[Bug tree-optimization/46787] New: Does not vectorize loop with load from scalar variable

[Bug tree-optimization/46787] New: Does not vectorize loop with load from scalar variable

[rguenth at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=46787

           Summary: Does not vectorize loop with load from scalar variable
           Product: gcc
           Version: 4.6.0
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: normal
          Priority: P3
         Component: tree-optimization
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: rguenth@gcc.gnu.org
                CC: irar@gcc.gnu.org


When looking at why GCC is so slow with the himeno benchmark in the usual
Phoronix testing I noticed that we do not vectorize

float *x;
float parm;
float
test (int start, int end)
{
  int i;
  for (i = start; i < end; ++i)
    {
      float tem = x[i];
      x[i] = parm * tem;
    }
}

because there is a scalar non-varying load of parm that, when the loop
rolls just a single time is aliased by the store to x[i].

We are though vectorizing with at least a vectorization factor of two,
which means that x cannot validly point to parm (and a vector store
would exceed the scalar variables size, something that after vectorization
alias-analysis would use to disambiguate the vector store and the load).

Thus we can treat loads of scalar decls as if they were done outside of
the loop and vectorize it as

     D.1234_3 = tem;
     vec_tmp_4 = { D.1234_3, D.1234_3 };

thus, as if D.1234_3 were a vect_external_def and mark the statement
itself as not relevant.

[Bug tree-optimization/46787] Does not vectorize loop with load from scalar variable

[rguenth at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=46787

--- Comment #1 from Richard Guenther <rguenth at gcc dot gnu.org> 2010-12-03 15:37:16 UTC ---
To fix:

      /* FIXME -- data dependence analysis does not work correctly for objects
         with invariant addresses in loop nests.  Let us fail here until the
         problem is fixed.  */
      if (dr_address_invariant_p (dr) && nest)
        {
          free_data_ref (dr);
          if (dump_file && (dump_flags & TDF_DETAILS))
            fprintf (dump_file, "\tFAILED as dr address is invariant\n");
          ret = false;
          break;
        }

for this to work and be a real improvement we need to pass down the
minimum iterations of the loop (thus, the minimum vectorization factor
in case of the vectorizer).

We can also fix it up locally in the vectorizer when
compute_data_dependences_for_loop would not re-scan the loop
for data references (but we'd do it in the vectorizer and remove
the scalar loads).

We can also avoid computing data dependences until
vect_analyze_data_ref_dependences then and save some compile-time for
non-vectorized loops.

[Bug tree-optimization/46787] Does not vectorize loop with load from scalar variable

[rguenth at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=46787

Richard Guenther <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |ASSIGNED
   Last reconfirmed|                            |2011.04.21 14:39:11
         AssignedTo|unassigned at gcc dot       |rguenth at gcc dot gnu.org
                   |gnu.org                     |
     Ever Confirmed|0                           |1

--- Comment #2 from Richard Guenther <rguenth at gcc dot gnu.org> 2011-04-21 14:39:11 UTC ---
Mine.

[Bug tree-optimization/46787] Does not vectorize loop with load from scalar variable

[rguenth at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=46787

Richard Guenther <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|ASSIGNED                    |RESOLVED
         Resolution|                            |FIXED
   Target Milestone|---                         |4.7.0

--- Comment #4 from Richard Guenther <rguenth at gcc dot gnu.org> 2011-06-30 13:28:06 UTC ---
Fixed.

[Bug tree-optimization/46787] Does not vectorize loop with load from scalar variable

[rguenth at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=46787

--- Comment #3 from Richard Guenther <rguenth at gcc dot gnu.org> 2011-06-30 13:27:47 UTC ---
Author: rguenth
Date: Thu Jun 30 13:27:43 2011
New Revision: 175704

URL: http://gcc.gnu.org/viewcvs?root=gcc&view=rev&rev=175704
Log:
2011-06-30  Richard Guenther  <rguenther@suse.de>

    PR tree-optimization/46787
    * tree-data-ref.c (dr_address_invariant_p): Remove.
    (find_data_references_in_stmt): Invariant accesses are ok now.
    * tree-vect-stmts.c (vectorizable_load): Handle invariant
    loads.
    * tree-vect-data-refs.c (vect_analyze_data_ref_access): Allow
    invariant loads.

    * gcc.dg/vect/vect-121.c: New testcase.

Added:
    trunk/gcc/testsuite/gcc.dg/vect/vect-121.c
Modified:
    trunk/gcc/ChangeLog
    trunk/gcc/testsuite/ChangeLog
    trunk/gcc/tree-data-ref.c
    trunk/gcc/tree-vect-data-refs.c
    trunk/gcc/tree-vect-stmts.c