[PATCH] Add branch prediction favouring normal numbers

[Andreas Jaeger]

On 12/27/2012 11:41 AM, Siddhesh Poyarekar wrote:
> On Wed, Dec 26, 2012 at 12:33:16PM -0500, Mike Frysinger wrote:
>>
>> we have __glibc_unlikely & __glibc_likely now
>> -mike
>
> We have __glibc_unlikely, but not glibc_likely (and I realized that I
> had added it ;) ).  So here's a patch to add __glibc_likely.
>
>
> 	* misc/sys/cdefs.h(__glibc_likely): Wrap __builtin_expect for
> 	TRUE case.
>
> diff --git a/misc/sys/cdefs.h b/misc/sys/cdefs.h
> index fb6c959..1eee54e 100644
> --- a/misc/sys/cdefs.h
> +++ b/misc/sys/cdefs.h
> @@ -378,8 +378,10 @@
>
>   #if __GNUC__ >= 3
>   # define __glibc_unlikely(cond) __builtin_expect((cond), 0)
> +# define __glibc_likely(cond) __builtin_expect((cond), 1)
>   #else
>   # define __glibc_unlikely(cond) (cond)
> +# define __glibc_likely(cond) (cond)
>   #endif
>
>   #include <bits/wordsize.h>
>
> ---
>
> And then, here's the updated patch for the branch prediction:
>
>
> 	* sysdeps/ieee754/dbl-64/mpa.c (__mp_dbl): Favour normal
> 	numbers.
>
> diff --git a/sysdeps/ieee754/dbl-64/mpa.c b/sysdeps/ieee754/dbl-64/mpa.c
> index 1c93bdc..78afd99 100644
> --- a/sysdeps/ieee754/dbl-64/mpa.c
> +++ b/sysdeps/ieee754/dbl-64/mpa.c
> @@ -248,9 +248,10 @@ void __mp_dbl(const mp_no *x, double *y, int p) {
>
>     if (X[0] == ZERO)  {*y = ZERO;  return; }
>
> -  if      (EX> -42)                 norm(x,y,p);
> -  else if (EX==-42 && X[1]>=TWO10)  norm(x,y,p);
> -  else                              denorm(x,y,p);
> +  if (__glibc_likely (EX> -42 || (EX==-42 && X[1]>=TWO10)))
> +    norm(x,y,p);
> +  else
> +    denorm(x,y,p);
>   }
>   #endif


Both changes look fine to me, thanks,

Andreas

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  Andreas Jaeger aj@{suse.com,opensuse.org} Twitter/Identica: jaegerandi
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