Re: -O3 and -ftree-vectorize

[Xinliang David Li <davidxl@google.com>]

On Thu, Feb 6, 2014 at 2:21 PM, Tim Prince <n8tm@aol.com> wrote:
>
> On 2/6/2014 1:51 PM, Uros Bizjak wrote:
>>
>> Hello!
>>
>> 4.9 does not enable -ftree-vectorize for -O3 (and Ofast) anymore. Is
>> this intentional?
>>
>> $/ssd/uros/gcc-build/gcc/xgcc -B /ssd/uros/gcc-build/gcc -O3 -Q
>> --help=optimizers
>>
>> ...
>> -ftree-vectorize                      [disabled]
>> ...
>>
>>
> I'm seeing vectorization  but no output from -ftree-vectorizer-verbose,

use -fopt-info.

The old option is deprecated.

>and
> no dot product vectorization inside omp parallel regions, with gcc g++ or
> gfortran 4.9.  Primary targets are cygwin64 and linux x86_64.
> I've been unable to use -O3 vectorization with gcc, although it works with
> gfortran and g++, so use gcc -O2 -ftree-vectorize together with additional
> optimization flags which don't break.

Reproducible case?


David

> I've made source code changes to take advantage of the new vectorization
> with merge() and ? operators; while it's useful for -march=core-avx2, it's
> sometimes a loss for -msse4.1.
> gcc vectorization with #pragma omp parallel for simd is reasonably effective
> in my tests only on 12 or more cores.
> #pragma omp simd reduction(max: ) is giving correct results but poor
> performance in my tests.
>
> You've probably seen my gcc testresults posts.  The one major recent
> improvement is the ability to skip cilkplus tests on targets where it's
> totally unsupported.  Without cilk_for et al. even on "supported" targets
> cilkplus seems useless.
> There are still lots of failing stabs tests on targets where those
> apparently aren't supported.
>
> So there are some mysteries about what the developers intend.  I suppose
> this was posted on gcc list on account of such questions being ignored on
> gcc-help.
>
> --
> Tim Prince
>