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From: "spop at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/14741] graphite with loop blocking and interchanging doesn't optimize a matrix multiplication loop
Date: Sun, 21 Jul 2013 09:15:00 -0000
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=14741

--- Comment #21 from Sebastian Pop <spop at gcc dot gnu.org> ---
Scop detection does not detect this loop because we now require the scev of the
data references to be analyzable in all the loops around:

commit e97c4b0daa932558eae4d9b9794cdd549e6d40bd
Author: rguenth <rguenth@138bc75d-0d04-0410-961f-82ee72b054a4>
Date:   Tue Jan 10 09:14:51 2012 +0000

    2012-01-10  Richard Guenther  <rguenther@suse.de>

        PR tree-optimization/50913
        * graphite-scop-detection.c (stmt_has_simple_data_refs_p):
        Require data-refs to be representable by Graphite with respect
        to any loop nest.

        * gcc.dg/graphite/interchange-16.c: New testcase.
        * gcc.dg/graphite/scop-20.c: XFAIL.
        * gfortran.dg/graphite/interchange-1.f: Likewise.
        * gfortran.dg/graphite/block-1.f90: Likewise.
        * gfortran.dg/graphite/block-2.f: Likewise.

So for this gimple stmt:
# VUSE <.MEM_6>
_29 = *a_28(D)[_27];

the data reference when analyzed in the innermost loop is:
{(stride.12_14 + offset.13_15) + _19, +, stride.12_14}_3
and that cannot be represented in graphite as it has a non constant
(parametric) stride.

Note that this data reference can be represented in an outer loop, as the
stride is then a multidimensional access with constant strides.