RFR: 8264945: Optimize the code-gen for Math.pow(x, 0.5)

[Jie Fu]

On Fri, 9 Apr 2021 02:19:10 GMT, Jie Fu <jiefu at openjdk.org> wrote:

> Hi all,
> 
> I'd like to optimize the code-gen for Math.pow(x, 0.5).
> And 7x ~ 14x performance improvement is observed by the jmh micro-benchmarks.
> 
> While I was optimizing a machine learning program, I found both Math.pow(x, 2) and Math.pow(x, 0.5) are used.
> To my surprise, C2 just optimizes the case for Math.pow(x, 2) [1], but still not for Math.pow(x, 0.5) yet.
> 
> The patch just replace Math.pow(x, 0.5) with Math.sqrt(x).
> 
> Before:
> 
> Benchmark                     (seed)   Mode  Cnt      Score    Error   Units
> MathBench.powDouble0Dot5           0  thrpt    8  45525.117 ? 11.686  ops/ms
> MathBench.powDouble0Dot5Loop       0  thrpt    8      0.031 ?  0.001  ops/ms
> 
> Benchmark                     (seed)   Mode  Cnt      Score    Error   Units
> MathBench.powDouble0Dot5           0  thrpt    8  45509.317 ?  6.581  ops/ms
> MathBench.powDouble0Dot5Loop       0  thrpt    8      0.031 ?  0.001  ops/ms
> 
> 
> After:
> 
> Benchmark                     (seed)   Mode  Cnt       Score     Error   Units
> MathBench.powDouble0Dot5           0  thrpt    8  343354.892 ? 362.900  ops/ms
> MathBench.powDouble0Dot5Loop       0  thrpt    8       0.457 ?   0.001  ops/mso
> 
> Benchmark                     (seed)   Mode  Cnt       Score    Error   Units
> MathBench.powDouble0Dot5           0  thrpt    8  343421.559 ? 49.326  ops/ms
> MathBench.powDouble0Dot5Loop       0  thrpt    8       0.457 ?  0.001  ops/ms
> 
> 
> Testing:
>   - tier1~3 on Linux/x64
> 
> Thanks,
> Best regards,
> Jie
> 
> [1] https://github.com/openjdk/jdk/blob/master/src/hotspot/share/opto/library_call.cpp#L1680

Hi all,

This is the follow-up of JDK-8265325, which optimizes the code-gen of C2 for pow(x, 0.5).
Instead of calling into the StubRoutines::dpow(), the compiler can directly generate sqrt for all x >= 0.0.
1.5x ~ 4.2x performance improvement is observed after the opt.

Before

Benchmark                      (seed)   Mode  Cnt       Score       Error   Units
MathBench.powDouble0Dot5Const       0  thrpt    8  203004.302 ?   684.072  ops/ms
MathBench.powDouble0Dot5Loop        0  thrpt    8       0.121 ?     0.001  ops/ms


After

Benchmark                      (seed)   Mode  Cnt       Score      Error   Units
MathBench.powDouble0Dot5Const       0  thrpt    8  308771.237 ? 1604.567  ops/ms   <-- 1.5x
MathBench.powDouble0Dot5Loop        0  thrpt    8       0.508 ?    0.001  ops/ms   <-- 4.2x


Testing:
  tier1 ~ tier3 on Linux/x64

Thanks.
Best regards,
Jie

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

PR: https://git.openjdk.java.net/jdk/pull/3404