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.