Re: fix memrchr for GCC 3.4

[Jakub Jelinek <jakub at redhat dot com>]

On Thu, Sep 04, 2003 at 12:07:55PM +0200, Andreas Jaeger wrote:
> 
> GCC 3.4 miscompiles memrchr (the testsuite fails).  The GCC developers
> closed my bugs as invalid:
> http://gcc.gnu.org/bugzilla/show_bug.cgi?id=9196
> 
> with the following comment:
> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> The C semantics for pointer arithmetic mean that GCC is allowed to assume that
> this line
> 
>   return __res + 1;
> 
> never causes a null pointer to be returned.  The "mis-optimized" code has been
> optimized based on that assumption.  In short, the bug is in glibc.
> 
> The simplest fix is to write "incl %0" as the last instruction of the assembly
> block rather than trying to do this calculation in C. 

Can't you just make __res unsigned long int instead of void * and
add cast (ie. return (void *) (__res + 1); )?

Of course, best would be to add __builtin_memrchr to gcc and after making
sure gcc finally does better job than all bits/string*.h macros use just gcc
builtins.

	Jakub