RFR: 8265491: Math Signum optimization for x86 [v9]

[Marcus G K Williams]

On Tue, 27 Apr 2021 21:38:40 GMT, Marcus G K Williams <github.com+168222+mgkwill at openjdk.org> wrote:

>> x86 Math.Signum() uses two floating point compares and a copy sign operation involving data movement to gpr and XMM.
>> 
>> We can optimize to one floating point compare and sign computation in XMM. We observe ~25% performance improvement with this optimization.
>> 
>> Base:
>> 
>> Benchmark                       Mode Cnt Score Error Units
>> Signum._1_signumFloatTest avgt 5 4.660 ? 0.040 ns/op
>> Signum._2_overheadFloat avgt 5 3.314 ? 0.023 ns/op
>> Signum._3_signumDoubleTest avgt 5 4.809 ? 0.043 ns/op
>> Signum._4_overheadDouble avgt 5 3.313 ? 0.015 ns/op
>> 
>>  
>> Optimized:
>> signum intrinsic patch
>> 
>> Benchmark                       Mode  Cnt  Score   Error  Units
>> Signum._1_signumFloatTest       avgt    5  3.769 ? 0.015  ns/op
>> Signum._2_overheadFloat         avgt    5  3.312 ? 0.025  ns/op
>> Signum._3_signumDoubleTest      avgt    5  3.765 ? 0.005  ns/op
>> Signum._4_overheadDouble        avgt    5  3.309 ? 0.010  ns/op
>> 
>> 
>> Signed-off-by: Marcus G K Williams <marcus.williams at intel.com>
>
> Marcus G K Williams has updated the pull request incrementally with one additional commit since the last revision:
> 
>   sviswa7 review
>   
>   Signed-off-by: Marcus G K Williams <marcus.williams at intel.com>

Thanks for the review @sviswa7. 

I've updated to `StubRoutines::x86::float_sign_flip()` & `StubRoutines::x86::double_sign_flip()` as I was using when the code was in the src/hotspot/cpu/x86/x86.ad. If I understood your comment correctly this should use 128 bit aligned.

I've added comments as suggested.

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PR: https://git.openjdk.java.net/jdk/pull/3581