RFR: 8281375: Accelerate bitCount operation for AVX2 and AVX512 target.

[Jatin Bhateja]

On Wed, 9 Feb 2022 08:05:32 GMT, Eric Liu <eliu at openjdk.org> wrote:

>> Summary of changes:
>> 
>> - Patch extends existing vectorized bitCount optimization added with [JDK-8278868](https://bugs.openjdk.java.net/browse/JDK-8278868) and emits optimized JIT sequence for AVX2 and other AVX512 targets which do not support avx512_vpopcntdq feature.
>> - Since PopCountVI/PopCountVL node emit different instruction sequence based on the target features hence a rudimentary cost mode has been added which influences the SLP unrolling factor to prevent generating bloated main loops.
>> - Following are the performance results of an existing [JMH micro](https://github.com/jatin-bhateja/jdk/blob/master/test/micro/org/openjdk/bench/vm/compiler/VectorBitCount.java) over various X86 targets.
>> 
>> 
>> Benchmark | SIZE | Baseline AVX2 (ns/op) | Withopt AVX2 (ns/op) | Gain % | Baseline AVX3 (ns/op) | Withopt AVX3 (ns/op) | Gain % | Baseline AVX3 (VPOPCOUNTDQ) | Withopt AVX3 (VPOCOUNTDQ) | Gain %
>> -- | -- | -- | -- | -- | -- | -- | -- | -- | -- | --
>> VectorBitCount.WithSuperword.intBitCount | 1024 | 1089.799 | 420.156 | 159.3796114 | 1083.92 | 203.958 | 431.442748 | 88.958 | 60.096 | 48.02649095
>> VectorBitCount.WithSuperword.longBitCount | 1024 | 417.458 | 413.859 | 0.869619846 | 417.203 | 214.949 | 94.09394787 | 105.954 | 117.019 | -9.455729411
>> 
>> Please review and share your feedback.
>> 
>> Best Regards,
>> Jatin
>
> src/hotspot/share/opto/loopTransform.cpp line 976:
> 
>> 974:       case Op_PopCountVL: {
>> 975:         const TypeVect * vt = n->bottom_type()->is_vect();
>> 976:         body_size += Matcher::vector_op_cost(n->Opcode(), vt->element_basic_type(), vt->length());
> 
> For other platforms, I think `vector_op_cost` should be better to return 0 at this moment.

If a target support PopCountVI/VL minimum cost should 1, Zero will mean instruction has no cost at all.

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PR: https://git.openjdk.java.net/jdk/pull/7373