On(c)e more: optimizer failure

[Matt Godbolt]

I believe your example doesn't take into account that the values can be NaN
which compares false in all situations. If you allow the compiler to
optimize without supporting NaN (-ffast-math), I think it generates the
code you want: https://godbolt.org/z/1ra7zcsnd

--matt

On Sat, Aug 21, 2021 at 1:59 PM Stefan Kanthak <stefan.kanthak@nexgo.de>
wrote:

> Hi,
>
> the following snippet is from the nextafter() function of
> <http://www.netlib.no/netlib/toms/722>
>
> --- repro.c ---
> #define Zero 0.0
> double nextafter(double argx, double argy)
> {
>     double z = argx;
>
>     if (isnan(argx) || isnan(argy)) return argx + argy;
>
>     if (argx == argy) return argx;
>
>     if (argx != Zero)
>         if (((argx < Zero) && (argx < argy))
>          || ((argx > Zero) && (argx > argy)))
>             z += 1.0;
>         else
>             z -= 1.0;
>     return z;
> }
> --- EOF ---
>
> I expect that GCC knows DeMorgan's rules and is able to
> simplify/optimize the last if-statement to
>
>         if ((argx < Zero) == (argx < argy))
>
> Unfortunately GCC fails to do so: see the lines from
> label .L20: to label L7
>
> $ gcc -m64 -O3 -o- -S
> ...
> nextafter:
>         ucomisd %xmm1, %xmm0
>         jp      .L19
>         pxor    %xmm2, %xmm2
>         movl    $1, %edx
>         ucomisd %xmm2, %xmm0
>         setp    %al
>         cmovne  %edx, %eax
>         testb   %al, %al
>         je      .L3
>         ucomisd %xmm1, %xmm0
>         setp    %al
>         cmove   %eax, %edx
>         testb   %dl, %dl
>         jne     .L20
> .L3:
>         ret
> .L20:
>         comisd  %xmm0, %xmm2
>         ja      .L21
> .L4:
>         comisd  %xmm2, %xmm0
>         jbe     .L7
>         comisd  %xmm1, %xmm0
>         jbe     .L7
> .L6:
>         addsd   .LC1(%rip), %xmm0
>         ret
> .L21:
>         comisd  %xmm0, %xmm1
>         ja      .L6
>         jmp     .L4
> .L7:
>         subsd   .LC1(%rip), %xmm0
>         ret
> .L19:
>         addsd   %xmm1, %xmm0
>         ret
> .LC1:
>         .long   0
>         .long   1072693248
>
> Stefan
>


-- 
Matt
(he/him)