On(c)e more: optimizer failure

[Matt Godbolt]

Ok! Thanks; sorry for the misunderstanding on my side.

--matt

On Sat, Aug 21, 2021 at 2:53 PM Stefan Kanthak <stefan.kanthak@nexgo.de>
wrote:

> Matt Godbolt <matt@godbolt.org> wrote:
>
> > I believe your example doesn't take into account that the values can be
> NaN
> > which compares false in all situations.
>
> That's a misbelief!
> Please notice the first if-clause, which rules out NaNs for both arguments.
> Also notice that GCC did NOT generate JP after the 4 COMISD instructions
> in question, i.e. it knew that NaNs had been ruled out.
> I included the 3 initial if-clauses just to give GCC enough rope to hang
> himself.
>
> > 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
>
> Replace
>      if (isnan(argx) || isnan(argy)) return argx + argy;
> with
>      if ((argx != argx) || (argy != argy)) return argx + argy;
> then feed the changed snippet to compiler explorer again, with and without
> -ffast-math
>
> Stefan
>
> > --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)
> >
>


-- 
Matt
(he/him)