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Re: [PATCH, i386]: Use missing gcc math builtins

Quoting Ulrich Drepper <>:

> > This patch enhances libc's mathinline.h header to use some missing
> builtins
> > (mainly rounding builtins) that were added during 4.0 timeframe.
> But those builtins seem not to do much.  Just look at
> #include <stdio.h>
> int
> main (void)
> {
>   double x = 4.5;
>   printf ("%g\n", __builtin_exp2l (x));
>   return 0;
> }
> The __builtin_exp2l call is not expanded inline, it's expanded to a call
> to exp2l.  The definition in the libc header is complicated for a reason
> and I don't think gcc wants to hardcode that.

  It is OK this way. These builtins will be expanded to inlines only when
-ffast-math is added to compile flags. Otherwise, it will fall-back to function
call. Almost all x87 math builtins (including (float)->(int) conversions) are
enabled only with -ffast-math in gcc.

> I haven't looked at the remainder of the patch.  If there are some
> builtins which are indeed useful and there is a difference in the code
> generation, send a new patch.

  If -ffast-math is specified, all these builtins will be expanded to their
inlines. This patch mainly gets #defines from mathinline.h out of the way, when
correct version of gcc is used (patch is based on the same approach as some
earlier Jakub Jelinek's patch in this area).


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