8333393: PhaseCFG::insert_anti_dependences can fail to raise LCAs and to add necessary anti-dependence edges

[dlunde]

Issue Summary

When searching for load anti-dependences in GCM, the memory state for the load is sometimes represented not only by the memory node input of the load, but also other memory nodes. Because PhaseCFG::insert_anti_dependences searches for anti-dependences only from the load's memory input, it is, therefore, possible to sometimes overlook anti-dependences. The result is that loads are potentially scheduled too late, after stores that redefine the memory states of the loads.

Changeset

It is not yet clear why multiple nodes sometimes represent the memory state of a load, nor if this is expected. We can, however, resolve all the miscompiled test cases seen in this issue by improving the idealization of Phi nodes. Specifically, there is an idealization where we split Phis through input MergeMems, that we, prior to this changeset, applied too conservatively.

To illustrate the idealization and how it resolves this issue, consider the example below.

64 membar_release is a critical anti-dependence for 183 loadI. The anti-dependence search starts at the load's direct memory input, 107 Phi, and stops immediately at Phis. Therefore, the search ends at 106 Phi and we never find 64 membar_release.

We can apply the split-through-MergeMem Phi idealization to 119 Phi. This idealization pushes 119 Phi through 120 MergeMem and 121 MergeMem, splitting it into the individual inputs of the MergeMems in the process. As a result, we replace 119 Phi with two new Phis. One of these generated Phis has identical inputs to 107 Phi (106 Phi and 104 Phi), and further idealizations will merge this new Phi and 107 Phi. As a result, 107 Phi then has a Phi-free path to 64 membar_release and we correctly discover the anti-dependence.

The changeset consists of the following changes.

• Add an analysis that allows applying the split-through-MergeMem idealization in more cases than before (including in the above example) while still ensuring termination.
• Add a missing ResourceMark in PhiNode::split_out_instance.
• Add multiple new regression tests in TestGCMLoadPlacement.java.

For reference, here is a previous PR with an alternative fix that we decided to discard in favor of the fix in this PR.

Testing

• GitHub Actions
• tier1 to tier4 (and additional Oracle-internal testing) on Windows x64, Linux x64, Linux aarch64, macOS x64, and macOS aarch64.
• Performance testing using DaCapo, SPECjbb, and SPECjvm on Windows x64, Linux x64, Linux aarch64, macOS x64, and macOS aarch64. There are no significant improvements nor regressions.
• C2 compilation time testing using DaCapo. There are no significant improvements nor regressions.

---

Progress

• Change must be properly reviewed (1 review required, with at least 1 Reviewer)
• Change must not contain extraneous whitespace
• Commit message must refer to an issue

Issue

• JDK-8333393: PhaseCFG::insert_anti_dependences can fail to raise LCAs and to add necessary anti-dependence edges (Bug - P3)

Contributors

• Roberto Castañeda Lozano <rcastanedalo@openjdk.org>
• Christian Hagedorn <chagedorn@openjdk.org>
• Tobias Hartmann <thartmann@openjdk.org>

Reviewing

Using git

Checkout this PR locally:
$ git fetch https://git.openjdk.org/jdk.git pull/23691/head:pull/23691
$ git checkout pull/23691

Update a local copy of the PR:
$ git checkout pull/23691
$ git pull https://git.openjdk.org/jdk.git pull/23691/head

Using Skara CLI tools

Checkout this PR locally:
$ git pr checkout 23691

View PR using the GUI difftool:
$ git pr show -t 23691

Using diff file

Download this PR as a diff file:
https://git.openjdk.org/jdk/pull/23691.diff

Using Webrev

Link to Webrev Comment

[bridgekeeper]

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[openjdk]

❗ This change is not yet ready to be integrated.
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[openjdk]

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[dlunde]

Pinging the reviewers of the prior pull request: @chhagedorn @eme64 @robcasloz @merykitty

/contributor add rcastanedalo
/contributor add chagedorn
/contributor add thartmann

[openjdk]

@dlunde
Contributor Roberto Castañeda Lozano <rcastanedalo@openjdk.org> successfully added.

[openjdk]

@dlunde
Contributor Christian Hagedorn <chagedorn@openjdk.org> successfully added.

[openjdk]

@dlunde
Contributor Tobias Hartmann <thartmann@openjdk.org> successfully added.

[mlbridge]

Webrevs

• 01: Full - Incremental (7f702a68)
• 00: Full (893bfb30)