Re: Popcount optimization using AVX512

On Fri, Apr 05, 2024 at 07:58:44AM -0500, Nathan Bossart wrote:
> On Fri, Apr 05, 2024 at 10:33:27AM +0300, Ants Aasma wrote:
>> The main issue I saw was that clang was able to peel off the first
>> iteration of the loop and then eliminate the mask assignment and
>> replace masked load with a memory operand for vpopcnt. I was not able
>> to convince gcc to do that regardless of optimization options.
>> Generated code for the inner loop:
>> 
>> clang:
>> <L2>:
>>       50:      add rdx, 64
>>       54:      cmp rdx, rdi
>>       57:      jae <L1>
>>       59:      vpopcntq zmm1, zmmword ptr [rdx]
>>       5f:      vpaddq zmm0, zmm1, zmm0
>>       65:      jmp <L2>
>> 
>> gcc:
>> <L1>:
>>       38:      kmovq k1, rdx
>>       3d:      vmovdqu8 zmm0 {k1} {z}, zmmword ptr [rax]
>>       43:      add rax, 64
>>       47:      mov rdx, -1
>>       4e:      vpopcntq zmm0, zmm0
>>       54:      vpaddq zmm0, zmm0, zmm1
>>       5a:      vmovdqa64 zmm1, zmm0
>>       60:      cmp rax, rsi
>>       63:      jb <L1>
>> 
>> I'm not sure how much that matters in practice. Attached is a patch to
>> do this manually giving essentially the same result in gcc. As most
>> distro packages are built using gcc I think it would make sense to
>> have the extra code if it gives a noticeable benefit for large cases.
> 
> Yeah, I did see this, but I also wasn't sure if it was worth further
> complicating the code.  I can test with and without your fix and see if it
> makes any difference in the benchmarks.

This seems to provide a small performance boost, so I've incorporated it
into v27.

-- 
Nathan Bossart
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