[Bug tree-optimization/71414] 2x slower than clang summing small float array, GCC should consider larger vectorization factor for "unrolling" reductions

[rguenther at suse dot de]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=71414

--- Comment #8 from rguenther at suse dot de <rguenther at suse dot de> ---
On Tue, 7 Jun 2016, yyc1992 at gmail dot com wrote:

> 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
> ```

Yeah, but -fvariable-expansion-in-unroller is quite late.