RFR: 8247645: ChaCha20 intrinsics

[Jamil Nimeh]

On Tue, 22 Mar 2022 23:07:30 GMT, Jamil Nimeh <jnimeh at openjdk.org> wrote:

>> src/hotspot/cpu/x86/stubGenerator_x86_64.cpp line 5842:
>> 
>>> 5840:     __ evpscatterdd(Address(result, zmm_scratch, Address::times_4, 32), writeMask, zmm_cVec, Assembler::AVX_512bit);
>>> 5841:     __ knotwl(writeMask, writeMask);
>>> 5842:     __ evpscatterdd(Address(result, zmm_scratch, Address::times_4, 48), writeMask, zmm_dVec, Assembler::AVX_512bit);
>> 
>> Using the vextracti32x4 instead of evpscatterdd would give better performance:
>>     __ vextracti32x4(Address(result, 0), zmm_aVec, 0);
>>     __ vextracti32x4(Address(result, 64), zmm_aVec, 1);
>>     __ vextracti32x4(Address(result, 128), zmm_aVec, 2);
>>     __ vextracti32x4(Address(result, 192), zmm_aVec, 3);
>>     __ vextracti32x4(Address(result, 16), zmm_bVec, 0);
>>     __ vextracti32x4(Address(result, 80), zmm_bVec, 1);
>>     __ vextracti32x4(Address(result, 144), zmm_bVec, 2);
>>     __ vextracti32x4(Address(result, 208), zmm_bVec, 3);
>>     __ vextracti32x4(Address(result, 32), zmm_cVec, 0);
>>     __ vextracti32x4(Address(result, 96), zmm_cVec, 1);
>>     __ vextracti32x4(Address(result, 160), zmm_cVec, 2);
>>     __ vextracti32x4(Address(result, 224), zmm_cVec, 3);
>>     __ vextracti32x4(Address(result, 48), zmm_dVec, 0);
>>     __ vextracti32x4(Address(result, 112), zmm_dVec, 1);
>>     __ vextracti32x4(Address(result, 176), zmm_dVec, 2);
>>     __ vextracti32x4(Address(result, 240), zmm_dVec, 3);
>
> I have been wondering about this approach for a while now, since I did something similar for the AVX2 version.  I had assumed that using evpscatterdd used less instructions and therefore would be more efficient, but I'm more than happy to move to the vextracti32x4 approach.  I'll be eager to see how it impacts performance along with the increased storage of intermediate data on additional XMMRegister objects.

The changes you recommended yielded about a 10-15% performance improvement on the system I was using for benchmarks.  Thanks for the suggestions!

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PR: https://git.openjdk.org/jdk/pull/7702