToken</span><span class="p">(</span><span class="nb">object</span><span class="p">):</span>
<span class="k">pass</span>
<span class="k">class</span> <span class="nc">ExternToken</span><span class="p">(</span><span class="nb">object</span><span class="p">):</span>
<span class="k">pass</span>
<span class="k">class</span> <span class="nc">IfToken</span><span class="p">(</span><span class="nb">object</span><span class="p">):</span>
<span class="k">pass</span>
<span class="k">class</span> <span class="nc">ThenToken</span><span class="p">(</span><span class="nb">object</span><span class="p">):</span>
<span class="k">pass</span>
<span class="k">class</span> <span class="nc">ElseToken</span><span class="p">(</span><span class="nb">object</span><span class="p">):</span>
<span class="k">pass</span>
<span class="k">class</span> <span class="nc">ForToken</span><span class="p">(</span><span class="nb">object</span><span class="p">):</span>
<span class="k">pass</span>
<span class="k">class</span> <span class="nc">InToken</span><span class="p">(</span><span class="nb">object</span><span class="p">):</span>
<span class="k">pass</span>
<span class="k">class</span> <span class="nc">IdentifierToken</span><span class="p">(</span><span class="nb">object</span><span class="p">):</span>
<span class="k">def</span> <span class="nf">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">name</span><span class="p">):</span>
<span class="bp">self</span><span class="o">.</span><span class="n">name</span> <span class="o">=</span> <span class="n">name</span>
<span class="k">class</span> <span class="nc">NumberToken</span><span class="p">(</span><span class="nb">object</span><span class="p">):</span>
<span class="k">def</span> <span class="nf">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">value</span><span class="p">):</span>
<span class="bp">self</span><span class="o">.</span><span class="n">value</span> <span class="o">=</span> <span class="n">value</span>
<span class="k">class</span> <span class="nc">CharacterToken</span><span class="p">(</span><span class="nb">object</span><span class="p">):</span>
<span class="k">def</span> <span class="nf">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">char</span><span class="p">):</span>
<span class="bp">self</span><span class="o">.</span><span class="n">char</span> <span class="o">=</span> <span class="n">char</span>
<span class="k">def</span> <span class="nf">__eq__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">other</span><span class="p">):</span>
<span class="k">return</span> <span class="nb">isinstance</span><span class="p">(</span><span class="n">other</span><span class="p">,</span> <span class="n">CharacterToken</span><span class="p">)</span> <span class="ow">and</span> <span class="bp">self</span><span class="o">.</span><span class="n">char</span> <span class="o">==</span> <span class="n">other</span><span class="o">.</span><span class="n">char</span>
<span class="k">def</span> <span class="nf">__ne__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">other</span><span class="p">):</span>
<span class="k">return</span> <span class="ow">not</span> <span class="bp">self</span> <span class="o">==</span> <span class="n">other</span>
<span class="c"># Regular expressions that tokens and comments of our language.</span>
<span class="n">REGEX_NUMBER</span> <span class="o">=</span> <span class="n">re</span><span class="o">.</span><span class="n">compile</span><span class="p">(</span><span class="s">'[0-9]+(?:\.[0-9]+)?'</span><span class="p">)</span>
<span class="n">REGEX_IDENTIFIER</span> <span class="o">=</span> <span class="n">re</span><span class="o">.</span><span class="n">compile</span><span class="p">(</span><span class="s">'[a-zA-Z][a-zA-Z0-9]*'</span><span class="p">)</span>
<span class="n">REGEX_COMMENT</span> <span class="o">=</span> <span class="n">re</span><span class="o">.</span><span class="n">compile</span><span class="p">(</span><span class="s">'#.*'</span><span class="p">)</span>
<span class="k">def</span> <span class="nf">Tokenize</span><span class="p">(</span><span class="n">string</span><span class="p">):</span>
<span class="k">while</span> <span class="n">string</span><span class="p">:</span>
<span class="c"># Skip whitespace.</span>
<span class="k">if</span> <span class="n">string</span><span class="p">[</span><span class="mi">0</span><span class="p">]</span><span class="o">.</span><span class="n">isspace</span><span class="p">():</span>
<span class="n">string</span> <span class="o">=</span> <span class="n">string</span><span class="p">[</span><span class="mi">1</span><span class="p">:]</span>
<span class="k">continue</span>
<span class="c"># Run regexes.</span>
<span class="n">comment_match</span> <span class="o">=</span> <span class="n">REGEX_COMMENT</span><span class="o">.</span><span class="n">match</span><span class="p">(</span><span class="n">string</span><span class="p">)</span>
<span class="n">number_match</span> <span class="o">=</span> <span class="n">REGEX_NUMBER</span><span class="o">.</span><span class="n">match</span><span class="p">(</span><span class="n">string</span><span class="p">)</span>
<span class="n">identifier_match</span> <span class="o">=</span> <span class="n">REGEX_IDENTIFIER</span><span class="o">.</span><span class="n">match</span><span class="p">(</span><span class="n">string</span><span class="p">)</span>
<span class="c"># Check if any of the regexes matched and yield the appropriate result.</span>
<span class="k">if</span> <span class="n">comment_match</span><span class="p">:</span>
<span class="n">comment</span> <span class="o">=</span> <span class="n">comment_match</span><span class="o">.</span><span class="n">group</span><span class="p">(</span><span class="mi">0</span><span class="p">)</span>
<span class="n">string</span> <span class="o">=</span> <span class="n">string</span><span class="p">[</span><span class="nb">len</span><span class="p">(</span><span class="n">comment</span><span class="p">):]</span>
<span class="k">elif</span> <span class="n">number_match</span><span class="p">:</span>
<span class="n">number</span> <span class="o">=</span> <span class="n">number_match</span><span class="o">.</span><span class="n">group</span><span class="p">(</span><span class="mi">0</span><span class="p">)</span>
<span class="k">yield</span> <span class="n">NumberToken</span><span class="p">(</span><span class="nb">float</span><span class="p">(</span><span class="n">number</span><span class="p">))</span>
<span class="n">string</span> <span class="o">=</span> <span class="n">string</span><span class="p">[</span><span class="nb">len</span><span class="p">(</span><span class="n">number</span><span class="p">):]</span>
<span class="k">elif</span> <span class="n">identifier_match</span><span class="p">:</span>
<span class="n">identifier</span> <span class="o">=</span> <span class="n">identifier_match</span><span class="o">.</span><span class="n">group</span><span class="p">(</span><span class="mi">0</span><span class="p">)</span>
<span class="c"># Check if we matched a keyword.</span>
<span class="k">if</span> <span class="n">identifier</span> <span class="o">==</span> <span class="s">'def'</span><span class="p">:</span>
<span class="k">yield</span> <span class="n">DefToken</span><span class="p">()</span>
<span class="k">elif</span> <span class="n">identifier</span> <span class="o">==</span> <span class="s">'extern'</span><span class="p">:</span>
<span class="k">yield</span> <span class="n">ExternToken</span><span class="p">()</span>
<span class="k">elif</span> <span class="n">identifier</span> <span class="o">==</span> <span class="s">'if'</span><span class="p">:</span>
<span class="k">yield</span> <span class="n">IfToken</span><span class="p">()</span>
<span class="k">elif</span> <span class="n">identifier</span> <span class="o">==</span> <span class="s">'then'</span><span class="p">:</span>
<span class="k">yield</span> <span class="n">ThenToken</span><span class="p">()</span>
<span class="k">elif</span> <span class="n">identifier</span> <span class="o">==</span> <span class="s">'else'</span><span class="p">:</span>
<span class="k">yield</span> <span class="n">ElseToken</span><span class="p">()</span>
<span class="k">elif</span> <span class="n">identifier</span> <span class="o">==</span> <span class="s">'for'</span><span class="p">:</span>
<span class="k">yield</span> <span class="n">ForToken</span><span class="p">()</span>
<span class="k">elif</span> <span class="n">identifier</span> <span class="o">==</span> <span class="s">'in'</span><span class="p">:</span>
<span class="k">yield</span> <span class="n">InToken</span><span class="p">()</span>
<span class="k">else</span><span class="p">:</span>
<span class="k">yield</span> <span class="n">IdentifierToken</span><span class="p">(</span><span class="n">identifier</span><span class="p">)</span>
<span class="n">string</span> <span class="o">=</span> <span class="n">string</span><span class="p">[</span><span class="nb">len</span><span class="p">(</span><span class="n">identifier</span><span class="p">):]</span>
<span class="k">else</span><span class="p">:</span>
<span class="c"># Yield the ASCII value of the unknown character.</span>
<span class="k">yield</span> <span class="n">CharacterToken</span><span class="p">(</span><span class="n">string</span><span class="p">[</span><span class="mi">0</span><span class="p">])</span>
<span class="n">string</span> <span class="o">=</span> <span class="n">string</span><span class="p">[</span><span class="mi">1</span><span class="p">:]</span>
