[Bug c/93878] New: -Wfloat-equal complains even in safe cases

[roland.illig at gmx dot de]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=93878

            Bug ID: 93878
           Summary: -Wfloat-equal complains even in safe cases
           Product: gcc
           Version: 10.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: roland.illig at gmx dot de
  Target Milestone: ---

$ cat float.c
#include <math.h>

int f(float a, float b) {
    float rounded = roundf(b);
    if (a < 0) return 1;
    if (a == 3.0) return 2;
    if (rounded == 3.0) return 3;
    if (a == b) return 4;
    return 5;
}

$ gcc -Wall -Os -Wextra -Wfloat-equal -c float.c
float.c: In function ‘f’:
float.c:6:11: warning: comparing floating point with == or != is unsafe
[-Wfloat-equal]
    6 |     if (a == 3.0) return 2;
      |           ^~
float.c:7:17: warning: comparing floating point with == or != is unsafe
[-Wfloat-equal]
    7 |     if (rounded == 3.0) return 3;
      |                 ^~
float.c:8:11: warning: comparing floating point with == or != is unsafe
[-Wfloat-equal]
    8 |     if (a == b) return 4;
      |           ^~

The case "a == 3.0" may be debatable. But the case "rounded == 3.0" is
perfectly safe since the number is rounded before being compared.

These test cases should be added to the test suite for this diagnostic, to
demonstrate the current behavior and limitations.

There are probably many other situations that are safe as well, therefore it's
good that -Wfloat-equal is neither included in -Wall nor in -Wextra. Thanks for
that. :)