Re: [PATCH 2/*] Optimize generic strchrnul and strchr

[OndÅej BÃlka <neleai at seznam dot cz>]

On Thu, May 28, 2015 at 05:36:04PM +0000, Joseph Myers wrote:
> On Wed, 27 May 2015, Wilco Dijkstra wrote:
> 
> > Finally first_nonzero_byte should just use __builtin_ffsl (yet another
> > function that should be inlined by default in the generic string.h...).
> 
> Will GCC always inline __builtin_ffsl (or call a libgcc function) rather 
> than generating a call to ffsl (user namespace) on some architectures?  If 
> it can ever call ffsl you need to do something similar to how we handle 
> __mempcpy calling __builtin_mempcpy (include/string.h redeclares mempcpy 
> with __asm__ ("__mempcpy"), so that libc-internal calls to __mempcpy 
> really do call that function at the assembler level if not inlined, rather 
> than calling mempcpy and having namespace issues).
> 
However it doesn't do it that well, I reported bug about that somewhere.
So after all you need to make assembly dump and fix gcc mistakes.

It don't eliminate ureachable checks when you put zero there, like for

int 
foo(int x)
{
  if (!x)
    return bar();
  return __builtin_ffsl(x);
}

You get following assembly:

    .cfi_startproc
        testl   %edi, %edi
        je      .L4
        movslq  %edi, %rax
        movq    $-1, %rdx
        bsfq    %rax, %rax
        cmove   %rdx, %rax
        addq    $1, %rax
        ret