[Bug c/66736] New: float rounding differences when using constant literal versus variable

["dhekir at gmail dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66736

            Bug ID: 66736
           Summary: float rounding differences when using constant literal
                    versus variable
           Product: gcc
           Version: 5.1.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: dhekir at gmail dot com
  Target Milestone: ---

Calling function "log10f(3)" with a constant literal or via a variable, such as
"float f = 3; log10f(f)" gives different rounding results, which are incorrect
in the latter case.

Note that the bug is not about imprecision in the result, but inconsistency
between two statements which should be equivalent.

The difference only appears with no optimization flag or with -O0; activating
-O1 or greater makes the difference disappear.

It is especially annoying (although not forbidden) that the rounding
differences in this case do not respect usual order (i.e. changing the rounding
mode allows one to see that FE_DOWNWARD is larger than FE_TONEAREST in the
version using the variable).

This behavior has been observed in several GCCs, from 4.8.4 (Ubuntu) to 5.1.1
(Fedora), including a 5.0.0 compiled from trunk, and using different versions
of glibc (2.19, and also tried compiling 2.21). All of them produced the same
result.

Also, there are several constants for which this happen, but 3 would be one of
the most notable ones.

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

int main() {
  float r = log10f(3);
  printf("literal constant: %g (%a)\n", r, r);
  float x = 3;
  r = log10f(x);
  printf("with variable:    %g (%a)\n", r, r);
  return 0;
}