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

[Edward Lam <edward@xxxxxxxxxx>]

PS. I should note that adding whole program optimization (even for a single .cpp 
file!) causes VS2017 to suddenly not generate FMA instructions again. So it's 
important to *NOT* use /GL with cl.exe.

On 2/8/2018 2:14 PM, Edward Lam wrote:
> On 2/8/2018 10:19 AM, Christoph Hertzberg wrote:
> > Could you try writing a small AVX program which uses the 
> > `_mm256_fmadd_ps(a,b,c)` intrinsic and see if it compiles with MSVC? Perhaps 
> > only our `#ifdef __FMA__` test does not work with MSVC. (It would be 
> > interesting to know how to detect FMA support then)
>
> That works! For detection, the documentation at 
> https://msdn.microsoft.com/en-us/library/b0084kay.aspx suggests that perhaps 
> this will work:
>
> #if defined(_MSC_VER) && defined(__AVX2__)
> #define __FMA__
> #endif
>
> For reference, recompiling the earlier test with the best options plus -D__FMA__ 
> produces:
>
> $ ./gemm_test # 325 fmadd instructions produced
> 1124 1215 1465
> col major (checksum: 0) elapsed_ms: 962
> row major (checksum: 0) elapsed_ms: 1021
> --------
> 1730 1235 1758
> col major (checksum: 0) elapsed_ms: 1798
> row major (checksum: 0) elapsed_ms: 1805
> --------
> 1116 1736 868
> col major (checksum: 0) elapsed_ms: 712
> row major (checksum: 0) elapsed_ms: 712
> --------
> 1278 1323 788
> col major (checksum: 0) elapsed_ms: 578
> row major (checksum: 0) elapsed_ms: 584
> --------
>
> Compared to the same compiler options *without* -D__FMA__ :
>
> $ ./gemm_test # 125 fmadd instructions produced
> 1124 1215 1465
> col major (checksum: 0) elapsed_ms: 1245
> row major (checksum: 0) elapsed_ms: 1160
> --------
> 1730 1235 1758
> col major (checksum: 0) elapsed_ms: 2071
> row major (checksum: 0) elapsed_ms: 2066
> --------
> 1116 1736 868
> col major (checksum: 0) elapsed_ms: 905
> row major (checksum: 0) elapsed_ms: 905
> --------
> 1278 1323 788
> col major (checksum: 0) elapsed_ms: 711
> row major (checksum: 0) elapsed_ms: 720
> --------
>
>
> Cheers,
> -Edward