https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107879
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=sse -fexcess-precision=standard
-Wall -Wextra && ./a
...
OK
$ gcc-13/bin/gcc -O1 -lm a.c -o a -mfpmath=sse -fexcess-precision=standard
-Wall -Wextra && ./a
...
BUG
*/
#include <stdio.h>
__attribute__((noinline, noipa))
static double build_filter(double * y) {
volatile int ph = 0;
volatile double vf = 1.0;
double factor = vf;
double x = - (double)ph * factor; /* should be -0.0 */
fprintf(stderr, "z = %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 == 0) *y = 1.0; /* should be 1.0 or .. */
else *y = 1.0 / x; /* -inf? */
fprintf(stderr, "*y = %f\n", *y); /* printf 1.0, ok */
double w = 2.0 * x / factor; /* should be -0.0 */
fprintf(stderr, "w = %f\n", w); /* prints -0.0, ok */
double omww = 1 - w; /* should be 1.0 */
fprintf(stderr, "omww = %f\n", omww); /* printf 1.0, ok */
double ffm = (omww > 0.0) ? omww : 0.0; /* should be 1.0 */
fprintf(stderr, "ffm = %f\n", ffm); /* printf 0.0 or 1.0, BUG */
return ffm;
}
int main()
{
double y = 42.0;
double filter = build_filter(&y);
fprintf(stderr, "f = %.20f; y = %.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=sse -fexcess-precision=standard
-Wall -Wextra && ./a
z = -0.000000
*y = 1.000000
w = -0.000000
omww = 1.000000
ffm = 1.000000
f = 1.00000000000000000000; y = 1.00000000000000000000
OK
# -O1, bad:
$ gcc-HEAD/bin/gcc -O1 -lm a.c -o a -mfpmath=sse -fexcess-precision=standard
-Wall -Wextra && ./a
z = -0.000000
*y = 1.000000
w = -0.000000
omww = 1.000000
ffm = 0.000000
f = 0.00000000000000000000; y = 1.00000000000000000000
BUG
I think the code does not have anything controversial. Probably a code
generation bug?
$ LANG=C gcc-HEAD/bin/gcc -v
Using built-in specs.
COLLECT_GCC=/<<NIX>>/gcc-13.0.0/bin/gcc
COLLECT_LTO_WRAPPER=/<<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)
