[Bug middle-end/93939] New: missing optimization for floating-point expression converted to integer whose result is known at compile time

[vincent-gcc at vinc17 dot net]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=93939

            Bug ID: 93939
           Summary: missing optimization for floating-point expression
                    converted to integer whose result is known at compile
                    time
           Product: gcc
           Version: 10.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: middle-end
          Assignee: unassigned at gcc dot gnu.org
          Reporter: vincent-gcc at vinc17 dot net
  Target Milestone: ---

Consider the following example.

#include <stdio.h>

typedef double T;

int main (void)
{
  volatile T a = 8;
  T b = a;
  int i;

  i = 3 * b;
  printf ("%d\n", (int) i);

  if (b == 8)
    {
      i = 3 * b;
      printf ("%d\n", i == 24);
    }

  return 0;
}

Even if one compiles it with -O3, a comparison instruction for i == 24 is
generated, while its result 1 is known at compile time.

If I change T to int, or change the type of i to double, or comment out the
first printf (so that the first i is not used), then the i == 24 is optimized
as expected.

Tested under Debian/unstable / x86_64 with:

gcc-10 (Debian 10-20200222-1) 10.0.1 20200222 (experimental) [master revision
01af7e0a0c2:487fe13f218:e99b18cf7101f205bfdd9f0f29ed51caaec52779]