RFR: 8266028: C2 computes -0.0 for Math.pow(-0.0, 0.5)

[Jie Fu]

Hi all,

C2 computes -0.0 for Math.pow(-0.0, 0.5) which should be 0.0 according to the API specs.
This bug was found while I was implementing the same optimization for x86_32 (JDK-8265940).

The reason is that CmpDNode [1] doesn't distinguish -0.0 from 0.0.
Math.pow(-0.0, 0.5) was replaced with Math.sqrt(-0.0) incorrectly since CmpDNode says -0.0 < 0.0 if false.

The fix excludes C2's optimization for x=0.0/-0.0. 
And the jtreg test has been improved to make sure the corner cases are covered by the compilers.

Thanks.
Best regards,
Jie

[1] https://github.com/openjdk/jdk/blob/master/src/hotspot/share/opto/library_call.cpp#L1656

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Commit messages:
 - 8266028: C2 computes -0.0 for Math.pow(-0.0, 0.5)

Changes: https://git.openjdk.java.net/jdk/pull/3712/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3712&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8266028
  Stats: 21 lines in 2 files changed: 9 ins; 11 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3712.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3712/head:pull/3712

PR: https://git.openjdk.java.net/jdk/pull/3712