[Bug tree-optimization/66975] New: parloops to handle signed int reductions

[Bug tree-optimization/66975] New: parloops to handle signed int reductions

[vries at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66975

            Bug ID: 66975
           Summary: parloops to handle signed int reductions
           Product: gcc
           Version: 6.0
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: vries at gcc dot gnu.org
  Target Milestone: ---

With this ( https://gcc.gnu.org/ml/gcc-cvs/2015-07/msg00896.html ) fix we
introduce xfails for signed int reductions:
...
        * gcc.dg/autopar/outer-4.c: Add xfail.
        * gcc.dg/autopar/outer-5.c: Same.
        * gcc.dg/autopar/outer-6.c: Same.
        * gcc.dg/autopar/reduc-2.c: Same.
        * gcc.dg/autopar/reduc-2char.c: Same.
        * gcc.dg/autopar/reduc-2short.c: Same.
        * gcc.dg/autopar/reduc-8.c: Same.
...

The problem is that when parallelizing reductions in parloops, we change the
order of evaluation, which can mean that we introduce an overflow, where there
was none before. This only yields the same result if the overflow behaviour can
be assumed to wrap. The fix adds this missing check.

However, we can convert the signed reduction to an unsigned one, correctly
parallelize it, and cast the result back to signed.

[Bug tree-optimization/66975] parloops to handle signed int reductions

[vries at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66975

--- Comment #1 from vries at gcc dot gnu.org ---
This ( https://gcc.gnu.org/ml/gcc-patches/2015-07/msg02064.html ) patches fixes
the problem for operations that do not overflow, f.i. min and max.

[Bug tree-optimization/66975] parloops to handle signed int reductions

[vries at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66975

--- Comment #2 from vries at gcc dot gnu.org ---
(In reply to vries from comment #1)
> This ( https://gcc.gnu.org/ml/gcc-patches/2015-07/msg02064.html ) patches
> fixes the problem for operations that do not overflow, f.i. min and max.

Updated patch: https://gcc.gnu.org/ml/gcc-patches/2015-07/msg02450.html

[Bug tree-optimization/66975] parloops to handle signed int reductions

[vries at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66975

--- Comment #3 from vries at gcc dot gnu.org ---
https://gcc.gnu.org/ml/gcc-cvs/2015-08/msg00003.html :

Author: vries
Date: Sat Aug  1 08:29:29 2015
New Revision: 226463

URL: https://gcc.gnu.org/viewcvs?rev=226463&root=gcc&view=rev
Log:
Allow non-overflow ops in reductions

2015-08-01  Tom de Vries  <tom@codesourcery.com>

        * tree.c (operation_can_overflow, operation_no_trapping_overflow): New
        function.
        * tree.h (operation_can_overflow, operation_no_trapping_overflow):
        Declare.
        * tree-vect-loop.c (vect_is_simple_reduction_1): Use
        operation_no_trapping_overflow.  Allow non-overflow operations.
        * graphite-sese-to-poly.c (is_reduction_operation_p): Allow
non-overflow
        operations.

        * gcc.dg/autopar/reduc-2char.c (init_arrays): Mark with attribute
        optimize ("-ftree-parallelize-loops=0").
        Add successful scans for 2 detected reductions.  Add xfail scans for 3
        detected reductions.
        * gcc.dg/autopar/reduc-2short.c: Same.
        * gcc.dg/autopar/reduc-8.c (init_arrays): Mark with attribute
        optimize ("-ftree-parallelize-loops=0").  Add successful scans for 2
        detected reductions.
        * gcc.dg/vect/trapv-vect-reduc-4.c: Update scan to match vectorized min
        and max reductions.

Modified:
    trunk/gcc/ChangeLog
    trunk/gcc/graphite-sese-to-poly.c
    trunk/gcc/testsuite/ChangeLog
    trunk/gcc/testsuite/gcc.dg/autopar/reduc-2char.c
    trunk/gcc/testsuite/gcc.dg/autopar/reduc-2short.c
    trunk/gcc/testsuite/gcc.dg/autopar/reduc-8.c
    trunk/gcc/testsuite/gcc.dg/vect/trapv-vect-reduc-4.c
    trunk/gcc/tree-vect-loop.c
    trunk/gcc/tree.c
    trunk/gcc/tree.h

[Bug tree-optimization/66975] parloops to handle signed int reductions

[vries at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66975

vries at gcc dot gnu.org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2015-08-01
     Ever confirmed|0                           |1

--- Comment #4 from vries at gcc dot gnu.org ---
https://gcc.gnu.org/ml/gcc-patches/2015-07/msg01817.html :
...
The "proper" solution (see other passes) is to rewrite the reduction
to a wrapping
one (cast to unsigned for the reduction op).
...