[Bug c/63783] New: gcc-4.9: Miscompilation of boolean negation on SH4 using -O2

["glaubitz at physik dot fu-berlin.de" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63783

            Bug ID: 63783
           Summary: gcc-4.9: Miscompilation of boolean negation on SH4
                    using -O2
           Product: gcc
           Version: 4.9.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: glaubitz at physik dot fu-berlin.de

Hello!

Recently we ran into linker issues on our SH4 buildds in Debian [1]. We
initially blamed the linker for the problem, but after some more investigation,
Michael Karcher was eventually able to track down the problem being a
regression in gcc [2].

When compiling the following code snippet with optimization -O2 enabled, the
assertation will fail but succeed without:

#include <assert.h>

int decision_result;
int truecount = 0;
int val;

void buggy(int flag)
{
  int condition;
  if(flag == 0)
    condition = val != 0;
  else
    condition = !decision_result;
  if (condition)
  {
     truecount++;
  }
}

int main(void)
{
  decision_result = 1;
  buggy(1);
  assert(truecount == 0);
}

The issue did not arise with gcc-4.6, for example, so it appears to be a
regression with one of the later versions of gcc.

Cheers,
Adrian

> [1] https://sourceware.org/bugzilla/show_bug.cgi?id=17553
> [2] https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=768574