Re: wineport: Add fast builtin & asm versions of ffs() + ctz() where supported.

[Alexandre Julliard]

Adam Martinson <amartinson(a)codeweavers.com> writes:
> @@ -236,7 +241,40 @@ extern int getopt_long_only (int ___argc, char *const *___argv,
>  #endif  /* HAVE_GETOPT_LONG */
>
> #ifndef HAVE_FFS
> -int ffs( int x );
> +    #if defined(__i386__) || defined(__x86_64__)
> +        #define HAVE_FFS
> +        static inline int ffs( int x )
> +        {
> +            if (!x)
> +            {
> +                return 0;
> +            }
> +            else
> +            {
> +                int ret;
> +                __asm__("bsfl %1, %0; incl %0" : "=r" (ret) : "r" (x));
> +                return ret;
> +            }
> +        }
> +    #elif defined(__GNUC__) && GCC_VERSION >= 29503
> +        #define HAVE_FFS
> +        #define ffs(x) __builtin_ffs(x)
> +    #else
> +        int ffs( int x );
> +    #endif
> +#endif

You'd have to show benchmarks to prove that this complexity is
necessary. Given that ffs() should already be inlined on all decent
platforms, I doubt you'd be able to demonstrate a difference (if
anything, your version would be slower because of the extra increment).

-- 
Alexandre Julliard
julliard(a)winehq.org

wineport: Add fast builtin & asm versions of ffs() + ctz() where supported.

[David Laight]

On Fri, Mar 11, 2011 at 02:28:27PM +0100, Alexandre Julliard wrote:
> Adam Martinson <amartinson(a)codeweavers.com> writes:
> > @@ -236,7 +241,40 @@ extern int getopt_long_only (int ___argc, char *const *___argv,
> >  #endif  /* HAVE_GETOPT_LONG */
> >
> > #ifndef HAVE_FFS
> > -int ffs( int x );
> > +    #if defined(__i386__) || defined(__x86_64__)
> > ...
> > +                __asm__("bsfl %1, %0; incl %0" : "=r" (ret) : "r" (x));
> > +                return ret;
> > ...
> > +    #elif defined(__GNUC__) && GCC_VERSION >= 29503
> > +        #define HAVE_FFS
> > +        #define ffs(x) __builtin_ffs(x)
> > +    #else
> > +        int ffs( int x );
> > +    #endif
> > +#endif
>
> You'd have to show benchmarks to prove that this complexity is
> necessary. Given that ffs() should already be inlined on all decent
> platforms, I doubt you'd be able to demonstrate a difference (if
> anything, your version would be slower because of the extra increment).

Never mind the fact that it is possible to write code that is (probably)
faster than the bsfl instruction on most x86 processors!
You'd also get a gain from getting the 'if (x == 0)' statically predicted
correctly (probably for not zero) - that is also likely to be large.

	David

-- 
David Laight: david(a)l8s.co.uk

wineport: Add fast builtin & asm versions of ffs() + ctz() where supported.

[Adam Martinson]

On 03/11/2011 07:28 AM, Alexandre Julliard wrote:
> Adam Martinson<amartinson(a)codeweavers.com>  writes:
> > @@ -236,7 +241,40 @@ extern int getopt_long_only (int ___argc, char *const *___argv,
> >   #endif  /* HAVE_GETOPT_LONG */
> >
> > #ifndef HAVE_FFS
> > -int ffs( int x );
> > +    #if defined(__i386__) || defined(__x86_64__)
> > +        #define HAVE_FFS
> > +        static inline int ffs( int x )
> > +        {
> > +            if (!x)
> > +            {
> > +                return 0;
> > +            }
> > +            else
> > +            {
> > +                int ret;
> > +                __asm__("bsfl %1, %0; incl %0" : "=r" (ret) : "r" (x));
> > +                return ret;
> > +            }
> > +        }
> > +    #elif defined(__GNUC__)&&  GCC_VERSION>= 29503
> > +        #define HAVE_FFS
> > +        #define ffs(x) __builtin_ffs(x)
> > +    #else
> > +        int ffs( int x );
> > +    #endif
> > +#endif
> > You'd have to show benchmarks to prove that this complexity is
> > necessary. Given that ffs() should already be inlined on all decent
> > platforms, I doubt you'd be able to demonstrate a difference (if
> > anything, your version would be slower because of the extra increment).

I did this because it was easy and I was doing ctz() anyhow; I don't 
actually need these versions of ffs() for anything.  On any system with 
HAVE_FFS the system version takes precedence.