Przemyslaw Czerpak (druzus/at/priv.onet.pl)
Harbour compile time optimizations.

1. Function call optimization.

Just like Clipper Harbour compiler can optimize few function calls if
parameters are well known constant values. Here is the list of functions
optimized at compile time:

   - Clipper compatible:

      At( <cConst1>, <cConst2> ) // Clipper wrongly calculates
                                 // At( "", <cConst> ) as 1
      Asc( <cConst> [ , ... ] )
      Chr( <nConst> )
      Len( <cConst> | <aConst> | <hConst> )  // <hConst> is Harbour extension
      Upper( <cConst> ) // <cConst> cannot contain characters different then
                           [0-9A-Za-z ]

   - Harbour extension:

      Int( <nConst> )
      Min( <xConst1>, <xConst2> )   // <xConstN> is N, D or L value
      Max( <xConst1>, <xConst2> )   // <xConstN> is N, D or L value
      Empty( <aConst> | <hConst> | <cConst> | <bConst> |
             <nConst> | <dConst>| <lConst> | NIL )
      CToD( "" [ , ... ] )
      DToS( <dConst> ] )
      SToD( [ <cConst> ] )
      hb_SToD( [ <cConst> ] )
      hb_SToT( [ <cConst> ] )
      hb_bitNot( <nConst> [, ... ] )
      hb_bitAnd( <nConst1>, <nConst2> [, <nConstN> ] )
      hb_bitOr( <nConst1>, <nConst2> [, <nConstN> ] )
      hb_bitXor( <nConst1>, <nConst2> [, <nConstN> ] )
      hb_bitTest( <nConst1>, <nConst2> [, ... ] )
      hb_bitSet( <nConst1>, <nConst2> [, ... ] )
      hb_bitReset( <nConst1>, <nConst2> [, ... ] )
      hb_bitShift( <nConst1>, <nConst2> [, ... ] )

   - Harbour special functions:

      HB_I18N_GETTEXT_NOOP( <cConst1> [ , <cConst2> ] )
      HB_I18N_NGETTEXT_NOOP( <nCount>, <cConst1> | <acConst1> [ , <cConst2> ] )
      HB_I18N_GETTEXT_NOOP_*( <cConst1> [ , <cConst2> ] )
      HB_I18N_NGETTEXT_NOOP_*( <nCount>, <cConst1> | <acConst1> [ , <cConst2> ] )
      hb_mutexCreate()


2. Expresion optimization:

Just like Clipper Harbour compiler can optimize some expresions if
arguments are well known and can be calculated at compile time:

   - Clipper compatible:

      <nConst1> + <nConst2>   => <nConst>
      <nConst1> + <dConst2>   => <dConst>
      <dConst1> + <nConst2>   => <dConst>
      <cConst1> + <cConst2>   => <cConst>
                                 // In Clipper neither <cConst1> nor <cConst2>
                                 // can contain '&' character. Harbour checks
                                 // if concatenation can change existing valid
                                 // macro name or ignore '&' when -kM compiler
                                 // switch which disables macro substitution
                                 // is used
      <nConst1> - <nConst2>   => <nConst>
      <dConst1> - <dConst2>   => <dConst>
      <dConst1> - <nConst2>   => <dConst>
      <cConst1> - <cConst2>   => <cConst>
                                 // In Clipper neither <cConst1> nor <cConst2>
                                 // can contain '&' character. Harbour checks
                                 // if concatenation can change existing valid
                                 // macro name or ignore '&' when -kM compiler
                                 // switch which disables macro substitution
                                 // is used
      <nConst1> * <nConst2>   => <nConst>
      <nConst1> / <nConst2>   => <nConst> // Clipper optimize only integers
      <nConst1> % <nConst2>   => <nConst>
      <cConst1> $ <cConst2>   => <lConst>
                                 // Clipper wrongly calculates
                                 // "" $ <cConst> as .T.
                                 // In Clipper neither <cConst1> nor <cConst2>
                                 // can contain '&' character. Harbour checks
                                 // if after '&' is potentially valid macro
                                 // name or ignore '&' when -kM compiler switch
                                 // which disables macro substitution is used
      <lConst1> == <lConst2>  => <lConst>
      <nConst1> == <nConst2>  => <lConst>
      <dConst1> == <dConst2>  => <lConst>
      <cConst1> == <cConst2>  => <lConst>
                                 // In Clipper neither <cConst1> nor <cConst2>
                                 // can contain '&' character. Harbour checks
                                 // if after '&' is potentially valid macro
                                 // name or ignore '&' when -kM compiler switch
                                 // which disables macro substitution is used

