Re: [eigen] Matrix multiplication much slower on MSVC than on g++/clang

[Edward Lam <edward@xxxxxxxxxx>]

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).

-Edward