[Bug tree-optimization/69719] New: wrong code at -O3 on x86_64-linux-gnu

[su at cs dot ucdavis.edu]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=69719

            Bug ID: 69719
           Summary: wrong code at -O3 on x86_64-linux-gnu
           Product: gcc
           Version: 6.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: su at cs dot ucdavis.edu
  Target Milestone: ---

The current gcc trunk and 5.3.x miscompile the following code on
x86_64-linux-gnu at -O3 in both 32-bit and 64-bit modes. 

This is a regression from 5.2.x.


$ 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/6.0.0/lto-wrapper
Target: x86_64-pc-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 6.0.0 20160207 (experimental) [trunk revision 233200] (GCC) 
$ 
$ gcc-trunk -O2 small.c; ./a.out
$ gcc-5.2 -O3 small.c; ./a.out
$ 
$ gcc-trunk -O3 small.c
$ ./a.out
Aborted (core dumped)
$ gcc-5.3 -O3 small.c
$ ./a.out
Aborted (core dumped)
$ 


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


int b, c = 1, e, f; 
int a[6][5] = { {0, 0, 0, 0, 0}, 
                {0, 0, 0, 0, 0}, 
                {0, 1, 0, 0, 0} };

void 
fn1 ()
{
  int d;
  for (b = 0; b < 5; b++)
    for (d = 4; d; d--)
      a[c + 1][b] = a[d + 1][d];
}

int
main ()
{
  fn1 ();

  if (a[2][1] != 0) 
    __builtin_abort (); 

  return 0; 
}