      NIL       == <xConst>   => <lConst>
      <xConst>  == NIL        => <lConst>
      <lConst1> = <lConst2>   => <lConst>
      <nConst1> = <nConst2>   => <lConst>
      <dConst1> = <dConst2>   => <lConst>
      NIL       = <xConst>    => <lConst>
      <xConst>  = NIL         => <lConst>
      ""        = ""          => .T.
      <lConst1> != <lConst2>  => <lConst>
      <nConst1> != <nConst2>  => <lConst>
      <dConst1> != <dConst2>  => <lConst>
      NIL       != <xConst>   => <lConst>
      <xConst>  != NIL        => <lConst>
      ""        != ""         => .F.
      <lConst1> >= <lConst2>  => <lConst>
      <nConst1> >= <nConst2>  => <lConst>
      <dConst1> >= <dConst2>  => <lConst>
      <lConst1> <= <lConst2>  => <lConst>
      <nConst1> <= <nConst2>  => <lConst>
      <dConst1> <= <dConst2>  => <lConst>
      <lConst1> > <lConst2>   => <lConst>
      <nConst1> > <nConst2>   => <lConst>
      <dConst1> > <dConst2>   => <lConst>
      <lConst1> < <lConst2>   => <lConst>
      <nConst1> < <nConst2>   => <lConst>
      <dConst1> < <dConst2>   => <lConst>
      .NOT. .T.               => .F.
      .NOT. .F.               => .T.
      <lConst1> .AND. <lConst2> => <lConst>
      <lConst1> .OR. <lConst2> => <lConst>
      iif( .T., <expr1>, <expr2> ) => <expr1>
      iif( .F., <expr1>, <expr2> ) => <expr2>

   - optimizations which can be disabled by -z compiler switch

      .T. .AND. <expr>        => <expr>
      <expr> .AND. .T.        => <expr>
      .F. .OR. <expr>         => <expr>
      <expr> .OR. .F.         => <expr>
      .F. .AND. <expr>        => .F.
      <expr> .AND. .F.        => .F.
      .T. .OR. <expr>         => .T.
      <expr> .OR. .T.         => .T.

   - Harbour extension:

      <nConst1> ^ <nConst2>   => <nConst>
      <aValue> [ <nConst> ]   => <xArrayItem>
      ( <expr> )              => <expr>         // it allows to optimize
                                                // expresions like: 1+(2)

   - Harbour extensions which may disable RT errors in wrong expressions
     or can change used operators using basic math rules. Enabled by -ko
     compiler switch:
         .NOT. .NOT. <expr>      => <expr>
         - -  <expr>             => <expr>
         <expr> + 0              => <expr>
         0 + <expr>              => <expr>
         <expr> + ""             => <expr>
         "" + <expr>             => <expr>
         ( "<alias>" )->         => <alias>->
         <expr> == .T.           => <expr>
         .T. == <expr>           => <expr>
         <expr> == .F.           => !<expr>
         .F. == <expr>           => !<expr>
         <expr> =  .T.           => <expr>
         .T. =  <expr>           => <expr>
         <expr> =  .F.           => !<expr>
         .F. =  <expr>           => !<expr>
         <expr> != .T.           => !<expr>
         .T. != <expr>           => !<expr>
         <expr> != .F.           => <expr>
         .F. != <expr>           => <expr>

     In cases when result is meaningless Harbour compiler can skip code
     for operation, i.e. for such line of .prg code:
         ( <exp1> <op> <exp2> )
     where result of <op> operation is ignored Harbour reduces the code
     to:
         ( <exp1>, <exp2> )

In Clipper in some places optimization is not enabled, f.e. Clipper
does not optimize <exp> in expressions like:
   <exp> : msg( ... )
Unlike Clipper Harbour tries to optimize all expressions.

If some code needs strict Clipper behavior then it can be forced by using
-kc Harbour compiler switch. It disables Harbour extensions and enables
replicating some Clipper bugs like optimizing "" $ <cConst> to .T. at
compile time (at runtime and in macrocompiler it's always .F. in Clipper
and Harbour).

Expressions fully optimized to constant values at compile time can be used
to intialize static variables, f.e.:
   static s_var := ( 1 + 2 / 3 )

Clipper does not optimize expression used in LOCAL, PRIVATE and
PUBLIC variables declarations but it optimize expressions for STATIC
declarations. This code illustrates it:

   proc main()
      memvar v, p
      local l := "" $ ""
      static s := "" $ ""
      private v := "" $ ""
      public p := "" $ ""
      ? l, s, v, p, "" $ ""
   return

This behavior is not replicated in Harbour even if -kc switch is used
and Harbour optimize expressions in all declarations.

Harbour supports macro expansion for expressions with declared symbols.
This functionality can be enabled by -kd compiler switch:
   -kd => accept macros with declared symbols
It only allows to write code like:
   cbVar := {|| &cLocal + cPrivate }
or:
   cbVar := {|| &cLocalPref.func&cPriv1( cPriv2 ) }
or:
   ? &cLocalPref.func&cPriv1( cPriv2, &cStatic )
etc.
If possible then for macrocodeblocks Harbour compiler tries to
generate early eval code in which macros are expanded when codeblock
is created. Otherwise macros are expanded each time codeblock is
evaluated.
This feature can be useful also in porting some other xbase compatible
code to Harbour because some compilers just like xHarbour accepted
in some limited way officially unsupported syntax with macros using
declared symbols.

Harbour has additional optimization phase which operates on generated PCODE.
It can also reduce expressions, joins jumps, removes death or meaningless
code which can appear after all other optimizations and were not optimized
by expression optimizer. It can also optimize readonly local variables
keeping the QSelf() value. QSelf() is not real function call but very fast
single PCODE often used in OOP code. Harbour can replace local variables
keeping it by direct QSelf() usage.