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

[Jie Fu]

On Tue, 27 Apr 2021 09:06:55 GMT, Andrew Haley <aph at openjdk.org> wrote:

>> 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
>
> Marked as reviewed by aph (Reviewer).

Thanks @theRealAph for your review.

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