RFR(M):8214751: X86: Support for VNNI instruction

[Deshpande, Vivek R]

Hi All

Could you please review the VNNI VPDPWSSD instruction support with autovectorization.
It can vectorize this operation in the loop:
out[i] += ((in1[2*i] * in2[2*i]) + (in1[2*i+1] * in2[2*i+1]));
More information on VNNI can be found here:
https://software.intel.com/sites/default/files/managed/c5/15/architecture-instruction-set-extensions-programming-reference.pdf

The initial performance gains with micro on skylake with AVX3 is 10.8x.
 and it generates
vmovdqu xmm3, xmmword ptr [rbp+r8*2+0x10]
vmovdqu xmm6, xmmword ptr [rdx+r8*2+0x10]
vpmaddwd xmm3, xmm6, xmm3
vpaddd xmm3, xmm3, xmmword ptr [r9+rdi*4+0x10]
vmovdqu xmmword ptr [r9+rdi*4+0x10], xmm3

It can generate vpdpwssd instruction on cascadelake.

The webrev is here:
http://cr.openjdk.java.net/~vdeshpande/8214751/VNNI/webrev.00/
The jbs entry for the same is here:
https://bugs.openjdk.java.net/browse/JDK-8214751

Regards,
Vivek
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