Re: bizarre ifdef logic in generated parser

[Tim Van Holder]

On Mon, 2003-01-27 at 11:33, Aharon Robbins wrote:
> Greetings.
> 
> I note the following:
> 
>       # if YYSTACK_USE_ALLOCA
>       #  define YYSTACK_ALLOC alloca
>       # else
> --->  #  ifndef YYSTACK_USE_ALLOCA
>       #   if defined (alloca) || defined (_ALLOCA_H)
>       #    define YYSTACK_ALLOC alloca
>       #   else
>       #    ifdef __GNUC__
>       #     define YYSTACK_ALLOC __builtin_alloca
>       #    endif
>       #   endif
> --->  #  endif
>       # endif
> 
> It seems that the marked test can be omitted, given that it's already
> in the else part of `if YYSTACK_USE_ALLOCA', we know it's not
> defined.

No we don't - there are 3 distinct situations:
1. #define YYSTACK_USE_ALLOCA 0
2. #define YYSTACK_USE_ALLOCA 1
3. /* no definition for YYSTACK_USE_ALLOCA */

#if YYSTACK_USE_ALLOCA
  case 2
#else
# ifdef YYSTACK_USE_ALLOCA
  case 1
# else
  case 3
# endif
#endif

In this case the snippet of code means that if alloca has not been
explicitly enabled or disabled, that alloca will be used if possible
(i.e. when alloca.h is available, or we're being compiled with gcc).

-- 
Tim Van Holder <address@hidden>