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From: "slyfox at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug middle-end/107879] New: [13 Regression] ffmpeg-4 test suite
 fails on FPU arithmetics
Date: Sat, 26 Nov 2022 15:46:49 +0000
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X-Bugzilla-Version: 13.0
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D107879

            Bug ID: 107879
           Summary: [13 Regression] ffmpeg-4 test suite fails on FPU
                    arithmetics
           Product: gcc
           Version: 13.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: middle-end
          Assignee: unassigned at gcc dot gnu.org
          Reporter: slyfox at gcc dot gnu.org
  Target Milestone: ---

Test failure is initially observed on ffmpeg-4.4.2 test suite where a bunch=
 of
tests started failing comparison against golden values. Extracted minimized
example:

// $ cat a.c
/*
Extracted from ffmpeg-4 test failure initially.

How to reproduce:

    $ gcc-13/bin/gcc -O0 -lm a.c -o a -mfpmath=3Dsse -fexcess-precision=3Ds=
tandard
-Wall -Wextra && ./a
    ...
    OK

    $ gcc-13/bin/gcc -O1 -lm a.c -o a -mfpmath=3Dsse -fexcess-precision=3Ds=
tandard
-Wall -Wextra && ./a
    ...
    BUG
*/
#include <stdio.h>

__attribute__((noinline, noipa))
static double build_filter(double * y) {
    volatile int ph =3D 0;
    volatile double vf =3D 1.0;
    double factor =3D vf;

    double x =3D - (double)ph * factor; /* should be -0.0 */
    fprintf(stderr, "z =3D %f\n", x);   /* prints -0.0, ok */

    /* This 'if / else' code should not affect calculation of 'ffm', but
       removing it does change things. */
    if (x =3D=3D 0) *y =3D 1.0;    /* should be 1.0 or .. */
    else *y =3D 1.0 / x;       /* -inf? */
    fprintf(stderr, "*y =3D %f\n", *y); /* printf 1.0, ok */

    double w =3D 2.0 * x / factor; /* should be -0.0 */
    fprintf(stderr, "w =3D %f\n", w); /* prints -0.0, ok */

    double omww =3D 1 - w;                  /* should be 1.0 */
    fprintf(stderr, "omww =3D %f\n", omww); /* printf 1.0, ok */
    double ffm =3D (omww > 0.0) ? omww : 0.0; /* should be 1.0 */
    fprintf(stderr, "ffm =3D %f\n", ffm); /* printf 0.0 or 1.0, BUG */

    return ffm;
}

int main()
{
    double y =3D 42.0;
    double filter =3D build_filter(&y);

    fprintf(stderr, "f =3D %.20f; y =3D %.20f\n", filter, y);
    /* Should be 1.0, sometimes returns 0.0. */
    fprintf(stderr, "%s\n", (filter > 0.5) ? "OK" : "BUG");
}

How to reproduce:

# -O0, good:
$ gcc-HEAD/bin/gcc -O0 -lm a.c -o a -mfpmath=3Dsse -fexcess-precision=3Dsta=
ndard
-Wall -Wextra && ./a
z =3D -0.000000
*y =3D 1.000000
w =3D -0.000000
omww =3D 1.000000
ffm =3D 1.000000
f =3D 1.00000000000000000000; y =3D 1.00000000000000000000
OK

# -O1, bad:
$ gcc-HEAD/bin/gcc -O1 -lm a.c -o a -mfpmath=3Dsse -fexcess-precision=3Dsta=
ndard
-Wall -Wextra && ./a
z =3D -0.000000
*y =3D 1.000000
w =3D -0.000000
omww =3D 1.000000
ffm =3D 0.000000
f =3D 0.00000000000000000000; y =3D 1.00000000000000000000
BUG

I think the code does not have anything controversial. Probably a code
generation bug?

$ LANG=3DC gcc-HEAD/bin/gcc -v
Using built-in specs.
COLLECT_GCC=3D/<<NIX>>/gcc-13.0.0/bin/gcc
COLLECT_LTO_WRAPPER=3D/<<NIX>>/gcc-13.0.0/libexec/gcc/x86_64-unknown-linux-=
gnu/13.0.0/lto-wrapper
Target: x86_64-unknown-linux-gnu
Configured with:
Thread model: posix
Supported LTO compression algorithms: zlib
gcc version 13.0.0 20221126 (experimental) (GCC)=