<span class="k">yield</span> <span class="n">EOFToken</span><span class="p">()</span>
</pre></div>
</div>
</div>
<div class="section" id="abstract-syntax-tree-aka-parse-tree">
<h3>5.4.3. Abstract Syntax Tree (aka Parse Tree)<a class="headerlink" href="#abstract-syntax-tree-aka-parse-tree" title="Permalink to this headline">¶</a></h3>
<div class="highlight-python"><div class="highlight"><pre><span class="c"># Base class for all expression nodes.</span>
<span class="k">class</span> <span class="nc">ExpressionNode</span><span class="p">(</span><span class="nb">object</span><span class="p">):</span>
<span class="k">pass</span>
<span class="c"># Expression class for numeric literals like "1.0".</span>
<span class="k">class</span> <span class="nc">NumberExpressionNode</span><span class="p">(</span><span class="n">ExpressionNode</span><span class="p">):</span>
<span class="k">def</span> <span class="nf">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">value</span><span class="p">):</span>
<span class="bp">self</span><span class="o">.</span><span class="n">value</span> <span class="o">=</span> <span class="n">value</span>
<span class="k">def</span> <span class="nf">CodeGen</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="k">return</span> <span class="n">Constant</span><span class="o">.</span><span class="n">real</span><span class="p">(</span><span class="n">Type</span><span class="o">.</span><span class="n">double</span><span class="p">(),</span> <span class="bp">self</span><span class="o">.</span><span class="n">value</span><span class="p">)</span>
<span class="c"># Expression class for referencing a variable, like "a".</span>
<span class="k">class</span> <span class="nc">VariableExpressionNode</span><span class="p">(</span><span class="n">ExpressionNode</span><span class="p">):</span>
<span class="k">def</span> <span class="nf">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">name</span><span class="p">):</span>
<span class="bp">self</span><span class="o">.</span><span class="n">name</span> <span class="o">=</span> <span class="n">name</span>
<span class="k">def</span> <span class="nf">CodeGen</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="k">if</span> <span class="bp">self</span><span class="o">.</span><span class="n">name</span> <span class="ow">in</span> <span class="n">g_named_values</span><span class="p">:</span>
<span class="k">return</span> <span class="n">g_named_values</span><span class="p">[</span><span class="bp">self</span><span class="o">.</span><span class="n">name</span><span class="p">]</span>
<span class="k">else</span><span class="p">:</span>
<span class="k">raise</span> <span class="ne">RuntimeError</span><span class="p">(</span><span class="s">'Unknown variable name: '</span> <span class="o">+</span> <span class="bp">self</span><span class="o">.</span><span class="n">name</span><span class="p">)</span>
<span class="c"># Expression class for a binary operator.</span>
<span class="k">class</span> <span class="nc">BinaryOperatorExpressionNode</span><span class="p">(</span><span class="n">ExpressionNode</span><span class="p">):</span>
<span class="k">def</span> <span class="nf">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">operator</span><span class="p">,</span> <span class="n">left</span><span class="p">,</span> <span class="n">right</span><span class="p">):</span>
<span class="bp">self</span><span class="o">.</span><span class="n">operator</span> <span class="o">=</span> <span class="n">operator</span>
<span class="bp">self</span><span class="o">.</span><span class="n">left</span> <span class="o">=</span> <span class="n">left</span>
<span class="bp">self</span><span class="o">.</span><span class="n">right</span> <span class="o">=</span> <span class="n">right</span>
<span class="k">def</span> <span class="nf">CodeGen</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="n">left</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">left</span><span class="o">.</span><span class="n">CodeGen</span><span class="p">()</span>
<span class="n">right</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">right</span><span class="o">.</span><span class="n">CodeGen</span><span class="p">()</span>
<span class="k">if</span> <span class="bp">self</span><span class="o">.</span><span class="n">operator</span> <span class="o">==</span> <span class="s">'+'</span><span class="p">:</span>
<span class="k">return</span> <span class="n">g_llvm_builder</span><span class="o">.</span><span class="n">fadd</span><span class="p">(</span><span class="n">left</span><span class="p">,</span> <span class="n">right</span><span class="p">,</span> <span class="s">'addtmp'</span><span class="p">)</span>
<span class="k">elif</span> <span class="bp">self</span><span class="o">.</span><span class="n">operator</span> <span class="o">==</span> <span class="s">'-'</span><span class="p">:</span>
<span class="k">return</span> <span class="n">g_llvm_builder</span><span class="o">.</span><span class="n">fsub</span><span class="p">(</span><span class="n">left</span><span class="p">,</span> <span class="n">right</span><span class="p">,</span> <span class="s">'subtmp'</span><span class="p">)</span>
<span class="k">elif</span> <span class="bp">self</span><span class="o">.</span><span class="n">operator</span> <span class="o">==</span> <span class="s">'*'</span><span class="p">:</span>
<span class="k">return</span> <span class="n">g_llvm_builder</span><span class="o">.</span><span class="n">fmul</span><span class="p">(</span><span class="n">left</span><span class="p">,</span> <span class="n">right</span><span class="p">,</span> <span class="s">'multmp'</span><span class="p">)</span>
<span class="k">elif</span> <span class="bp">self</span><span class="o">.</span><span class="n">operator</span> <span class="o">==</span> <span class="s">'<'</span><span class="p">:</span>
<span class="n">result</span> <span class="o">=</span> <span class="n">g_llvm_builder</span><span class="o">.</span><span class="n">fcmp</span><span class="p">(</span><span class="n">FCMP_ULT</span><span class="p">,</span> <span class="n">left</span><span class="p">,</span> <span class="n">right</span><span class="p">,</span> <span class="s">'cmptmp'</span><span class="p">)</span>
<span class="c"># Convert bool 0 or 1 to double 0.0 or 1.0.</span>
<span class="k">return</span> <span class="n">g_llvm_builder</span><span class="o">.</span><span class="n">uitofp</span><span class="p">(</span><span class="n">result</span><span class="p">,</span> <span class="n">Type</span><span class="o">.</span><span class="n">double</span><span class="p">(),</span> <span class="s">'booltmp'</span><span class="p">)</span>
<span class="k">else</span><span class="p">:</span>
<span class="k">raise</span> <span class="ne">RuntimeError</span><span class="p">(</span><span class="s">'Unknown binary operator.'</span><span class="p">)</span>
<span class="c"># Expression class for function calls.</span>
<span class="k">class</span> <span class="nc">CallExpressionNode</span><span class="p">(</span><span class="n">ExpressionNode</span><span class="p">):</span>
<span class="k">def</span> <span class="nf">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">callee</span><span class="p">,</span> <span class="n">args</span><span class="p">):</span>
<span class="bp">self</span><span class="o">.</span><span class="n">callee</span> <span class="o">=</span> <span class="n">callee</span>
<span class="bp">self</span><span class="o">.</span><span class="n">args</span> <span class="o">=</span> <span class="n">args</span>
<span class="k">def</span> <span class="nf">CodeGen</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="c"># Look up the name in the global module table.</span>
<span class="n">callee</span> <span class="o">=</span> <span class="n">g_llvm_module</span><span class="o">.</span><span class="n">get_function_named</span><span class="p">(</span><span class="bp">self</span><span class="o">.</span><span class="n">callee</span><span class="p">)</span>
<span class="c"># Check for argument mismatch error.</span>
<span class="k">if</span> <span class="nb">len</span><span class="p">(</span><span class="n">callee</span><span class="o">.</span><span class="n">args</span><span class="p">)</span> <span class="o">!=</span> <span class="nb">len</span><span class="p">(</span><span class="bp">self</span><span class="o">.</span><span class="n">args</span><span class="p">):</span>
<span class="k">raise</span> <span class="ne">RuntimeError</span><span class="p">(</span><span class="s">'Incorrect number of arguments passed.'</span><span class="p">)</span>
<span class="n">arg_values</span> <span class="o">=</span> <span class="p">[</span><span class="n">i</span><span class="o">.</span><span class="n">CodeGen</span><span class="p">()</span> <span class="k">for</span> <span class="n">i</span> <span class="ow">in</span> <span class="bp">self</span><span class="o">.</span><span class="n">args</span><span class="p">]</span>
<span class="k">return</span> <span class="n">g_llvm_builder</span><span class="o">.</span><span class="n">call</span><span class="p">(</span><span class="n">callee</span><span class="p">,</span> <span class="n">arg_values</span><span class="p">,</span> <span class="s">'calltmp'</span><span class="p">)</span>
<span class="c"># Expression class for if/then/else.</span>
<span class="k">class</span> <span class="nc">IfExpressionNode</span><span class="p">(</span><span class="n">ExpressionNode</span><span class="p">):</span>
