RFR: 8264054: Bad XMM performance on java.lang.MathBench.sqrtDouble [v5]

[Sandhya Viswanathan]

> For the j.l.Math JMH at https://github.com/openjdk/jmh-jdk-microbenchmarks/blob/master/micros-jdk11/src/main/java/org/openjdk/bench/java/lang/MathBench.java, the performance for sqrt benchmark could be improved.  Thanks a lot to Eric Caspole for finding this issue.
> 
> Benchmark:
>     @Benchmark
>     public double  sqrtDouble() {
>         return  Math.sqrt(double4Dot1);
>     }
> 
> Current code generated (linux format) by c2 JIT is:
>      vsqrtsd 0x50(%r10),%xmm0,%xmm0
> 
> The vsqrtsd instruction operation is specified as below: 
>      VSQRTSD (VEX.128 encoded version)
>      DEST[63:0] := SQRT(SRC2[63:0])
>      DEST[127:64] := SRC1[127:64]
>      DEST[MAXVL-1:128] := 0
> 
> The upper 127:64 bits are set from previous contents of xmm0. As the destination xmm0 register was not initialized prior to use by c2 JIT, this causes stall and lower performance.
> 
> By adding xmm0 initialization prior to use, the performance of the above benchmark improves significantly.
> 
> Code generated after patch:
>      vxorpd %xmm0,%xmm0,%xmm0
>      vsqrtsd 0x50(%r10),%xmm0,%xmm0
> 
> Performance before patch:
> Benchmark                    Mode  Cnt       Score      Error   Units
> MathBench.sqrtDouble        thrpt    8  193612.396 ±  95.807  ops/ms
> 
> Performance after patch:
> MathBench.sqrtDouble        thrpt    8  276388.024 ± 846.372  ops/ms
> 
> Best Regards,
> Sandhya

Sandhya Viswanathan has updated the pull request incrementally with one additional commit since the last revision:

  Corrected typo

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

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/3256/files
  - new: https://git.openjdk.java.net/jdk/pull/3256/files/903b7c41..9e90b452

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=3256&range=04
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=3256&range=03-04

  Stats: 4 lines in 1 file changed: 0 ins; 0 del; 4 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3256.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3256/head:pull/3256

PR: https://git.openjdk.java.net/jdk/pull/3256