RFR: 8265917: Different values computed by C2 and interpreter/C1 for Math.pow(x, 2.0) on x86_32 [v2]

[Jie Fu]

On Mon, 26 Apr 2021 06:06:07 GMT, Jie Fu <jiefu at openjdk.org> wrote:

>> Hi all,
>> 
>> C2 may produce different results for Math.pow(x, 2.0) compared with interpreter/C1 on x86_32.
>> 
>> E.g., for Math.pow(1.0 / 2047, 2.0):
>> 
>> interpreter: 2.38651580386563E-7 
>>          C2: 2.3865158038656307E-7
>> 
>> 
>> The reason is that C2 will replace Math.pow(x, 2.0) with x*x.
>> However, there is no such optimization for interpreter/C1 on x86_32.
>> 
>> The fix just disables C2's opt for Math.pow(x, 2.0) on x86_32 since nobody (or very few people) would run x86_32.
>> And we don't have a plan to implement such opt on x86_32.
>> 
>> Another reason to fix this bug is that we need this patch to extend C2's opt for Math.pow(x, 0.5) on other platforms.
>> 
>> Thanks.
>> Best regards,
>> Jie
>
> Jie Fu has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Optimize pow(x, 2.0) for x86_32

Testing:
  -tier1 ~ tier3 on Linux/x86_32, no regression

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PR: https://git.openjdk.java.net/jdk/pull/3677