<span class="k">def</span> <span class="nf">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">condition</span><span class="p">,</span> <span class="n">then_branch</span><span class="p">,</span> <span class="n">else_branch</span><span class="p">):</span>
<span class="bp">self</span><span class="o">.</span><span class="n">condition</span> <span class="o">=</span> <span class="n">condition</span>
<span class="bp">self</span><span class="o">.</span><span class="n">then_branch</span> <span class="o">=</span> <span class="n">then_branch</span>
<span class="bp">self</span><span class="o">.</span><span class="n">else_branch</span> <span class="o">=</span> <span class="n">else_branch</span>
<span class="k">def</span> <span class="nf">CodeGen</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="n">condition</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">condition</span><span class="o">.</span><span class="n">CodeGen</span><span class="p">()</span>
<span class="c"># Convert condition to a bool by comparing equal to 0.0.</span>
<span class="n">condition_bool</span> <span class="o">=</span> <span class="n">g_llvm_builder</span><span class="o">.</span><span class="n">fcmp</span><span class="p">(</span>
<span class="n">FCMP_ONE</span><span class="p">,</span> <span class="n">condition</span><span class="p">,</span> <span class="n">Constant</span><span class="o">.</span><span class="n">real</span><span class="p">(</span><span class="n">Type</span><span class="o">.</span><span class="n">double</span><span class="p">(),</span> <span class="mi">0</span><span class="p">),</span> <span class="s">'ifcond'</span><span class="p">)</span>
<span class="n">function</span> <span class="o">=</span> <span class="n">g_llvm_builder</span><span class="o">.</span><span class="n">basic_block</span><span class="o">.</span><span class="n">function</span>
<span class="c"># Create blocks for the then and else cases. Insert the 'then' block at the</span>
<span class="c"># end of the function.</span>
<span class="n">then_block</span> <span class="o">=</span> <span class="n">function</span><span class="o">.</span><span class="n">append_basic_block</span><span class="p">(</span><span class="s">'then'</span><span class="p">)</span>
<span class="n">else_block</span> <span class="o">=</span> <span class="n">function</span><span class="o">.</span><span class="n">append_basic_block</span><span class="p">(</span><span class="s">'else'</span><span class="p">)</span>
<span class="n">merge_block</span> <span class="o">=</span> <span class="n">function</span><span class="o">.</span><span class="n">append_basic_block</span><span class="p">(</span><span class="s">'ifcond'</span><span class="p">)</span>
<span class="n">g_llvm_builder</span><span class="o">.</span><span class="n">cbranch</span><span class="p">(</span><span class="n">condition_bool</span><span class="p">,</span> <span class="n">then_block</span><span class="p">,</span> <span class="n">else_block</span><span class="p">)</span>
<span class="c"># Emit then value.</span>
<span class="n">g_llvm_builder</span><span class="o">.</span><span class="n">position_at_end</span><span class="p">(</span><span class="n">then_block</span><span class="p">)</span>
<span class="n">then_value</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">then_branch</span><span class="o">.</span><span class="n">CodeGen</span><span class="p">()</span>
<span class="n">g_llvm_builder</span><span class="o">.</span><span class="n">branch</span><span class="p">(</span><span class="n">merge_block</span><span class="p">)</span>
<span class="c"># Codegen of 'Then' can change the current block; update then_block for the</span>
<span class="c"># PHI node.</span>
<span class="n">then_block</span> <span class="o">=</span> <span class="n">g_llvm_builder</span><span class="o">.</span><span class="n">basic_block</span>
<span class="c"># Emit else block.</span>
<span class="n">g_llvm_builder</span><span class="o">.</span><span class="n">position_at_end</span><span class="p">(</span><span class="n">else_block</span><span class="p">)</span>
<span class="n">else_value</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">else_branch</span><span class="o">.</span><span class="n">CodeGen</span><span class="p">()</span>
<span class="n">g_llvm_builder</span><span class="o">.</span><span class="n">branch</span><span class="p">(</span><span class="n">merge_block</span><span class="p">)</span>
<span class="c"># Codegen of 'Else' can change the current block, update else_block for the</span>
<span class="c"># PHI node.</span>
<span class="n">else_block</span> <span class="o">=</span> <span class="n">g_llvm_builder</span><span class="o">.</span><span class="n">basic_block</span>
<span class="c"># Emit merge block.</span>
<span class="n">g_llvm_builder</span><span class="o">.</span><span class="n">position_at_end</span><span class="p">(</span><span class="n">merge_block</span><span class="p">)</span>
<span class="n">phi</span> <span class="o">=</span> <span class="n">g_llvm_builder</span><span class="o">.</span><span class="n">phi</span><span class="p">(</span><span class="n">Type</span><span class="o">.</span><span class="n">double</span><span class="p">(),</span> <span class="s">'iftmp'</span><span class="p">)</span>
<span class="n">phi</span><span class="o">.</span><span class="n">add_incoming</span><span class="p">(</span><span class="n">then_value</span><span class="p">,</span> <span class="n">then_block</span><span class="p">)</span>
<span class="n">phi</span><span class="o">.</span><span class="n">add_incoming</span><span class="p">(</span><span class="n">else_value</span><span class="p">,</span> <span class="n">else_block</span><span class="p">)</span>
<span class="k">return</span> <span class="n">phi</span>
<span class="c"># Expression class for for/in.</span>
<span class="k">class</span> <span class="nc">ForExpressionNode</span><span class="p">(</span><span class="n">ExpressionNode</span><span class="p">):</span>
<span class="k">def</span> <span class="nf">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">loop_variable</span><span class="p">,</span> <span class="n">start</span><span class="p">,</span> <span class="n">end</span><span class="p">,</span> <span class="n">step</span><span class="p">,</span> <span class="n">body</span><span class="p">):</span>
<span class="bp">self</span><span class="o">.</span><span class="n">loop_variable</span> <span class="o">=</span> <span class="n">loop_variable</span>
<span class="bp">self</span><span class="o">.</span><span class="n">start</span> <span class="o">=</span> <span class="n">start</span>
<span class="bp">self</span><span class="o">.</span><span class="n">end</span> <span class="o">=</span> <span class="n">end</span>
<span class="bp">self</span><span class="o">.</span><span class="n">step</span> <span class="o">=</span> <span class="n">step</span>
<span class="bp">self</span><span class="o">.</span><span class="n">body</span> <span class="o">=</span> <span class="n">body</span>
<span class="k">def</span> <span class="nf">CodeGen</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="c"># Output this as:</span>
<span class="c"># ...</span>
<span class="c"># start = startexpr</span>
<span class="c"># goto loop</span>
<span class="c"># loop:</span>
<span class="c"># variable = phi [start, loopheader], [nextvariable, loopend]</span>
<span class="c"># ...</span>
<span class="c"># bodyexpr</span>
<span class="c"># ...</span>
<span class="c"># loopend:</span>
<span class="c"># step = stepexpr</span>
<span class="c"># nextvariable = variable + step # endcond = endexpr # br endcond, loop, endloop</span>
<span class="c"># outloop:</span>
<span class="c"># Emit the start code first, without 'variable' in scope.</span>
<span class="n">start_value</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">start</span><span class="o">.</span><span class="n">CodeGen</span><span class="p">()</span>
<span class="c"># Make the new basic block for the loop header, inserting after current</span>
<span class="c"># block.</span>
<span class="n">function</span> <span class="o">=</span> <span class="n">g_llvm_builder</span><span class="o">.</span><span class="n">basic_block</span><span class="o">.</span><span class="n">function</span>
<span class="n">pre_header_block</span> <span class="o">=</span> <span class="n">g_llvm_builder</span><span class="o">.</span><span class="n">basic_block</span>
<span class="n">loop_block</span> <span class="o">=</span> <span class="n">function</span><span class="o">.</span><span class="n">append_basic_block</span><span class="p">(</span><span class="s">'loop'</span><span class="p">)</span>
<span class="c"># Insert an explicit fallthrough from the current block to the loop_block.</span>
<span class="n">g_llvm_builder</span><span class="o">.</span><span class="n">branch</span><span class="p">(</span><span class="n">loop_block</span><span class="p">)</span>
<span class="c"># Start insertion in loop_block.</span>
<span class="n">g_llvm_builder</span><span class="o">.</span><span class="n">position_at_end</span><span class="p">(</span><span class="n">loop_block</span><span class="p">)</span>
<span class="c"># Start the PHI node with an entry for start.</span>
<span class="n">variable_phi</span> <span class="o">=</span> <span class="n">g_llvm_builder</span><span class="o">.</span><span class="n">phi</span><span class="p">(</span><span class="n">Type</span><span class="o">.</span><span class="n">double</span><span class="p">(),</span> <span class="bp">self</span><span class="o">.</span><span class="n">loop_variable</span><span class="p">)</span>
