https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95329
Bug ID: 95329
Summary: fmaxf(inf, nan) does not always work
Product: gcc
Version: 8.3.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: florian at schanda dot org.uk
Target Milestone: ---
Created attachment 48601
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=48601&action=edit
Reduced testcase of the fmaxf issue
I've been creating some floating point benchmarks for SMT-LIB. I have noticed
that gcc (or something else) does not produce the same result in some specific
examples. I am on amd64 Debian, but I've also reproduced this on a recent amd64
Ubuntu. Below is a reduced test-case showing this:
#include <stdio.h>
#include <stdint.h>
#include <math.h>
#include <string.h>
int main()
{
uint32_t bv = 0x7FACBA7E;
// (0 11111111 01011001011101001111110)
float a, b;
a = -INFINITY;
memcpy(&b, &bv, 4);
printf("max(%f, %f) = %f\n", a, b, fmaxf(a, b));
b = NAN;
printf("max(%f, %f) = %f\n", a, b, fmaxf(a, b));
}
When compiling and running this program you get:
max(-inf, nan) = nan
max(-inf, nan) = -inf
I've tried with both:
gcc bug.c -lm
and
gcc -std=c99 -frounding-math -fsignaling-nans -ffp-contract=off -mfma
-mno-fma4 -pedantic -Wall -W -msse2 -mfpmath=sse -fstrict-aliasing bug.c -lm
However, IEEE-754 says that max(something not NaN, NaN) should get you the
thing that is not NaN. With the built-in constant it works, but with my
hand-crafted NaN it does not.
I've also tried replacing fmaxf by __builtin_fmaxf, with no success.
I also realise this likely may not be a gcc bug, but something deeper down in
libm or elsewhere. I do note that with clang-9 this works though. However I
suspect you guys will know where exactly this report needs to go to.
