RFR: 8345485: C2 MergeLoads: merge adjacent array/native memory loads into larger load [v12]

[kuaiwei]

On Thu, 17 Apr 2025 09:30:56 GMT, Emanuel Peter <epeter at openjdk.org> wrote:

>> kuaiwei has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains 14 commits:
>> 
>>  - Merge remote-tracking branch 'origin/master' into dev/merge_loads
>>  - Remove unused code
>>  - Move code to addnode.cpp and add more tests
>>  - Merge remote-tracking branch 'origin/master' into dev/merge_loads
>>  - Fix test
>>  - Add more tests
>>  - Enable StressIGVN and riscv platform
>>  - Change tests as review comments
>>  - Fix test failure and change for review comments
>>  - Revert extract value and add more tests
>>  - ... and 4 more: https://git.openjdk.org/jdk/compare/660b17a6...f6518b26
>
> src/hotspot/share/opto/addnode.cpp line 800:
> 
>> 798:  *
>> 799:  * Because array value is logical and with constant 0xff/0xffff, LoadS/LoadB is converted to an unsigned load
>> 800:  * and 'And' node is eliminated in previous IGVN phase. Please check AndINode::Ideal for reference
> 
> What do you mean by `array value is logical`?
> Do you have tests where we do not have the `And` in the code, and there would be implicit sign extension?

I mean `array value is masked with constant 0xff/0xffff`. I will change the comment. Tests without `And operator` will be added. I think the sign extension is checked before merging.

> src/hotspot/share/opto/addnode.cpp line 828:
> 
>> 826: };
>> 827: 
>> 828: typedef GrowableArray<MergeLoadInfo*> MergeLoadInfoList;
> 
> Do you need to have this Resource allocated? Why not just have the elements in-place? And then make the `MergeLoadInfo` a `StackObj`. That would remove the extra pointer indirection.

I choose `GrowableArray` for convenience. And it can be changed as a stack allocate data.

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PR Review Comment: https://git.openjdk.org/jdk/pull/24023#discussion_r2052273358
PR Review Comment: https://git.openjdk.org/jdk/pull/24023#discussion_r2052274772