[Bug tree-optimization/63605] 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=63605

            Bug ID: 63605
           Summary: wrong code at -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 (as well as 4.8.x and 4.9.x) miscompiles the following
code on x86_64-linux at -O3 in both 32-bit and 64-bit modes.  

This is a regression from 4.7.x. 

The miscompilation seems to be caused by the tree vectorizer as
-fno-tree-vectorize makes it disappear. 

$ 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 20141018 (experimental) [trunk revision 216429] (GCC) 

$ gcc-trunk -O2 small.c; a.out
1
$ gcc-4.7 -O3 small.c; a.out
1
$ 
$ gcc-trunk -O3 small.c; a.out
0
$ 


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


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

int a, b[8] = { 2, 0, 0, 0, 0, 0, 0, 0 }, c[8];

int
main ()
{
  int d;
  for (; a < 8; a++)
    {
      d = b[a] >> 1;
      c[a] = d != 0;
    }
  printf ("%d\n", c[0]);
  return 0;
}