[Bug tree-optimization/56624] New: Vectorizer gives up on a group-access if it contains stores to the same location

["michael.v.zolotukhin at gmail dot com" <gcc-bugzilla@gcc.gnu.org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56624

             Bug #: 56624
           Summary: Vectorizer gives up on a group-access if it contains
                    stores to the same location
    Classification: Unclassified
           Product: gcc
           Version: 4.8.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: michael.v.zolotukhin@gmail.com
             Build: 4.8.0 20130313


Created attachment 29672
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=29672
Reproducer

GCC can't vectorize such loop:
void foo (double *a)
{
  int i;
  for (i = 0; i < 100; i+=2)
    {
      a[i+1] = 2;
      a[i] = 3;
      a[i+1] = 2;
      a[i] = 3;
    }
}
Vectorizer reports following:

note: === vect_analyze_data_ref_accesses === 
note: Detected interleaving of size 2 
note: Two store stmts share the same dr. 
note: not vectorized: complicated access pattern.

Obviously, in this given case vectorization is possible because the first
stores have no effect.

This test is a reproducer of similar problem encountered on Spec2006/470.lbm -
there if-conversion could produce stores to the same location which will stop
vectorizer.

The test is attached, command line to reproduce:
gcc group_access.c -O3 -c -ftree-vectorizer-verbose=15