[Bug tree-optimization/59045] New: wrong code at -O3 on x86_64-linux-gnu

[su at cs dot ucdavis.edu]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=59045

            Bug ID: 59045
           Summary: wrong code at -O3 on x86_64-linux-gnu
           Product: gcc
           Version: 4.9.0
            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 testcase on x86_64-linux-gnu at
-O3 (in both 32-bit and 64-bit modes). 

This is a regression from 4.8.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/4.9.0/lto-wrapper
Target: x86_64-unknown-linux-gnu
Configured with: ../gcc-trunk/configure
--enable-languages=c,c++,objc,obj-c++,fortran,lto --disable-werror
--enable-checking=release --with-gmp=/usr/local/gcc-trunk
--with-mpfr=/usr/local/gcc-trunk --with-mpc=/usr/local/gcc-trunk
--with-cloog=/usr/local/gcc-trunk --prefix=/usr/local/gcc-trunk
Thread model: posix
gcc version 4.9.0 20131107 (experimental) [trunk revision 204540] (GCC) 
$ 
$ gcc-trunk -O2 small.c; a.out
1
$ gcc-4.8.2 -O3 small.c; a.out
1
$ gcc-trunk -O3 small.c; a.out
0
$ 


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


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

int a[7][3] = { {0, 0, 0}, {0, 0, 0}, {0, 0, 0}, {0, 0, 0}, {0, 0, 0}, {0, 0,
0}, {1, 0, 0} };
int b, c, d;

int
main ()
{
  for (; b < 3; b++)
    for (c = 1; c >= 0; c--)
      {
    d = a[b + 4][c];
    a[b + 4][c] = 0;
      }
  printf ("%d\n", d);
  return 0;
}