Re: Patches for math subtree

[Rich Felker <dalias@libc.org>]

On Sat, Dec 07, 2019 at 09:15:34PM +0100, Stefan Kanthak wrote:
> Just some optimisations.
> 
> --- -/src/math/i386/remquo.s
> +++ +/src/math/i386/remquo.s
> @@ -23,23 +23,17 @@
>  remquo:
>          mov 20(%esp),%ecx
>          fldl 12(%esp)
>          fldl 4(%esp)
>          mov 19(%esp),%dh
>          xor 11(%esp),%dh
>  1:      fprem1
>          fnstsw %ax
>          sahf
>          jp 1b
>          fstp %st(1)
> -        mov %ah,%dl
> -        shr %dl
> -        and $1,%dl
> -        mov %ah,%al
> -        shr $5,%al
> -        and $2,%al
> -        or %al,%dl
> -        mov %ah,%al
> -        shl $2,%al
> -        and $4,%al
> -        or %al,%dl
> +        setc %dl
> +        shl $2,%ah
> +        adc %dl,%dl
> +        shl $5,%ah
> +        adc %dl,%dl
>          test %dh,%dh
> 
> --- -/src/math/ceil.c
> +++ +/src/math/ceil.c
> @@ -18,10 +18,10 @@
> +        /* special case because of non-nearest rounding modes */
> +        if (e < 0x3ff) {
> +                FORCE_EVAL(y);
> +                return u.i >> 63 ? -0.0 : 1.0;
> +        }
>          /* y = int(x) - x, where int(x) is an integer neighbor of x */
>          if (u.i >> 63)
>                  y = x - toint + toint - x;
>          else
>                  y = x + toint - toint - x;
> -        /* special case because of non-nearest rounding modes */
> -        if (e <= 0x3ff-1) {
> -                FORCE_EVAL(y);
> -                return u.i >> 63 ? -0.0 : 1;
> -        }
> 
> --- -/src/math/floor.c
> +++ +/src/math/floor.c
> @@ -18,10 +18,10 @@
> +        /* special case because of non-nearest rounding modes */
> +        if (e < 0x3ff) {
> +                 FORCE_EVAL(y);
> +                 return u.i >> 63 ? -1.0 : 0.0;
> +        }
>          /* y = int(x) - x, where int(x) is an integer neighbor of x */
>          if (u.i >> 63)
>                  y = x - toint + toint - x;
>          else
>                  y = x + toint - toint - x;
> -        /* special case because of non-nearest rounding modes */
> -        if (e <= 0x3ff-1) {
> -                 FORCE_EVAL(y);
> -                 return u.i >> 63 ? -1 : 0;
> -        }

Do you have any explanation of why these are optimizations?

Rich