[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]

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: unassig...@gcc.gnu.org

        ReportedBy: michael.v.zolotuk...@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