RFR: 8303970: C2 can not merge homogeneous adjacent two If [v2]

[Yi Yang]

On Tue, 14 Mar 2023 10:42:49 GMT, Tobias Hartmann <thartmann at openjdk.org> wrote:

>> Yi Yang has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   dont apply Identity for dead bool
>
> src/hotspot/share/opto/subnode.cpp line 1502:
> 
>> 1500:     // During parsing, empty uses of bool is tolerable. During iterative GVN,
>> 1501:     // we don't aggressively replace bool whose use is empty with existing node.
>> 1502:     return this;
> 
> Why should we bother optimizing a dead Bool at all?

Because it is likely that bool is created but not immediately used during parsing(PhaseGVN), it can be optimized out.

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PR: https://git.openjdk.org/jdk/pull/12978