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From: "florian at schanda dot org.uk" <sourceware-bugzilla@sourceware.org>
To: glibc-bugs@sourceware.org
Subject: [Bug math/26045] New: fmaxf(inf, nan) does not always work
Date: Tue, 26 May 2020 12:43:10 +0000
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https://sourceware.org/bugzilla/show_bug.cgi?id=3D26045

            Bug ID: 26045
           Summary: fmaxf(inf, nan) does not always work
           Product: glibc
           Version: 2.28
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: math
          Assignee: unassigned at sourceware dot org
          Reporter: florian at schanda dot org.uk
  Target Milestone: ---

Created attachment 12570
  --> https://sourceware.org/bugzilla/attachment.cgi?id=3D12570&action=3Ded=
it
example showing fmaxf does not always work

Hi

The gcc people sent me here: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=
=3D95329

I believe the fmaxf function does not always work correctly (I am on Debian
stable, amd64). The following piece of code produces the wrong output when
compile with no optimisation (and hence fmaxf from libm is called):

$ gcc bug.c -lm
$ ./a.out

max(-inf, nan) =3D nan
max(-inf, nan) =3D -inf

The code in question uses a special hand-crafted NaN:

#include <stdio.h>
#include <stdint.h>
#include <math.h>
#include <string.h>

int main()
{
  uint32_t bv =3D 0x7FACBA7E;
  // (0 11111111 01011001011101001111110)

  float a, b;
  a =3D -INFINITY;
  memcpy(&b, &bv, 4);

  printf("max(%f, %f) =3D %f\n", a, b, fmaxf(a, b));

  b =3D NAN;
  printf("max(%f, %f) =3D %f\n", a, b, fmaxf(a, b));
}

Thanks in advance, and good luck! :)

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