-O3 and -ftree-vectorize

[Tim Prince]

On 02/07/2014 10:22 AM, Jakub Jelinek wrote:
> On Thu, Feb 06, 2014 at 05:21:00PM -0500, Tim Prince wrote:
>> I'm seeing vectorization  but no output from
>> -ftree-vectorizer-verbose, 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.
> Can you file a GCC bugzilla PR with minimal testcases for this (or point us
> at already filed bugreports)?
The question of problems with gcc -O3 (called from gfortran) have eluded 
me as to finding a minimal test case.  When I run under debug, it 
appears that somewhere prior to the crash some gfortran code is 
over-written with data by the gcc code, overwhelming my debugging 
skill.  I can get full performance with -O2 plus a bunch of intermediate 
flags.
As to non-vectorization of dot product in omp parallel region, 
-fopt-info (which I didn't know about) is reporting vectorization, but 
there are no parallel simd instructions in the generated code for the 
omp_fn.  I'll file a PR on that if it's still reproduced in a minimal case.

>
>> 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.
> Likewise.
Those are cases of 2 levels of loops from netlib "vector" benchmark 
where only one level is vectorizable and parallelizable. By putting the 
vectorizable loop on the outside the parallelization scales to a large 
number of cores.  I don't expect it to out-perform single thread 
optimized avx vectorization until 8 or more cores are in use, but it 
needs more than expected number of threads even relative to SSE 
vectorization.
>
>> #pragma omp simd reduction(max: ) is giving correct results but poor
>> performance in my tests.
> Likewise.
I'll file a PR on this, didn't know if there might be interest.  I have 
an Intel compiler issue "closed, will not be fixed" so the simd 
reduction(max: ) isn't viable for icc in the near term.
Thanks,