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From: "ysrumyan at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/57558] New: Issue with number of iterations calculation
Date: Fri, 07 Jun 2013 14:45:00 -0000
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57558

            Bug ID: 57558
           Summary: Issue with number of iterations calculation
           Product: gcc
           Version: 4.9.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: ysrumyan at gmail dot com

For the following simple test-case extracted from 254.gap (spec2000):

typedef unsigned long ul;
void foo (ul* __restrict x, ul* __restrict y, ul n)
{
  ul i;
  for (i=1; i<=n; i++, x++, y++)
    *x ^= *y;
}

gcc cannot determine number of iterations and failed to vectorize loop:

t1.c:5: note: === get_loop_niters ===Analyzing # of iterations of loop 1
  exit condition [2, + , 1] <= n_8(D)
  bounds on difference of bases: -1 ... 4294967293
  result:
    under assumptions n_8(D) != 4294967295
    # of iterations n_8(D) + 4294967295, bounded by 4294967294

t1.c:5: note: not vectorized: number of iterations cannot be computed.

Note that if we change for-stmt to
  for (i=0; i<n; i++, x++, y++)

it will be vectorized.