RFR: 8328865: [c2] No need to convert "(x+1)+y" into "(x+y)+1" when y is a CallNode [v2]

[Dean Long]

On Wed, 27 Mar 2024 08:45:55 GMT, SUN Guoyun <duke at openjdk.org> wrote:

>> This patch prohibits the conversion from  "(x+1)+y" into "(x+y)+1" when y is a CallNode to reduce unnecessary spillcode and ADDNode.
>> 
>> Testing: tier1-3 in x86_64 and LoongArch64
>> 
>> JMH in x86_64:
>> <pre>
>> before:
>> Benchmark           Mode  Cnt      Score   Error  Units
>> CallNode.test      thrpt    2  26397.733          ops/s
>> 
>> after:
>> Benchmark           Mode  Cnt      Score   Error  Units
>> CallNode.test      thrpt    2  27839.337          ops/s
>> </pre>
>
> SUN Guoyun has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains two commits:
> 
>  - 8328865: [c2] No need to convert "(x+1)+y" into "(x+y)+1" when y is a CallNode
>  - 8328865: [c2] No need to convert "(x+1)+y" into "(x+y)+1" when y is a CallNode

The difference in generate code above seems very x86-specific, depending on differences between 2-operand ADD vs 3-operand LEA.  It's not obvious to me why this change would necessarily generate better code, even in some cases.  And the requirement should be leaning towards better or no worse code in all cases on all platforms.

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PR Comment: https://git.openjdk.org/jdk/pull/18482#issuecomment-2026274309