[Bug tree-optimization/64682] New: wrong code at -O2 and -O3 on x86_64-linux-gnu

[su at cs dot ucdavis.edu]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64682

            Bug ID: 64682
           Summary: wrong code at -O2 and -O3 on x86_64-linux-gnu
           Product: gcc
           Version: 5.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: su at cs dot ucdavis.edu

The current gcc trunk miscompiles the following code on x86_64-linux at -O2 and
-O3 in both 32-bit and 64-bit modes. 

This is a regression from 4.9.x. 

$ gcc-trunk -v
Using built-in specs.
COLLECT_GCC=gcc-trunk
COLLECT_LTO_WRAPPER=/usr/local/gcc-trunk/libexec/gcc/x86_64-unknown-linux-gnu/5.0.0/lto-wrapper
Target: x86_64-unknown-linux-gnu
Configured with: ../gcc-trunk/configure --prefix=/usr/local/gcc-trunk
--enable-languages=c,c++ --disable-werror --enable-multilib
Thread model: posix
gcc version 5.0.0 20150119 (experimental) [trunk revision 219832] (GCC) 

$ 
$ gcc-trunk -Os small.c; a.out
5
$ gcc-4.9 -O2 small.c; a.out
5
$ 
$ gcc-trunk -O2 small.c; a.out
1
$ 


-----------------------------------


int printf (const char *, ...);

int a, b = 1;

int
main ()
{
  int i;
  for (i = 0; i < 56; i++)
    for (; a; a--)
      ;
  int *c = &b;
  if (*c)
    *c = 1 % (unsigned int) *c | 5;

  printf ("%d\n", b);

  return 0;
}