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

[Jie Fu]

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

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

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

Changes: https://git.openjdk.java.net/jdk/pull/3404/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3404&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8264945
  Stats: 28 lines in 2 files changed: 23 ins; 0 del; 5 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3404.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3404/head:pull/3404

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