<span class="n">variable_phi</span><span class="o">.</span><span class="n">add_incoming</span><span class="p">(</span><span class="n">start_value</span><span class="p">,</span> <span class="n">pre_header_block</span><span class="p">)</span>
<span class="c"># Within the loop, the variable is defined equal to the PHI node. If it</span>
<span class="c"># shadows an existing variable, we have to restore it, so save it now.</span>
<span class="n">old_value</span> <span class="o">=</span> <span class="n">g_named_values</span><span class="o">.</span><span class="n">get</span><span class="p">(</span><span class="bp">self</span><span class="o">.</span><span class="n">loop_variable</span><span class="p">,</span> <span class="bp">None</span><span class="p">)</span>
<span class="n">g_named_values</span><span class="p">[</span><span class="bp">self</span><span class="o">.</span><span class="n">loop_variable</span><span class="p">]</span> <span class="o">=</span> <span class="n">variable_phi</span>
<span class="c"># Emit the body of the loop. This, like any other expr, can change the</span>
<span class="c"># current BB. Note that we ignore the value computed by the body.</span>
<span class="bp">self</span><span class="o">.</span><span class="n">body</span><span class="o">.</span><span class="n">CodeGen</span><span class="p">()</span>
<span class="c"># Emit the step value.</span>
<span class="k">if</span> <span class="bp">self</span><span class="o">.</span><span class="n">step</span><span class="p">:</span>
<span class="n">step_value</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">step</span><span class="o">.</span><span class="n">CodeGen</span><span class="p">()</span>
<span class="k">else</span><span class="p">:</span>
<span class="c"># If not specified, use 1.0.</span>
<span class="n">step_value</span> <span class="o">=</span> <span class="n">Constant</span><span class="o">.</span><span class="n">real</span><span class="p">(</span><span class="n">Type</span><span class="o">.</span><span class="n">double</span><span class="p">(),</span> <span class="mi">1</span><span class="p">)</span>
<span class="n">next_value</span> <span class="o">=</span> <span class="n">g_llvm_builder</span><span class="o">.</span><span class="n">fadd</span><span class="p">(</span><span class="n">variable_phi</span><span class="p">,</span> <span class="n">step_value</span><span class="p">,</span> <span class="s">'next'</span><span class="p">)</span>
<span class="c"># Compute the end condition and convert it to a bool by comparing to 0.0.</span>
<span class="n">end_condition</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">end</span><span class="o">.</span><span class="n">CodeGen</span><span class="p">()</span>
<span class="n">end_condition_bool</span> <span class="o">=</span> <span class="n">g_llvm_builder</span><span class="o">.</span><span class="n">fcmp</span><span class="p">(</span>
<span class="n">FCMP_ONE</span><span class="p">,</span> <span class="n">end_condition</span><span class="p">,</span> <span class="n">Constant</span><span class="o">.</span><span class="n">real</span><span class="p">(</span><span class="n">Type</span><span class="o">.</span><span class="n">double</span><span class="p">(),</span> <span class="mi">0</span><span class="p">),</span> <span class="s">'loopcond'</span><span class="p">)</span>
<span class="c"># Create the "after loop" block and insert it.</span>
<span class="n">loop_end_block</span> <span class="o">=</span> <span class="n">g_llvm_builder</span><span class="o">.</span><span class="n">basic_block</span>
<span class="n">after_block</span> <span class="o">=</span> <span class="n">function</span><span class="o">.</span><span class="n">append_basic_block</span><span class="p">(</span><span class="s">'afterloop'</span><span class="p">)</span>
<span class="c"># Insert the conditional branch into the end of loop_end_block.</span>
<span class="n">g_llvm_builder</span><span class="o">.</span><span class="n">cbranch</span><span class="p">(</span><span class="n">end_condition_bool</span><span class="p">,</span> <span class="n">loop_block</span><span class="p">,</span> <span class="n">after_block</span><span class="p">)</span>
<span class="c"># Any new code will be inserted in after_block.</span>
<span class="n">g_llvm_builder</span><span class="o">.</span><span class="n">position_at_end</span><span class="p">(</span><span class="n">after_block</span><span class="p">)</span>
<span class="c"># Add a new entry to the PHI node for the backedge.</span>
<span class="n">variable_phi</span><span class="o">.</span><span class="n">add_incoming</span><span class="p">(</span><span class="n">next_value</span><span class="p">,</span> <span class="n">loop_end_block</span><span class="p">)</span>
<span class="c"># Restore the unshadowed variable.</span>
<span class="k">if</span> <span class="n">old_value</span><span class="p">:</span>
<span class="n">g_named_values</span><span class="p">[</span><span class="bp">self</span><span class="o">.</span><span class="n">loop_variable</span><span class="p">]</span> <span class="o">=</span> <span class="n">old_value</span>
<span class="k">else</span><span class="p">:</span>
<span class="k">del</span> <span class="n">g_named_values</span><span class="p">[</span><span class="bp">self</span><span class="o">.</span><span class="n">loop_variable</span><span class="p">]</span>
<span class="c"># for expr always returns 0.0.</span>
<span class="k">return</span> <span class="n">Constant</span><span class="o">.</span><span class="n">real</span><span class="p">(</span><span class="n">Type</span><span class="o">.</span><span class="n">double</span><span class="p">(),</span> <span class="mi">0</span><span class="p">)</span>
<span class="c"># This class represents the "prototype" for a function, which captures its name,</span>
<span class="c"># and its argument names (thus implicitly the number of arguments the function</span>
<span class="c"># takes).</span>
<span class="k">class</span> <span class="nc">PrototypeNode</span><span class="p">(</span><span class="nb">object</span><span class="p">):</span>
<span class="k">def</span> <span class="nf">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">name</span><span class="p">,</span> <span class="n">args</span><span class="p">):</span>
<span class="bp">self</span><span class="o">.</span><span class="n">name</span> <span class="o">=</span> <span class="n">name</span>
<span class="bp">self</span><span class="o">.</span><span class="n">args</span> <span class="o">=</span> <span class="n">args</span>
<span class="k">def</span> <span class="nf">CodeGen</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="c"># Make the function type, eg. double(double,double).</span>
<span class="n">funct_type</span> <span class="o">=</span> <span class="n">Type</span><span class="o">.</span><span class="n">function</span><span class="p">(</span>
<span class="n">Type</span><span class="o">.</span><span class="n">double</span><span class="p">(),</span> <span class="p">[</span><span class="n">Type</span><span class="o">.</span><span class="n">double</span><span class="p">()]</span> <span class="o">*</span> <span class="nb">len</span><span class="p">(</span><span class="bp">self</span><span class="o">.</span><span class="n">args</span><span class="p">),</span> <span class="bp">False</span><span class="p">)</span>
<span class="n">function</span> <span class="o">=</span> <span class="n">Function</span><span class="o">.</span><span class="n">new</span><span class="p">(</span><span class="n">g_llvm_module</span><span class="p">,</span> <span class="n">funct_type</span><span class="p">,</span> <span class="bp">self</span><span class="o">.</span><span class="n">name</span><span class="p">)</span>
<span class="c"># If the name conflicted, there was already something with the same name.</span>
<span class="c"># If it has a body, don't allow redefinition or reextern.</span>
<span class="k">if</span> <span class="n">function</span><span class="o">.</span><span class="n">name</span> <span class="o">!=</span> <span class="bp">self</span><span class="o">.</span><span class="n">name</span><span class="p">:</span>
<span class="n">function</span><span class="o">.</span><span class="n">delete</span><span class="p">()</span>
<span class="n">function</span> <span class="o">=</span> <span class="n">g_llvm_module</span><span class="o">.</span><span class="n">get_function_named</span><span class="p">(</span><span class="bp">self</span><span class="o">.</span><span class="n">name</span><span class="p">)</span>
<span class="c"># If the function already has a body, reject this.</span>
<span class="k">if</span> <span class="ow">not</span> <span class="n">function</span><span class="o">.</span><span class="n">is_declaration</span><span class="p">:</span>
<span class="k">raise</span> <span class="ne">RuntimeError</span><span class="p">(</span><span class="s">'Redefinition of function.'</span><span class="p">)</span>
<span class="c"># If the function took a different number of args, reject.</span>
<span class="k">if</span> <span class="nb">len</span><span class="p">(</span><span class="n">function</span><span class="o">.</span><span class="n">args</span><span class="p">)</span> <span class="o">!=</span> <span class="nb">len</span><span class="p">(</span><span class="bp">self</span><span class="o">.</span><span class="n">args</span><span class="p">):</span>
<span class="k">raise</span> <span class="ne">RuntimeError</span><span class="p">(</span><span class="s">'Redeclaration of a function with different number '</span>
