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From: "tprince at computer dot org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c/60117] New: simd reduction clause suppresses simd auto-vectorization when -fopenmp is set
Date: Sat, 08 Feb 2014 16:15:00 -0000
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=60117

            Bug ID: 60117
           Summary: simd reduction clause suppresses simd
                    auto-vectorization when -fopenmp is set
           Product: gcc
           Version: 4.9.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: tprince at computer dot org

Created attachment 32082
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=32082&action=edit
source code reproducer

gcc version 4.9.0 20140203
gcc -O2 -ftree-vectorize -std=c99 -march=core-avx2 -fopt-info -S -fopenmp
s314.c
This uses vmaxss instruction in the main loop body, in spite of the fairly
positive vectorization report.
-O3 makes no significant difference, so -O2 is used in practice for stability
elsewhere in gcc source code.
If the omp simd is disabled by removing -fopenmp, excellent code is produced
using vmaxps.  Performance test shows 10x speedup with max-unroll-times=2.