RFR: 8346664: C2: Optimize mask check with constant offset [v7]

[Matthias Ernst]

On Wed, 29 Jan 2025 17:23:40 GMT, Matthias Ernst <duke at openjdk.org> wrote:

>> Fixes [JDK-8346664](https://bugs.openjdk.org/browse/JDK-8346664): extends the optimization of masked sums introduced in #6697 to cover constant values, which currently break the optimization.
>> 
>> Such constant values arise in an expression of the following form, for example from `MemorySegmentImpl#isAlignedForElement`:
>> 
>> 
>> (base + (index + 1) << 8) & 255
>> => MulNode
>> (base + (index << 8 + 256)) & 255
>> => AddNode
>> ((base + index << 8) + 256) & 255
>> 
>> 
>> Currently, `256` is not being recognized as a shifted value. This PR enables further reduction:
>> 
>> 
>> ((base + index << 8) + 256) & 255
>> => MulNode (this PR)
>> (base + index << 8) & 255
>> => MulNode (PR #6697)
>> base & 255 (loop invariant)
>> 
>> 
>> Implementation notes:
>> * I verified that the originating issue "scaled varhandle indexed with i+1"  (https://mail.openjdk.org/pipermail/panama-dev/2024-December/020835.html) is resolved with this PR.
>> * ~in order to stay with the flow of the current implementation, I refrained from solving general (const & mask)==0 cases, but only those where const == _ << shift.~
>> * ~I modified existing test cases adding/subtracting from the index var (which would fail with current C2). Let me know if would like to see separate cases for these.~
>
> Matthias Ernst 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 20 additional commits since the last revision:
> 
>  - Merge branch 'openjdk:master' into mernst/JDK-8346664
>  - make the check more clear: shift >= mask_width
>  - fully randomized
>  - JLS: only the lower bits of the shift are taken into account (aka we don't assert).
>  - (c)
>  - (c)
>  - Assert that MulNode::Ideal already masks constant shift amounts for us.
>    Avoid accidental zero mask breaking test.
>  - "element".
>  - avoid redundant comment
>  - addConstNonConstMaskLong
>  - ... and 10 more: https://git.openjdk.org/jdk/compare/7b75b7c7...490cc2fb

> TestShiftConvertAndNotification
> TestAndShiftZeroCCP

Can repro. This is on me, I apparently did not run all tier1 after the assert. It has to do with mixed long/int operands. I will investigate.

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PR Comment: https://git.openjdk.org/jdk/pull/22856#issuecomment-2623868977