<span class="s">'of args.'</span><span class="p">)</span>
<span class="c"># Set names for all arguments and add them to the variables symbol table.</span>
<span class="k">for</span> <span class="n">arg</span><span class="p">,</span> <span class="n">arg_name</span> <span class="ow">in</span> <span class="nb">zip</span><span class="p">(</span><span class="n">function</span><span class="o">.</span><span class="n">args</span><span class="p">,</span> <span class="bp">self</span><span class="o">.</span><span class="n">args</span><span class="p">):</span>
<span class="n">arg</span><span class="o">.</span><span class="n">name</span> <span class="o">=</span> <span class="n">arg_name</span>
<span class="c"># Add arguments to variable symbol table.</span>
<span class="n">g_named_values</span><span class="p">[</span><span class="n">arg_name</span><span class="p">]</span> <span class="o">=</span> <span class="n">arg</span>
<span class="k">return</span> <span class="n">function</span>
<span class="c"># This class represents a function definition itself.</span>
<span class="k">class</span> <span class="nc">FunctionNode</span><span class="p">(</span><span class="nb">object</span><span class="p">):</span>
<span class="k">def</span> <span class="nf">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">prototype</span><span class="p">,</span> <span class="n">body</span><span class="p">):</span>
<span class="bp">self</span><span class="o">.</span><span class="n">prototype</span> <span class="o">=</span> <span class="n">prototype</span>
<span class="bp">self</span><span class="o">.</span><span class="n">body</span> <span class="o">=</span> <span class="n">body</span>
<span class="k">def</span> <span class="nf">CodeGen</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="c"># Clear scope.</span>
<span class="n">g_named_values</span><span class="o">.</span><span class="n">clear</span><span class="p">()</span>
<span class="c"># Create a function object.</span>
<span class="n">function</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">prototype</span><span class="o">.</span><span class="n">CodeGen</span><span class="p">()</span>
<span class="c"># Create a new basic block to start insertion into.</span>
<span class="n">block</span> <span class="o">=</span> <span class="n">function</span><span class="o">.</span><span class="n">append_basic_block</span><span class="p">(</span><span class="s">'entry'</span><span class="p">)</span>
<span class="k">global</span> <span class="n">g_llvm_builder</span>
<span class="n">g_llvm_builder</span> <span class="o">=</span> <span class="n">Builder</span><span class="o">.</span><span class="n">new</span><span class="p">(</span><span class="n">block</span><span class="p">)</span>
<span class="c"># Finish off the function.</span>
<span class="k">try</span><span class="p">:</span>
<span class="n">return_value</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">body</span><span class="o">.</span><span class="n">CodeGen</span><span class="p">()</span>
<span class="n">g_llvm_builder</span><span class="o">.</span><span class="n">ret</span><span class="p">(</span><span class="n">return_value</span><span class="p">)</span>
<span class="c"># Validate the generated code, checking for consistency.</span>
<span class="n">function</span><span class="o">.</span><span class="n">verify</span><span class="p">()</span>
<span class="c"># Optimize the function.</span>
<span class="n">g_llvm_pass_manager</span><span class="o">.</span><span class="n">run</span><span class="p">(</span><span class="n">function</span><span class="p">)</span>
<span class="k">except</span><span class="p">:</span>
<span class="n">function</span><span class="o">.</span><span class="n">delete</span><span class="p">()</span>
<span class="k">raise</span>
<span class="k">return</span> <span class="n">function</span>
</pre></div>
</div>
</div>
<div class="section" id="parser">
<h3>5.4.4. Parser<a class="headerlink" href="#parser" title="Permalink to this headline">¶</a></h3>
<div class="highlight-python"><div class="highlight"><pre><span class="k">class</span> <span class="nc">Parser</span><span class="p">(</span><span class="nb">object</span><span class="p">):</span>
<span class="k">def</span> <span class="nf">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">tokens</span><span class="p">,</span> <span class="n">binop_precedence</span><span class="p">):</span>
<span class="bp">self</span><span class="o">.</span><span class="n">tokens</span> <span class="o">=</span> <span class="n">tokens</span>
<span class="bp">self</span><span class="o">.</span><span class="n">binop_precedence</span> <span class="o">=</span> <span class="n">binop_precedence</span>
<span class="bp">self</span><span class="o">.</span><span class="n">Next</span><span class="p">()</span>
<span class="c"># Provide a simple token buffer. Parser.current is the current token the</span>
<span class="c"># parser is looking at. Parser.Next() reads another token from the lexer and</span>
<span class="c"># updates Parser.current with its results.</span>
<span class="k">def</span> <span class="nf">Next</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="bp">self</span><span class="o">.</span><span class="n">current</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">tokens</span><span class="o">.</span><span class="n">next</span><span class="p">()</span>
<span class="c"># Gets the precedence of the current token, or -1 if the token is not a binary</span>
<span class="c"># operator.</span>
<span class="k">def</span> <span class="nf">GetCurrentTokenPrecedence</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="k">if</span> <span class="nb">isinstance</span><span class="p">(</span><span class="bp">self</span><span class="o">.</span><span class="n">current</span><span class="p">,</span> <span class="n">CharacterToken</span><span class="p">):</span>
<span class="k">return</span> <span class="bp">self</span><span class="o">.</span><span class="n">binop_precedence</span><span class="o">.</span><span class="n">get</span><span class="p">(</span><span class="bp">self</span><span class="o">.</span><span class="n">current</span><span class="o">.</span><span class="n">char</span><span class="p">,</span> <span class="o">-</span><span class="mi">1</span><span class="p">)</span>
<span class="k">else</span><span class="p">:</span>
<span class="k">return</span> <span class="o">-</span><span class="mi">1</span>
<span class="c"># identifierexpr ::= identifier | identifier '(' expression* ')'</span>
<span class="k">def</span> <span class="nf">ParseIdentifierExpr</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="n">identifier_name</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">current</span><span class="o">.</span><span class="n">name</span>
<span class="bp">self</span><span class="o">.</span><span class="n">Next</span><span class="p">()</span> <span class="c"># eat identifier.</span>
<span class="k">if</span> <span class="bp">self</span><span class="o">.</span><span class="n">current</span> <span class="o">!=</span> <span class="n">CharacterToken</span><span class="p">(</span><span class="s">'('</span><span class="p">):</span> <span class="c"># Simple variable reference.</span>
<span class="k">return</span> <span class="n">VariableExpressionNode</span><span class="p">(</span><span class="n">identifier_name</span><span class="p">)</span>
<span class="c"># Call.</span>
<span class="bp">self</span><span class="o">.</span><span class="n">Next</span><span class="p">()</span> <span class="c"># eat '('.</span>
<span class="n">args</span> <span class="o">=</span> <span class="p">[]</span>
<span class="k">if</span> <span class="bp">self</span><span class="o">.</span><span class="n">current</span> <span class="o">!=</span> <span class="n">CharacterToken</span><span class="p">(</span><span class="s">')'</span><span class="p">):</span>
<span class="k">while</span> <span class="bp">True</span><span class="p">:</span>
<span class="n">args</span><span class="o">.</span><span class="n">append</span><span class="p">(</span><span class="bp">self</span><span class="o">.</span><span class="n">ParseExpression</span><span class="p">())</span>
<span class="k">if</span> <span class="bp">self</span><span class="o">.</span><span class="n">current</span> <span class="o">==</span> <span class="n">CharacterToken</span><span class="p">(</span><span class="s">')'</span><span class="p">):</span>
<span class="k">break</span>
<span class="k">elif</span> <span class="bp">self</span><span class="o">.</span><span class="n">current</span> <span class="o">!=</span> <span class="n">CharacterToken</span><span class="p">(</span><span class="s">','</span><span class="p">):</span>
<span class="k">raise</span> <span class="ne">RuntimeError</span><span class="p">(</span><span class="s">'Expected ")" or "," in argument list.'</span><span class="p">)</span>
<span class="bp">self</span><span class="o">.</span><span class="n">Next</span><span class="p">()</span>
<span class="bp">self</span><span class="o">.</span><span class="n">Next</span><span class="p">()</span> <span class="c"># eat ')'.</span>
<span class="k">return</span> <span class="n">CallExpressionNode</span><span class="p">(</span><span class="n">identifier_name</span><span class="p">,</span> <span class="n">args</span><span class="p">)</span>
<span class="c"># numberexpr ::= number</span>
