[Bug tree-optimization/52571] New: vectorizer changes alignment of common symbols

["ian at airs dot com" <gcc-bugzilla at gcc dot gnu dot org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52571

             Bug #: 52571
           Summary: vectorizer changes alignment of common symbols
    Classification: Unclassified
           Product: gcc
           Version: 4.7.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: ian@airs.com


Compile this file with current mainline with -O2 on an ELF system:

unsigned long int *p;

void
f (char **a)
{
  p = (unsigned long int *) a;
}

In the .s file I see this:

    .comm    p,8,8

Now compile it with -O3.  In the .s file I see this:

    .comm    p,8,16

The alignment of a common symbol should not change based on the compiler
optimization level.

This appears to happen in vect_compute_data_ref_alignment.

Perhaps vect_can_force_dr_alignment_p needs to check DECL_COMMON.