[Bug c/89573] New: -fexcess-precision=standard doesn't work for conversion to integer of multiplication

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=89573

            Bug ID: 89573
           Summary: -fexcess-precision=standard doesn't work for
                    conversion to integer of multiplication
           Product: gcc
           Version: 9.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: rguenth at gcc dot gnu.org
  Target Milestone: ---

The following, built with -m32 -march=i586 -fexcess-precision=standard on
x86_64-linux outputs

a: -75345
b: -75346
c: -75346

where the first result is off.  The IL looks like

    int r = (int) ((long double) log (p) * (long double) inv_log_of_base);

where possibly the missing cast to (double) is elided by bogus optimization
(didn't try to track down the issue yet).

-ffloat-store makes the testcase work as does any optimization (well, we
then get constant folding).

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

int main(int argc, char **argv)
{
  double p = 0.00053447623258905705;
  double inv_log_of_base = 10000.499991668185;

  int r = log(p) * inv_log_of_base;
  printf("a: %d\n", r);

  double gr = log(p) * inv_log_of_base;
  printf("b: %g\n", gr);

  double g = log(p);
  int c = g * inv_log_of_base;
  printf("c: %d\n", c);

  return 0;
}