RFR: 8325495: C2: implement optimization for series of Add of unique value [v2]

[Kangcheng Xu]

On Tue, 17 Sep 2024 09:37:31 GMT, Roland Westrelin <roland at openjdk.org> wrote:

>> Kangcheng Xu has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains 22 additional commits since the last revision:
>> 
>>  - Merge branch 'openjdk:master' into arithmetic-canonicalization
>>  - Merge pull request #1 from tabjy/arithmetic-canonicalization-v2
>>    
>>    Arithmetic canonicalization v2
>>  - remove dead code
>>  - fix potential void type const nodes
>>  - refactor and cleanup
>>  - add more test cases
>>  - re-implement depth limit on recursion
>>  - passes TestIRLShiftIdeal_XPlusX_LShiftC
>>  - passes AddI[L]NodeIdealizationTests
>>  - revert depth limits
>>  - ... and 12 more: https://git.openjdk.org/jdk/compare/72ec1422...c8fdb74c
>
> src/hotspot/share/opto/addnode.cpp line 427:
> 
>> 425:   }
>> 426: 
>> 427:   Node* con = (bt == T_INT) ? (Node*) phase->intcon((jint) factor) : (Node*) phase->longcon(factor);
> 
> You can use `integercon()` and pass `bt`

I disagree: `integercon()` internally uses `checked_cast<jint>(l)` to make prevent information loss during type conversion and asserts at runtime if the value is larger what a `jint` can hold. However, such an information loss is intended for integer arithmetic overflows. (e.g., `Integer.MAX_VALUE * a + a` is extracted to `((jlong) Integer.MAX_VALUE + (jlong) 1) * a`. Here we want `Integer.MAX_VALUE + 1` to overflow to `(int) Integer.MIN_VALUE`).

If I were to use `integercon()`, the best I could do is `intgercon(bt == T_INT ? (jint) factor : factor)` which is rather pointless.

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PR Review Comment: https://git.openjdk.org/jdk/pull/20754#discussion_r1765694385