| Re: [eigen] Matrix multiplication much slower on MSVC than on g++/clang |
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On 2018-02-09 14:56, Edward Lam wrote:
On 2/9/2018 3:16 AM, Gael Guennebaud wrote:
That works! For detection, the documentation at
https://msdn.microsoft.com/en-us/library/b0084kay.aspx
<https://msdn.microsoft.com/en-us/library/b0084kay.aspx> suggests
that
perhaps this will work:
#if defined(_MSC_VER) && defined(__AVX2__)
#define __FMA__
#endif
To implement that we need to make sure that on all architectures AVX2
=> FMA. This seems to be true for Intel's ones, but I'm not sure about
AMD.
According to
https://stackoverflow.com/questions/16348909/how-do-i-know-if-i-can-compile-with-fma-instruction-sets
, all AMD processors which support AVX2 support FMA. Unfortunately, I
couldn't easily confirm through official online resources. The wikipedia
page on Advanced_Vector_Extensions notes that only AMD Excavactor
processors (and up) support AVX2, and those definitely support FMA
(double-checked at
https://support.amd.com/TechDocs/47414_15h_sw_opt_guide.pdf).
It seems that there are AMD architectures which support FMA3, but not
AVX2. This would make the check above safe, but not optimal.
https://en.wikipedia.org/wiki/Piledriver_(microarchitecture)
Bulldozer supports FMA4 (and AVX1), but not FMA3 (nor AVX2).
https://en.wikipedia.org/wiki/Bulldozer_(microarchitecture)
It seems, Eigen generally only supports FMA3, even though FMA4 should be
relatively easy (just the name of the intrinsic should be different).
That said, I'm ok with the hack suggested above. We should probably
still document how to make sure that FMA is enabled (if AVX2 is not
available).
Christoph
-Edward
--
Dr.-Ing. Christoph Hertzberg
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