RFR: 8349138: Optimize Math.copySign API for Intel e-core targets

[Jasmine Karthikeyan]

On Fri, 31 Jan 2025 11:22:47 GMT, Jatin Bhateja <jbhateja at openjdk.org> wrote:

> Math.copySign is only intrinsified on x86 targets supporting the AVX512 feature.
> Intel E-core Xeons support only the AVX2 feature set and still compile Java implementation which is composed of logical operations.
> 
> Since there is a 3-cycle penalty for copying incoming float/double values to GPRs before being operated upon by logical operation there is an opportunity to optimize this using an efficient instruction sequence.
> 
> Patch uses ANDPS and ANDPD logical instruction to generate efficient instruction sequences to absorb domain copy over penalty. Also, performs minor tuning for existing AVX512 instruction sequence based on VPTERNLOG instruction.
> 
> Following are the performance numbers of the following existing microbenchmark  
> https://github.com/openjdk/jdk/blob/master/test/micro/org/openjdk/bench/vm/compiler/Signum.java
> 
> Patch passes following validation test
> [test/jdk/java/lang/Math/IeeeRecommendedTests.java
> ](https://github.com/openjdk/jdk/blob/master/test/jdk/java/lang/Math/IeeeRecommendedTests.java)
> 
> 
> Granite Rapids-AP (P-core Xeon)
> Baseline AVX512:
> Benchmark                      Mode  Cnt     Score   Error   Units
> Signum._5_copySignFloatTest   thrpt    2  1296.141          ops/ns
> Signum._7_copySignDoubleTest  thrpt    2   838.954          ops/ns
> 
> Withopt :
> Benchmark                      Mode  Cnt    Score   Error   Units
> Signum._5_copySignFloatTest   thrpt    2  940.240          ops/ns
> Signum._7_copySignDoubleTest  thrpt    2  967.370          ops/ns
> 
> Baseline AVX2:
> Benchmark                      Mode  Cnt   Score   Error   Units
> Signum._5_copySignFloatTest   thrpt    2  63.673          ops/ns
> Signum._7_copySignDoubleTest  thrpt    2  26.898          ops/ns
> 
> Withopt :
> Benchmark                      Mode  Cnt    Score   Error   Units
> Signum._5_copySignFloatTest   thrpt    2  785.801          ops/ns
> Signum._7_copySignDoubleTest  thrpt    2  558.710          ops/ns
> 
> Sierra Forest (E-core Xeon)
> Baseline:
> Benchmark                                       (seed)   Mode  Cnt        Score   Error   Units
> o.o.b.vm.compiler.Signum._5_copySignFloatTest      N/A  thrpt    2       40.528          ops/ns
> o.o.b.vm.compiler.Signum._7_copySignDoubleTest     N/A  thrpt    2       25.101          ops/ns
> 
> Withopt:
> Benchmark                                       (seed)   Mode  Cnt        Score   Error   Units
> o.o.b.vm.compiler.Signum._5_copySignFloatTest      N/A  thrpt    2      676.101          ops/ns
> o.o.b.vm.compiler.Signum._7_copySignDoubleTest     N/A  thrpt    2    ...

I think this is a good improvement! Having more intrinsics available for AVX2 targets is nice. I've left some comments below.

src/hotspot/cpu/x86/x86.ad line 1613:

> 1611:     case Op_CopySignD:
> 1612:     case Op_CopySignF:
> 1613:       if (UseAVX < 1 || !is_LP64)  {

Should it be limited to just AVX2, or can the new rules work on AVX1 as well? Since they only use instructions that are available to AVX1.

src/hotspot/cpu/x86/x86.ad line 6769:

> 6767: 
> 6768: instruct copySignF_reg_avx(regF dst, regF src, regF xtmp1, regF xtmp2) %{
> 6769:   predicate(!VM_Version::supports_avx512vl());

Suggestion:

  predicate(UseAVX > 0 && !VM_Version::supports_avx512vl());

Just to be a bit more explicit (and same for the one below).

-------------

PR Review: https://git.openjdk.org/jdk/pull/23386#pullrequestreview-2588420458
PR Review Comment: https://git.openjdk.org/jdk/pull/23386#discussion_r1938356114
PR Review Comment: https://git.openjdk.org/jdk/pull/23386#discussion_r1938356134