[Bug tree-optimization/71439] New: wrong code at -O3 in 32-bit and 64-bit mode on x86_64-linux-gnu

["chengniansun at gmail dot com" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=71439

            Bug ID: 71439
           Summary: wrong code at -O3 in 32-bit and 64-bit mode on
                    x86_64-linux-gnu
           Product: gcc
           Version: 7.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: chengniansun at gmail dot com
  Target Milestone: ---

The following code is miscompiled by the trunk at -O3 in both 32-bit and 64-bit
modes on x86_64-linux-gnu. 

$: gcc-trunk -v
Using built-in specs.
COLLECT_GCC=gcc-trunk
COLLECT_LTO_WRAPPER=/usr/local/gcc-trunk/libexec/gcc/x86_64-pc-linux-gnu/7.0.0/lto-wrapper
Target: x86_64-pc-linux-gnu
Configured with: ../gcc-source-trunk/configure --enable-languages=c,c++,lto
--prefix=/usr/local/gcc-trunk --disable-bootstrap
Thread model: posix
gcc version 7.0.0 20160606 (experimental) [trunk revision 237156] (GCC) 
$: 
$: gcc-trunk -O3 small.c ; ./a.out
0
$: gcc-trunk -O2 small.c ; ./a.out
6
$: 
$: cat small.c
int printf(const char*, ...);
int a, b, c;
short fn1(int p1, int p2) { return p1 + p2; }

int main() {
  a = 0;
  for (; a < 30; a = fn1(a, 4)) {
    c = b;
    b = 6;
  }
  printf("%d\n", c);
  return 0;
}
$: