RFR: 8333334: C2: Make result of `Node::dominates` more precise to enhance scalar replacement [v8]

[Christian Hagedorn]

On Thu, 22 Aug 2024 09:02:47 GMT, Qizheng Xing <qxing at openjdk.org> wrote:

>> This patch changes the algorithm of `Node::dominates` to make the result more precise, and allows the iterators of `ConcurrentHashMap` to be scalar replaced.
>> 
>> The previous algorithm will return a conservative result when encountering a dead control flow, and only try the first two input paths of a multi-input Region node, which may prevent the scalar replacement in some cases.
>> 
>> For example, with G1 GC enabled, C2 generates GC barriers for `ConcurrentHashMap` iteration operations at some early phases, and then eliminates them in a later IGVN, but `LoadNode` is also idealized in the same IGVN. This causes `LoadNode::Ideal` to see some dead barrier control flows, and refuse to split some instance field loads through Phi due to the conservative result of `Node::dominates`, and thus the scalar replacement can not be applied to iterators in the later macro elimination phase.
>> 
>> This patch allows `Node::dominates` to try other paths of the last multi-input Region node when the first path is dead, and makes `ConcurrentHashMap` iteration ~30% faster:
>> 
>> 
>> Benchmark                            (nkeys)  Mode  Cnt        Score       Error  Units
>> Maps.testConcurrentHashMapIterators    10000  avgt   15   414099.085 ± 33230.945  ns/op   # baseline
>> Maps.testConcurrentHashMapIterators    10000  avgt   15   315490.281 ±  3037.056  ns/op   # patch
>> 
>> 
>> Testing: tier1-4.
>
> Qizheng Xing has updated the pull request incrementally with four additional commits since the last revision:
> 
>  - Add wrapper method for checking `DomResult` of `all_controls_dominate`.
>  - Remove redundant `applyIf` and fix style for IR test.
>  - Fix style.
>  - Add brackets around modified if/else branches.

Otherwise, the update looks good, thanks!

src/hotspot/share/opto/memnode.cpp line 1717:

> 1715:       // Wait for the dead code to be removed.
> 1716:       // The dead code will eventually be removed in IGVN,
> 1717:       // so we have an unambiguous result whether it's dominated or not.

Suggestion:

      // There is some dead code which eventually will be removed in IGVN. Once this is the case, we get an unambiguous 
      // dominance result. Push the node to the worklist again until the dead code is removed.

-------------

Marked as reviewed by chagedorn (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/19496#pullrequestreview-2253980782
PR Review Comment: https://git.openjdk.org/jdk/pull/19496#discussion_r1726697616