Integrated: 8273409: Receiver type narrowed by CCP does not always trigger post-parse call devirtualization

[Tobias Hartmann]

On Wed, 8 Sep 2021 14:41:54 GMT, Tobias Hartmann <thartmann at openjdk.org> wrote:

> While working on [JDK-8273323](https://bugs.openjdk.java.net/browse/JDK-8273323), I noticed that even if CCP is able to narrow the receiver type of a virtual call to an exact type, post-parse call devirtualization is not always triggered. The problem is that CCP only adds nodes to the IGVN worklist if their types have been improved:
> https://github.com/openjdk/jdk/blob/4023646ed1bcb821b1d18f7e5104f04995e8171d/src/hotspot/share/opto/phaseX.cpp#L1961-L1965
> 
> And in turn, IGVN only adds user nodes to the worklist if the type of the parent node could be further improved. As a result, a `CallNode` is not always added to the worklist if the type of its receiver node has been improved. Often, we are lucky and macro expansion or another optimization phase after CCP adds the `CallNode` to the worklist. However, as `testDynamicCallWithCCP` shows, that's not always the case.
> 
> The fix is to explicitly add `CallStaticJavaNode` and `CallDynamicJava` to the worklist after CCP to give call devirtualization a chance to run:
> https://github.com/openjdk/jdk/blob/4023646ed1bcb821b1d18f7e5104f04995e8171d/src/hotspot/share/opto/callnode.cpp#L1054-L1057
> https://github.com/openjdk/jdk/blob/4023646ed1bcb821b1d18f7e5104f04995e8171d/src/hotspot/share/opto/callnode.cpp#L1137-L1140
> 
> I also considered moving the optimization from `Ideal` to `Value` (which in contrast to `Ideal` is already executed **during** CCP) but we can't trust receiver types during CCP (because CCP goes from optimistic types to pessimistic ones). Also, the current implementation relies on setting the `_generator` field which is not possible from `Value` which is declared `const`:
> https://github.com/openjdk/jdk/blob/4023646ed1bcb821b1d18f7e5104f04995e8171d/src/hotspot/share/opto/callnode.cpp#L1169-L1172
> 
> All the newly added tests fail IR verification if post-parse call devirtualization is disabled (`-XX:-IncrementalInlineVirtual -XX:-IncrementalInlineMH`). Without the fix, `testDynamicCallWithCCP` always fails.
> 
> I filed [JDK-8273496](https://bugs.openjdk.java.net/browse/JDK-8273496) to improve how CCP handles this.
> 
> Thanks,
> Tobias

This pull request has now been integrated.

Changeset: 4866eaa9
Author:    Tobias Hartmann <thartmann at openjdk.org>
URL:       https://git.openjdk.java.net/jdk/commit/4866eaa997b2ee2a47bdcd0d96202f220fb2774d
Stats:     205 lines in 3 files changed: 204 ins; 0 del; 1 mod

8273409: Receiver type narrowed by CCP does not always trigger post-parse call devirtualization

Reviewed-by: vlivanov, neliasso

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PR: https://git.openjdk.java.net/jdk/pull/5418