<span class="k">def</span> <span class="nf">ParseNumberExpr</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="n">result</span> <span class="o">=</span> <span class="n">NumberExpressionNode</span><span class="p">(</span><span class="bp">self</span><span class="o">.</span><span class="n">current</span><span class="o">.</span><span class="n">value</span><span class="p">)</span>
<span class="bp">self</span><span class="o">.</span><span class="n">Next</span><span class="p">()</span> <span class="c"># consume the number.</span>
<span class="k">return</span> <span class="n">result</span>
<span class="c"># parenexpr ::= '(' expression ')'</span>
<span class="k">def</span> <span class="nf">ParseParenExpr</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="bp">self</span><span class="o">.</span><span class="n">Next</span><span class="p">()</span> <span class="c"># eat '('.</span>
<span class="n">contents</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">ParseExpression</span><span class="p">()</span>
<span class="k">if</span> <span class="bp">self</span><span class="o">.</span><span class="n">current</span> <span class="o">!=</span> <span class="n">CharacterToken</span><span class="p">(</span><span class="s">')'</span><span class="p">):</span>
<span class="k">raise</span> <span class="ne">RuntimeError</span><span class="p">(</span><span class="s">'Expected ")".'</span><span class="p">)</span>
<span class="bp">self</span><span class="o">.</span><span class="n">Next</span><span class="p">()</span> <span class="c"># eat ')'.</span>
<span class="k">return</span> <span class="n">contents</span>
<span class="c"># ifexpr ::= 'if' expression 'then' expression 'else' expression</span>
<span class="k">def</span> <span class="nf">ParseIfExpr</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="bp">self</span><span class="o">.</span><span class="n">Next</span><span class="p">()</span> <span class="c"># eat the if.</span>
<span class="c"># condition.</span>
<span class="n">condition</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">ParseExpression</span><span class="p">()</span>
<span class="k">if</span> <span class="ow">not</span> <span class="nb">isinstance</span><span class="p">(</span><span class="bp">self</span><span class="o">.</span><span class="n">current</span><span class="p">,</span> <span class="n">ThenToken</span><span class="p">):</span>
<span class="k">raise</span> <span class="ne">RuntimeError</span><span class="p">(</span><span class="s">'Expected "then".'</span><span class="p">)</span>
<span class="bp">self</span><span class="o">.</span><span class="n">Next</span><span class="p">()</span> <span class="c"># eat the then.</span>
<span class="n">then_branch</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">ParseExpression</span><span class="p">()</span>
<span class="k">if</span> <span class="ow">not</span> <span class="nb">isinstance</span><span class="p">(</span><span class="bp">self</span><span class="o">.</span><span class="n">current</span><span class="p">,</span> <span class="n">ElseToken</span><span class="p">):</span>
<span class="k">raise</span> <span class="ne">RuntimeError</span><span class="p">(</span><span class="s">'Expected "else".'</span><span class="p">)</span>
<span class="bp">self</span><span class="o">.</span><span class="n">Next</span><span class="p">()</span> <span class="c"># eat the else.</span>
<span class="n">else_branch</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">ParseExpression</span><span class="p">()</span>
<span class="k">return</span> <span class="n">IfExpressionNode</span><span class="p">(</span><span class="n">condition</span><span class="p">,</span> <span class="n">then_branch</span><span class="p">,</span> <span class="n">else_branch</span><span class="p">)</span>
<span class="c"># forexpr ::= 'for' identifier '=' expr ',' expr (',' expr)? 'in'</span>
<span class="n">expression</span>
<span class="k">def</span> <span class="nf">ParseForExpr</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="bp">self</span><span class="o">.</span><span class="n">Next</span><span class="p">()</span> <span class="c"># eat the for.</span>
<span class="k">if</span> <span class="ow">not</span> <span class="nb">isinstance</span><span class="p">(</span><span class="bp">self</span><span class="o">.</span><span class="n">current</span><span class="p">,</span> <span class="n">IdentifierToken</span><span class="p">):</span>
<span class="k">raise</span> <span class="ne">RuntimeError</span><span class="p">(</span><span class="s">'Expected identifier after for.'</span><span class="p">)</span>
<span class="n">loop_variable</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">current</span><span class="o">.</span><span class="n">name</span>
<span class="bp">self</span><span class="o">.</span><span class="n">Next</span><span class="p">()</span> <span class="c"># eat the identifier.</span>
<span class="k">if</span> <span class="bp">self</span><span class="o">.</span><span class="n">current</span> <span class="o">!=</span> <span class="n">CharacterToken</span><span class="p">(</span><span class="s">'='</span><span class="p">):</span>
<span class="k">raise</span> <span class="ne">RuntimeError</span><span class="p">(</span><span class="s">'Expected "=" after for variable.'</span><span class="p">)</span>
<span class="bp">self</span><span class="o">.</span><span class="n">Next</span><span class="p">()</span> <span class="c"># eat the '='.</span>
<span class="n">start</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">ParseExpression</span><span class="p">()</span>
<span class="k">if</span> <span class="bp">self</span><span class="o">.</span><span class="n">current</span> <span class="o">!=</span> <span class="n">CharacterToken</span><span class="p">(</span><span class="s">','</span><span class="p">):</span>
<span class="k">raise</span> <span class="ne">RuntimeError</span><span class="p">(</span><span class="s">'Expected "," after for start value.'</span><span class="p">)</span>
<span class="bp">self</span><span class="o">.</span><span class="n">Next</span><span class="p">()</span> <span class="c"># eat the ','.</span>
<span class="n">end</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">ParseExpression</span><span class="p">()</span>
<span class="c"># The step value is optional.</span>
<span class="k">if</span> <span class="bp">self</span><span class="o">.</span><span class="n">current</span> <span class="o">==</span> <span class="n">CharacterToken</span><span class="p">(</span><span class="s">','</span><span class="p">):</span>
<span class="bp">self</span><span class="o">.</span><span class="n">Next</span><span class="p">()</span> <span class="c"># eat the ','.</span>
<span class="n">step</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">ParseExpression</span><span class="p">()</span>
<span class="k">else</span><span class="p">:</span>
<span class="n">step</span> <span class="o">=</span> <span class="bp">None</span>
<span class="k">if</span> <span class="ow">not</span> <span class="nb">isinstance</span><span class="p">(</span><span class="bp">self</span><span class="o">.</span><span class="n">current</span><span class="p">,</span> <span class="n">InToken</span><span class="p">):</span>
<span class="k">raise</span> <span class="ne">RuntimeError</span><span class="p">(</span><span class="s">'Expected "in" after for variable specification.'</span><span class="p">)</span>
<span class="bp">self</span><span class="o">.</span><span class="n">Next</span><span class="p">()</span> <span class="c"># eat 'in'.</span>
<span class="n">body</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">ParseExpression</span><span class="p">()</span>
<span class="k">return</span> <span class="n">ForExpressionNode</span><span class="p">(</span><span class="n">loop_variable</span><span class="p">,</span> <span class="n">start</span><span class="p">,</span> <span class="n">end</span><span class="p">,</span> <span class="n">step</span><span class="p">,</span> <span class="n">body</span><span class="p">)</span>
<span class="c"># primary ::= identifierexpr | numberexpr | parenexpr | ifexpr | forexpr</span>
<span class="k">def</span> <span class="nf">ParsePrimary</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="k">if</span> <span class="nb">isinstance</span><span class="p">(</span><span class="bp">self</span><span class="o">.</span><span class="n">current</span><span class="p">,</span> <span class="n">IdentifierToken</span><span class="p">):</span>
<span class="k">return</span> <span class="bp">self</span><span class="o">.</span><span class="n">ParseIdentifierExpr</span><span class="p">()</span>
<span class="k">elif</span> <span class="nb">isinstance</span><span class="p">(</span><span class="bp">self</span><span class="o">.</span><span class="n">current</span><span class="p">,</span> <span class="n">NumberToken</span><span class="p">):</span>
<span class="k">return</span> <span class="bp">self</span><span class="o">.</span><span class="n">ParseNumberExpr</span><span class="p">()</span>
<span class="k">elif</span> <span class="nb">isinstance</span><span class="p">(</span><span class="bp">self</span><span class="o">.</span><span class="n">current</span><span class="p">,</span> <span class="n">IfToken</span><span class="p">):</span>
<span class="k">return</span> <span class="bp">self</span><span class="o">.</span><span class="n">ParseIfExpr</span><span class="p">()</span>
<span class="k">elif</span> <span class="nb">isinstance</span><span class="p">(</span><span class="bp">self</span><span class="o">.</span><span class="n">current</span><span class="p">,</span> <span class="n">ForToken</span><span class="p">):</span>
