How to get best AVX2 performance from gfortran?

[Richard Biener]

On April 20, 2018 7:18:37 PM GMT+02:00, Jerry DeLisle <jvdelisle@charter.net> wrote:
>On 04/20/2018 08:51 AM, Thompson, Matt (GSFC-610.1)[SCIENCE SYSTEMS AND
>
>APPLICATIONS INC] wrote:
>> On 04/18/2018 11:40 PM, Jerry DeLisle wrote:
>>> On 04/18/2018 07:22 AM, Thompson, Matt (GSFC-610.1)[SCIENCE SYSTEMS 
>--- snip---
>> that from Jerry's flags:
>> 
>> Native O2 Delisle NoFMA NoFastMath:
>> 
>>     TRACER_2D 264.1194
>>     REMAPPING 113.4083
>> 
>> Well, nuts. Looks like -ffast-math was the special flag.
>
>Look in manual at gcc.gnu.org.  -ffast-math implies a few other flags 
>that you can further try separately.
>
>-ffast-math
>
>     Sets the options -fno-math-errno, -funsafe-math-optimizations, 
>-ffinite-math-only, -fno-rounding-math, -fno-signaling-nans, 
>-fcx-limited-range and -fexcess-precision=fast.

That doesn't seem to be a complete list. The Fortran FE also enables some of them unconditionally like -fno-math-errno. One important flag is -fno-trapping-math 

Richard. 

>Also, if you can run some profiles and narrow down the code in
>question, 
>maybe obfuscate it a bit or share privately with one of us, we can get 
>someone in the know to take a closer look.
>
>> 
>> I'll ask for a GCC 8 snapshot to be installed on our cluster to test 
>> with that at least to see if vectorization/speed is improved, though 
>> without reproducibility...
>> 
>> Thanks,
>> Matt
>> 
>>