Re: GNAT can't vectorize Real_Matrix multiplication from Ada.Numerics.Real_Arrays. What a surprise!

[Robert Eachus <rieachus@comcast.net>]

On Monday, February 19, 2018 at 9:31:26 PM UTC-5, Bojan Bozovic 

> I have looked at ATLAS, however it can't spawn more threads than specified at compile time, so there's lots of possibility to optimize there, by spawning as many threads as supported at run-time.

Aarrg!  Yes, there is a lot of work that needs to be done.  The intent is that you run ATLAS on your target environment, then use the best result as your blas library.

But the problem I am fighting with right now is that on the most recent (high-end) processors from Intel and AMD, you never want to use as many threads as the hardware tells you are available at run-time.  In fact, it is common that if you have a processor which supports 8-threads, you want to run four threads on all even or odd numbered threads.  The recent Threadripper and EPYC CPUs from AMD make it even more complex, as do any multisocket systems.  Usually you want to split the problem up completely and duplicate the data on each hardware CPU chip.