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From: "bmei at broadcom dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/59651] [4.9 Regression] Vectorizer failing to spot dependence causes incorrect code generation.
Date: Tue, 31 Dec 2013 22:42:00 -0000
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=59651

--- Comment #3 from Bingfeng Mei <bmei at broadcom dot com> ---
I can reproduce on aarch64. Still try to understand why. I constructed a
similar test but with positive loop step.

extern void abort (void);
int a[] = { 6, 0, 0, 0 };

int b;
int
main ()
{
  for (;;)
    {
      b = 0;
      for (; b<3; b += 1)
    a[b] = a[0] > 1;
      break;
    }
  if (a[2] != 0)
    abort ();
  return 0;
}

Actually GCC behaves similarly during vectorization and does vectorize the
loop. The only difference is around loop versioning. 

pr52943.c
  <bb 10>:
  if (1 != 0)
    goto <bb 11>;
  else
    goto <bb 12>;

bb 11 leads to vectorized version. So scalar version gets optimized out.

Above example:
  <bb 10>:
  if (0 != 0)
    goto <bb 11>;
  else
    goto <bb 12>;
So vectorized version goes away and only scalar version remains.