[Bug tree-optimization/56625] New: After if-conversion vectorizer doesn't recognize similar stores

[michael.v.zolotukhin at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56625

             Bug #: 56625
           Summary: After if-conversion vectorizer doesn't recognize
                    similar stores
    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
                CC: kirill.yukhin@intel.com,
                    michael.v.zolotukhin@gmail.com
             Build: 4.8.0 20130313


Created attachment 29673
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=29673
Reproducer

In the following example there are two stores a[i]<-b[i] after if-conversion.
void foo (double a[], double b[])
{
  int i;
  for (i = 0; i < 100; i++)
    {
      if (a[i] == 0)
        a[i] = b[i]*4;
      else
        a[i] = b[i]*3;
    }
}
As vectorizer knows nothing about dependencies between a and b, it needs a
runtime test for it. But in the given example, vectorizer generates two
runtime-tests instead of one:
note: mark for run-time aliasing test between *_11 and *_8
note: mark for run-time aliasing test between *_15 and *_8

The test is attached, command line to reproduce:
gcc if-conv-runtime-tests.c -O3 -c -ftree-vectorizer-verbose=15
-ftree-loop-if-convert-stores  -fdump-tree-vect