Re: wineport: Add support for ctz().

[Adam Martinson]

On 03/16/2011 08:34 AM, Alexandre Julliard wrote:
> Adam Martinson  writes:
> > @@ -239,6 +243,19 @@ extern int getopt_long_only (int ___argc, char *const *___argv,
> >   int ffs( int x );
> >   #endif
> >
> > +#if defined(__GNUC__)&&  (GCC_VERSION>= 30406)
> > - #define ctz(x) __builtin_ctz(x)
> >
> > +#elif defined(__GNUC__)&&  (defined(__i386__) || defined(__x86_64__))
> > - static inline int ctz( unsigned int x )
> > - {
> > -
> >    int ret;
> > -
> >    __asm__("bsfl %1, %0" : "=r" (ret) : "r" (x));
> > -
> >    return ret;
> > - }
> >
> > +#else
> > - #define ctz(x) (ffs(x)-1)
> >
> > +#endif
>
> There's no reason to add this. Just use ffs().

If I thought ffs() was adequate, I would.  I need this for iterating 
sparse bitsets.

__builtin_ctz() compiles to:
mov    0x8(%ebp),%eax
bsf    %eax,%eax

(ffs()-1) compiles to:
mov    $0xffffffff,%edx
bsf    0x8(%ebp),%eax
cmove  %edx,%eax
add    $0x1,%eax
sub    $0x1,%eax

... Fortunately -O2 catches the add/sub.  So yes, there is a reason, 
ctz() is at least 50% faster.