Re: [Qemu-devel] [PATCH v2 5/7] softfloat: Rework `*_is_*_nan' functions

[Peter Maydell]

On 12 December 2014 at 19:35, Maciej W. Rozycki <address@hidden> wrote:
> Precompute the possible results, and then pick the suitable one.  The
> calculation of the unused result will be optimized away by the compiler
> at any reasonable optimization level, so no run-time performance loss.
>
> Signed-off-by: Thomas Schwinge <address@hidden>
> Signed-off-by: Maciej W. Rozycki <address@hidden>

> --- qemu-git-trunk.orig/fpu/softfloat-specialize.h      2014-12-11 
> 22:41:47.118930659 +0000
> +++ qemu-git-trunk/fpu/softfloat-specialize.h   2014-12-11 22:42:29.128941855 
> +0000
> @@ -207,10 +207,13 @@ int float16_is_signaling_nan(float16 a_
>  int float16_is_quiet_nan(float16 a_ STATUS_PARAM)
>  {
>      uint16_t a = float16_val(a_);
> +    int __attribute__ ((unused)) x, y;
> +    x = (((a >> 9) & 0x3F) == 0x3E) && (a & 0x1FF);
> +    y = (a & ~0x8000) >= 0x7c80;
>  #if SNAN_BIT_IS_ONE
> -    return (((a >> 9) & 0x3F) == 0x3E) && (a & 0x1FF);
> +    return x;
>  #else
> -    return ((a & ~0x8000) >= 0x7c80);
> +    return y;
>  #endif
>  }

I don't really see the point in this patch -- the existing
version of the code seems better to me (doesn't need the
extra __attribute__ ((unused)) variables, for instance).
What's the benefit?

thanks
-- PMM