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[Bug tree-optimization/71414] 2x slower than clang summing small float array, GCC should consider larger vectorization factor for "unrolling" reductions
- From: "yyc1992 at gmail dot com" <gcc-bugzilla at gcc dot gnu dot org>
- To: gcc-bugs at gcc dot gnu dot org
- Date: Tue, 07 Jun 2016 14:01:03 +0000
- Subject: [Bug tree-optimization/71414] 2x slower than clang summing small float array, GCC should consider larger vectorization factor for "unrolling" reductions
- Auto-submitted: auto-generated
- References: <bug-71414-4 at http dot gcc dot gnu dot org/bugzilla/>
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=71414
--- Comment #7 from Yichao Yu <yyc1992 at gmail dot com> ---
If I add `-fvariable-expansion-in-unroller` (omg this options is like half the
command line ;-p ...), the performance matches the clang one after the clang
3.8 regression.
```
% gcc -funroll-loops -fvariable-expansion-in-unroller -Ofast -march=core-avx2
benchmark.c -o benchmark2
% ./benchmark2
45.588861
% ./benchmark-gcc
80.518152
% ./benchmark-clang38
41.920054
% ./benchmark-clang37
25.093145
```