[Bug tree-optimization/58228] New: wrong code (with vectorization?) at -O3 on x86_64-linux-gnu

[su at cs dot ucdavis.edu]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58228

            Bug ID: 58228
           Summary: wrong code (with vectorization?) at -O3 on
                    x86_64-linux-gnu
           Product: gcc
           Version: unknown
            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, as well as gcc 4.7 and 4.8, produces wrong code for the
following testcase on x86_64-linux when compiled at -O3 in both 32-bit and
64-bit modes. This is a regression from 4.6.x.

This seems to do with vectorization, as it goes away with -fno-tree-vectorize. 

$ gcc-trunk -v
gcc version 4.9.0 20130822 (experimental) [trunk revision 201915] (GCC) 
$ gcc-4.6 -O3 reduced.c
$ a.out
1
$ gcc-4.7 -O3 reduced.c
$ a.out
0
$ gcc-4.8 -O3 reduced.c
$ a.out
0
$ gcc-trunk -O3 reduced.c
$ a.out
0
$ gcc-trunk -O3 -fno-tree-vectorize reduced.c
$ a.out
1
$ 


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


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

int a[8][8] = {{1}};
int b, c, d, e;

int main ()
{
  for (c = 0; c < 8; c++)
    for (b = 0; b < 2; b++)
      a[b + 4][c] = a[c][0];
  printf ("%d\n", a[4][4]);
  return 0;
}