<span class="k">return</span> <span class="bp">self</span><span class="o">.</span><span class="n">ParseForExpr</span><span class="p">()</span>
<span class="k">elif</span> <span class="bp">self</span><span class="o">.</span><span class="n">current</span> <span class="o">==</span> <span class="n">CharacterToken</span><span class="p">(</span><span class="s">'('</span><span class="p">):</span>
<span class="k">return</span> <span class="bp">self</span><span class="o">.</span><span class="n">ParseParenExpr</span><span class="p">()</span>
<span class="k">else</span><span class="p">:</span>
<span class="k">raise</span> <span class="ne">RuntimeError</span><span class="p">(</span><span class="s">'Unknown token when expecting an expression.'</span><span class="p">)</span>
<span class="c"># binoprhs ::= (operator primary)*</span>
<span class="k">def</span> <span class="nf">ParseBinOpRHS</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">left</span><span class="p">,</span> <span class="n">left_precedence</span><span class="p">):</span>
<span class="c"># If this is a binary operator, find its precedence.</span>
<span class="k">while</span> <span class="bp">True</span><span class="p">:</span>
<span class="n">precedence</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">GetCurrentTokenPrecedence</span><span class="p">()</span>
<span class="c"># If this is a binary operator that binds at least as tightly as the</span>
<span class="c"># current one, consume it; otherwise we are done.</span>
<span class="k">if</span> <span class="n">precedence</span> <span class="o"><</span> <span class="n">left_precedence</span><span class="p">:</span>
<span class="k">return</span> <span class="n">left</span>
<span class="n">binary_operator</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">current</span><span class="o">.</span><span class="n">char</span>
<span class="bp">self</span><span class="o">.</span><span class="n">Next</span><span class="p">()</span> <span class="c"># eat the operator.</span>
<span class="c"># Parse the primary expression after the binary operator.</span>
<span class="n">right</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">ParsePrimary</span><span class="p">()</span>
<span class="c"># If binary_operator binds less tightly with right than the operator after</span>
<span class="c"># right, let the pending operator take right as its left.</span>
<span class="n">next_precedence</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">GetCurrentTokenPrecedence</span><span class="p">()</span>
<span class="k">if</span> <span class="n">precedence</span> <span class="o"><</span> <span class="n">next_precedence</span><span class="p">:</span>
<span class="n">right</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">ParseBinOpRHS</span><span class="p">(</span><span class="n">right</span><span class="p">,</span> <span class="n">precedence</span> <span class="o">+</span> <span class="mi">1</span><span class="p">)</span>
<span class="c"># Merge left/right.</span>
<span class="n">left</span> <span class="o">=</span> <span class="n">BinaryOperatorExpressionNode</span><span class="p">(</span><span class="n">binary_operator</span><span class="p">,</span> <span class="n">left</span><span class="p">,</span> <span class="n">right</span><span class="p">)</span>
<span class="c"># expression ::= primary binoprhs</span>
<span class="k">def</span> <span class="nf">ParseExpression</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="n">left</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">ParsePrimary</span><span class="p">()</span>
<span class="k">return</span> <span class="bp">self</span><span class="o">.</span><span class="n">ParseBinOpRHS</span><span class="p">(</span><span class="n">left</span><span class="p">,</span> <span class="mi">0</span><span class="p">)</span>
<span class="c"># prototype ::= id '(' id* ')'</span>
<span class="k">def</span> <span class="nf">ParsePrototype</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="k">if</span> <span class="ow">not</span> <span class="nb">isinstance</span><span class="p">(</span><span class="bp">self</span><span class="o">.</span><span class="n">current</span><span class="p">,</span> <span class="n">IdentifierToken</span><span class="p">):</span>
<span class="k">raise</span> <span class="ne">RuntimeError</span><span class="p">(</span><span class="s">'Expected function name in prototype.'</span><span class="p">)</span>
<span class="n">function_name</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">current</span><span class="o">.</span><span class="n">name</span>
<span class="bp">self</span><span class="o">.</span><span class="n">Next</span><span class="p">()</span> <span class="c"># eat function name.</span>
<span class="k">if</span> <span class="bp">self</span><span class="o">.</span><span class="n">current</span> <span class="o">!=</span> <span class="n">CharacterToken</span><span class="p">(</span><span class="s">'('</span><span class="p">):</span>
<span class="k">raise</span> <span class="ne">RuntimeError</span><span class="p">(</span><span class="s">'Expected "(" in prototype.'</span><span class="p">)</span>
<span class="bp">self</span><span class="o">.</span><span class="n">Next</span><span class="p">()</span> <span class="c"># eat '('.</span>
<span class="n">arg_names</span> <span class="o">=</span> <span class="p">[]</span>
<span class="k">while</span> <span class="nb">isinstance</span><span class="p">(</span><span class="bp">self</span><span class="o">.</span><span class="n">current</span><span class="p">,</span> <span class="n">IdentifierToken</span><span class="p">):</span>
<span class="n">arg_names</span><span class="o">.</span><span class="n">append</span><span class="p">(</span><span class="bp">self</span><span class="o">.</span><span class="n">current</span><span class="o">.</span><span class="n">name</span><span class="p">)</span>
<span class="bp">self</span><span class="o">.</span><span class="n">Next</span><span class="p">()</span>
<span class="k">if</span> <span class="bp">self</span><span class="o">.</span><span class="n">current</span> <span class="o">!=</span> <span class="n">CharacterToken</span><span class="p">(</span><span class="s">')'</span><span class="p">):</span>
<span class="k">raise</span> <span class="ne">RuntimeError</span><span class="p">(</span><span class="s">'Expected ")" in prototype.'</span><span class="p">)</span>
<span class="c"># Success.</span>
<span class="bp">self</span><span class="o">.</span><span class="n">Next</span><span class="p">()</span> <span class="c"># eat ')'.</span>
<span class="k">return</span> <span class="n">PrototypeNode</span><span class="p">(</span><span class="n">function_name</span><span class="p">,</span> <span class="n">arg_names</span><span class="p">)</span>
<span class="c"># definition ::= 'def' prototype expression</span>
<span class="k">def</span> <span class="nf">ParseDefinition</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="bp">self</span><span class="o">.</span><span class="n">Next</span><span class="p">()</span> <span class="c"># eat def.</span>
<span class="n">proto</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">ParsePrototype</span><span class="p">()</span>
<span class="n">body</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">ParseExpression</span><span class="p">()</span>
<span class="k">return</span> <span class="n">FunctionNode</span><span class="p">(</span><span class="n">proto</span><span class="p">,</span> <span class="n">body</span><span class="p">)</span>
<span class="c"># toplevelexpr ::= expression</span>
<span class="k">def</span> <span class="nf">ParseTopLevelExpr</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="n">proto</span> <span class="o">=</span> <span class="n">PrototypeNode</span><span class="p">(</span><span class="s">''</span><span class="p">,</span> <span class="p">[])</span>
<span class="k">return</span> <span class="n">FunctionNode</span><span class="p">(</span><span class="n">proto</span><span class="p">,</span> <span class="bp">self</span><span class="o">.</span><span class="n">ParseExpression</span><span class="p">())</span>
<span class="c"># external ::= 'extern' prototype</span>
<span class="k">def</span> <span class="nf">ParseExtern</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="bp">self</span><span class="o">.</span><span class="n">Next</span><span class="p">()</span> <span class="c"># eat extern.</span>
<span class="k">return</span> <span class="bp">self</span><span class="o">.</span><span class="n">ParsePrototype</span><span class="p">()</span>
<span class="c"># Top-Level parsing</span>
<span class="k">def</span> <span class="nf">HandleDefinition</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="bp">self</span><span class="o">.</span><span class="n">Handle</span><span class="p">(</span><span class="bp">self</span><span class="o">.</span><span class="n">ParseDefinition</span><span class="p">,</span> <span class="s">'Read a function definition:'</span><span class="p">)</span>
<span class="k">def</span> <span class="nf">HandleExtern</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="bp">self</span><span class="o">.</span><span class="n">Handle</span><span class="p">(</span><span class="bp">self</span><span class="o">.</span><span class="n">ParseExtern</span><span class="p">,</span> <span class="s">'Read an extern:'</span><span class="p">)</span>
<span class="k">def</span> <span class="nf">HandleTopLevelExpression</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="k">try</span><span class="p">:</span>
<span class="n">function</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">ParseTopLevelExpr</span><span class="p">()</span><span class="o">.</span><span class="n">CodeGen</span><span class="p">()</span>
<span class="n">result</span> <span class="o">=</span> <span class="n">g_llvm_executor</span><span class="o">.</span><span class="n">run_function</span><span class="p">(</span><span class="n">function</span><span class="p">,</span> <span class="p">[])</span>
<span class="k">print</span> <span class="s">'Evaluated to:'</span><span class="p">,</span> <span class="n">result</span><span class="o">.</span><span class="n">as_real</span><span class="p">(</span><span class="n">Type</span><span class="o">.</span><span class="n">double</span><span class="p">())</span>
<span class="k">except</span> <span class="ne">Exception</span><span class="p">,</span> <span class="n">e</span><span class="p">:</span>
<span class="k">print</span> <span class="s">'Error:'</span><span class="p">,</span> <span class="n">e</span>
<span class="k">try</span><span class="p">:</span>
<span class="bp">self</span><span class="o">.</span><span class="n">Next</span><span class="p">()</span> <span class="c"># Skip for error recovery.</span>
<span class="k">except</span><span class="p">:</span>
<span class="k">pass</span>
<span class="k">def</span> <span class="nf">Handle</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">function</span><span class="p">,</span> <span class="n">message</span><span class="p">):</span>
<span class="k">try</span><span class="p">:</span>
<span class="k">print</span> <span class="n">message</span><span class="p">,</span> <span class="n">function</span><span class="p">()</span><span class="o">.</span><span class="n">CodeGen</span><span class="p">()</span>
<span class="k">except</span> <span class="ne">Exception</span><span class="p">,</span> <span class="n">e</span><span class="p">:</span>
<span class="k">print</span> <span class="s">'Error:'</span><span class="p">,</span> <span class="n">e</span>
<span class="k">try</span><span class="p">:</span>
<span class="bp">self</span><span class="o">.</span><span class="n">Next</span><span class="p">()</span> <span class="c"># Skip for error recovery.</span>
<span class="k">except</span><span class="p">:</span>
<span class="k">pass</span>
</pre></div>
</div>
</div>
<div class="section" id="main-driver-code">
<h3>5.4.5. Main driver code.<a class="headerlink" href="#main-driver-code" title="Permalink to this headline">¶</a></h3>
<div class="highlight-python"><div class="highlight"><pre><span class="k">def</span> <span class="nf">main</span><span class="p">():</span>
<span class="c"># Set up the optimizer pipeline. Start with registering info about how the</span>
<span class="c"># target lays out data structures.</span>
<span class="n">g_llvm_pass_manager</span><span class="o">.</span><span class="n">add</span><span class="p">(</span><span class="n">g_llvm_executor</span><span class="o">.</span><span class="n">target_data</span><span class="p">)</span>
<span class="c"># Do simple "peephole" optimizations and bit-twiddling optzns.</span>
<span class="n">g_llvm_pass_manager</span><span class="o">.</span><span class="n">add</span><span class="p">(</span><span class="n">PASS_INSTRUCTION_COMBINING</span><span class="p">)</span>
<span class="c"># Reassociate expressions.</span>
<span class="n">g_llvm_pass_manager</span><span class="o">.</span><span class="n">add</span><span class="p">(</span><span class="n">PASS_REASSOCIATE</span><span class="p">)</span>
<span class="c"># Eliminate Common SubExpressions.</span>
<span class="n">g_llvm_pass_manager</span><span class="o">.</span><span class="n">add</span><span class="p">(</span><span class="n">PASS_GVN</span><span class="p">)</span>
<span class="c"># Simplify the control flow graph (deleting unreachable blocks, etc).</span>
<span class="n">g_llvm_pass_manager</span><span class="o">.</span><span class="n">add</span><span class="p">(</span><span class="n">PASS_CFG_SIMPLIFICATION</span><span class="p">)</span>
<span class="n">g_llvm_pass_manager</span><span class="o">.</span><span class="n">initialize</span><span class="p">()</span>
<span class="c"># Install standard binary operators.</span>
<span class="c"># 1 is lowest possible precedence. 40 is the highest.</span>
<span class="n">operator_precedence</span> <span class="o">=</span> <span class="p">{</span>
<span class="s">'<'</span><span class="p">:</span> <span class="mi">10</span><span class="p">,</span>
<span class="s">'+'</span><span class="p">:</span> <span class="mi">20</span><span class="p">,</span>
<span class="s">'-'</span><span class="p">:</span> <span class="mi">20</span><span class="p">,</span>
<span class="s">'*'</span><span class="p">:</span> <span class="mi">40</span>
<span class="p">}</span>
<span class="c"># Run the main "interpreter loop".</span>
<span class="k">while</span> <span class="bp">True</span><span class="p">:</span>
<span class="k">print</span> <span class="s">'ready>'</span><span class="p">,</span>
<span class="k">try</span><span class="p">:</span>
<span class="n">raw</span> <span class="o">=</span> <span class="nb">raw_input</span><span class="p">()</span>
<span class="k">except</span> <span class="ne">KeyboardInterrupt</span><span class="p">:</span>
<span class="k">break</span>
<span class="n">parser</span> <span class="o">=</span> <span class="n">Parser</span><span class="p">(</span><span class="n">Tokenize</span><span class="p">(</span><span class="n">raw</span><span class="p">),</span> <span class="n">operator_precedence</span><span class="p">)</span>
<span class="k">while</span> <span class="bp">True</span><span class="p">:</span>
<span class="c"># top ::= definition | external | expression | EOF</span>
<span class="k">if</span> <span class="nb">isinstance</span><span class="p">(</span><span class="n">parser</span><span class="o">.</span><span class="n">current</span><span class="p">,</span> <span class="n">EOFToken</span><span class="p">):</span>
<span class="k">break</span>
<span class="k">if</span> <span class="nb">isinstance</span><span class="p">(</span><span class="n">parser</span><span class="o">.</span><span class="n">current</span><span class="p">,</span> <span class="n">DefToken</span><span class="p">):</span>
<span class="n">parser</span><span class="o">.</span><span class="n">HandleDefinition</span><span class="p">()</span>
<span class="k">elif</span> <span class="nb">isinstance</span><span class="p">(</span><span class="n">parser</span><span class="o">.</span><span class="n">current</span><span class="p">,</span> <span class="n">ExternToken</span><span class="p">):</span>
<span class="n">parser</span><span class="o">.</span><span class="n">HandleExtern</span><span class="p">()</span>
<span class="k">else</span><span class="p">:</span>
<span class="n">parser</span><span class="o">.</span><span class="n">HandleTopLevelExpression</span><span class="p">()</span>
<span class="c"># Print out all of the generated code. print '', g_llvm_module</span>
<span class="k">if</span> <span class="n">__name__</span> <span class="o">==</span> <span class="s">'__main__'</span><span class="p">:</span>
<span class="n">main</span><span class="p">()</span>
</pre></div>
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<h3><a href="../../index.html">Table Of Contents</a></h3>
<ul>
<li><a class="reference internal" href="#">5. Chapter 5: Extending the Language: Control Flow</a><ul>
<li><a class="reference internal" href="#introduction">5.1. Introduction</a></li>
<li><a class="reference internal" href="#if-then-else">5.2. If/Then/Else</a><ul>
<li><a class="reference internal" href="#lexer-extensions-for-if-then-else">5.2.1. Lexer Extensions for If/Then/Else</a></li>
<li><a class="reference internal" href="#ast-extensions-for-if-then-else">5.2.2. AST Extensions for If/Then/Else</a></li>
<li><a class="reference internal" href="#parser-extensions-for-if-then-else">5.2.3. Parser Extensions for If/Then/Else</a></li>
<li><a class="reference internal" href="#llvm-ir-for-if-then-else">5.2.4. LLVM IR for If/Then/Else</a></li>
<li><a class="reference internal" href="#code-generation-for-if-then-else">5.2.5. Code Generation for If/Then/Else</a></li>
</ul>
</li>
<li><a class="reference internal" href="#for-loop-expression">5.3. ‘for’ Loop Expression</a><ul>
<li><a class="reference internal" href="#lexer-extensions-for-the-for-loop">5.3.1. Lexer Extensions for the ‘for’ Loop</a></li>
<li><a class="reference internal" href="#ast-extensions-for-the-for-loop">5.3.2. AST Extensions for the ‘for’ Loop</a></li>
<li><a class="reference internal" href="#parser-extensions-for-the-for-loop">5.3.3. Parser Extensions for the ‘for’ Loop</a></li>
<li><a class="reference internal" href="#llvm-ir-for-the-for-loop">5.3.4. LLVM IR for the ‘for’ Loop</a></li>
<li><a class="reference internal" href="#code-generation-for-the-for-loop">5.3.5. Code Generation for the ‘for’ Loop</a></li>
</ul>
</li>
<li><a class="reference internal" href="#full-code-listing">5.4. Full Code Listing</a><ul>
<li><a class="reference internal" href="#globals">5.4.1. Globals</a></li>
<li><a class="reference internal" href="#lexer">5.4.2. Lexer</a></li>
<li><a class="reference internal" href="#abstract-syntax-tree-aka-parse-tree">5.4.3. Abstract Syntax Tree (aka Parse Tree)</a></li>
<li><a class="reference internal" href="#parser">5.4.4. Parser</a></li>
<li><a class="reference internal" href="#main-driver-code">5.4.5. Main driver code.</a></li>
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