8307927: C2: "malformed control flow" with irreducible loop #14522

rwestrel · 2023-06-16T15:59:40Z

The test contains a loop nest with 2 loops. The outer loop is an
irreducible loop. The safepoint for that loop is also in the inner
loop. Because IdealLoopTree::check_safepts() ignores irreducible
loops, that safepoint is not marked as being required and is
eliminated from the inner loop. The inner loop is then optimized out
and the outer loop becomes an infinite loop with no safepoint (a
single node loop). That, in turn, causes the loop to be eliminated
because it has not use and the assert fires.

The fix I propose is to make IdealLoopTree::check_safepts() work
with irreducible loops. I think
IdealLoopTree::allpaths_check_safepts() can be used for that. When
working on this I wondered if that method could be called with a loop
whose head has more than 3 inputs. I couldn't write a test case with
an irreducible loop whose head had more than 3 inputs but I added an
assert in the method and ran some testing. That assert fired so I also
propose to tweak the method so it's robust in that case.

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Issue

JDK-8307927: C2: "malformed control flow" with irreducible loop (Bug - P4)

Reviewers

Tobias Hartmann (@TobiHartmann - Reviewer) ⚠️ Review applies to 3bb72120
Emanuel Peter (@eme64 - Reviewer)

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Update a local copy of the PR:
$ git checkout pull/14522
$ git pull https://git.openjdk.org/jdk.git pull/14522/head

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Checkout this PR locally:
$ git pr checkout 14522

View PR using the GUI difftool:
$ git pr show -t 14522

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https://git.openjdk.org/jdk/pull/14522.diff

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bridgekeeper · 2023-06-16T16:08:45Z

👋 Welcome back roland! A progress list of the required criteria for merging this PR into master will be added to the body of your pull request. There are additional pull request commands available for use with this pull request.

openjdk · 2023-06-16T16:11:10Z

@rwestrel The following label will be automatically applied to this pull request:

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mlbridge · 2023-06-16T16:15:16Z

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01: Full - Incremental (bbf4d581)
00: Full (3bb72120)

TobiHartmann

Looks reasonable to me. All tests passed.

@eme64 Please have a look as well.

openjdk · 2023-06-23T06:54:01Z

@rwestrel This change now passes all automated pre-integration checks.

ℹ️ This project also has non-automated pre-integration requirements. Please see the file CONTRIBUTING.md for details.

After integration, the commit message for the final commit will be:

8307927: C2: "malformed control flow" with irreducible loop

Reviewed-by: thartmann, epeter

You can use pull request commands such as /summary, /contributor and /issue to adjust it as needed.

At the time when this comment was updated there had been 87 new commits pushed to the master branch:

be64d3a: 8310299: C2: 8275201 broke constant folding of array store check in some cases
b2eae16: 8295191: IR framework timeout options expect ms instead of s
af319d9: 8311064: Windows builds fail without precompiled headers after JDK-8310728
cbf418a: 8311020: Typo cleanup in Classfile API
f4b900b: 8310902: (fc) FileChannel.transferXXX async close and interrupt issues
cf8d706: 8308463: Refactor regenerated class handling in lambdaFormInvokers.cpp
6f58ab2: 8301569: jmod list option and jimage list --help not interpreted correctly on turkish locale
8f5a384: 8311032: Empty value for java.protocol.handler.pkgs system property can lead to unnecessary classloading attempts of protocol handlers
ded1370: 8309811: BytecodePrinter cannot handle unlinked classes
02b17d7: 8310264: In PhaseChaitin::Split defs and phis are leaked
... and 77 more: https://git.openjdk.org/jdk/compare/7628da200833d0d26215a91afdd1bf7fc110da1d...master

As there are no conflicts, your changes will automatically be rebased on top of these commits when integrating. If you prefer to avoid this automatic rebasing, please check the documentation for the /integrate command for further details.

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eme64 · 2023-06-23T08:38:09Z

src/hotspot/share/opto/loopnode.cpp

@@ -3514,9 +3514,9 @@ bool IdealLoopTree::beautify_loops( PhaseIdealLoop *phase ) {
 // encountered.  Helper for check_safepts.
 void IdealLoopTree::allpaths_check_safepts(VectorSet &visited, Node_List &stack) {


@rwestrel you should update the description here. Suggestion:

Allpaths backwards scan. Starting at the head, traversing all backedges, and the body. Terminating each path at first safepoint encountered. Helper for check_safepts.

Also the line above is not accurate enough anymore:
_required_safept->push(n); // save the one closest to the tail

For one: could there not be multiple such SafePoints? If so: what does it mean to take "the closest"? And we may have multiple backedges / tails, now that we allow irreducible loops.

Other than that the fix looks reasonable to me, thanks for the fix @rwestrel !

rwestrel · 2023-06-29T07:39:01Z

@eme64 thanks for reviewing this (and making suggestions). Does the updated change look ok?

eme64

Thanks for the changes @rwestrel , looks good now!

rwestrel · 2023-06-29T07:43:14Z

@TobiHartmann @eme64 thanks for the reviews.

rwestrel · 2023-06-29T07:43:23Z

/integrate

openjdk · 2023-06-29T07:43:52Z

Going to push as commit 690d626.
Since your change was applied there have been 87 commits pushed to the master branch:

be64d3a: 8310299: C2: 8275201 broke constant folding of array store check in some cases
b2eae16: 8295191: IR framework timeout options expect ms instead of s
af319d9: 8311064: Windows builds fail without precompiled headers after JDK-8310728
cbf418a: 8311020: Typo cleanup in Classfile API
f4b900b: 8310902: (fc) FileChannel.transferXXX async close and interrupt issues
cf8d706: 8308463: Refactor regenerated class handling in lambdaFormInvokers.cpp
6f58ab2: 8301569: jmod list option and jimage list --help not interpreted correctly on turkish locale
8f5a384: 8311032: Empty value for java.protocol.handler.pkgs system property can lead to unnecessary classloading attempts of protocol handlers
ded1370: 8309811: BytecodePrinter cannot handle unlinked classes
02b17d7: 8310264: In PhaseChaitin::Split defs and phis are leaked
... and 77 more: https://git.openjdk.org/jdk/compare/7628da200833d0d26215a91afdd1bf7fc110da1d...master

Your commit was automatically rebased without conflicts.

openjdk · 2023-06-29T07:44:01Z

@rwestrel Pushed as commit 690d626.

💡 You may see a message that your pull request was closed with unmerged commits. This can be safely ignored.

test

3bb7212

openjdk bot added the rfr Pull request is ready for review label Jun 16, 2023

openjdk bot added the hotspot-compiler hotspot-compiler-dev@openjdk.org label Jun 16, 2023

TobiHartmann approved these changes Jun 23, 2023

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eme64 reviewed Jun 23, 2023

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rwestrel added 2 commits June 28, 2023 17:41

Merge branch 'master' into JDK-8307927

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eme64 approved these changes Jun 29, 2023

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8307927: C2: "malformed control flow" with irreducible loop #14522

8307927: C2: "malformed control flow" with irreducible loop #14522

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openjdk bot commented Jun 29, 2023

		@@ -3514,9 +3514,9 @@ bool IdealLoopTree::beautify_loops( PhaseIdealLoop *phase ) {
		// encountered. Helper for check_safepts.
		void IdealLoopTree::allpaths_check_safepts(VectorSet &visited, Node_List &stack) {

8307927: C2: "malformed control flow" with irreducible loop #14522

8307927: C2: "malformed control flow" with irreducible loop #14522

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rwestrel commented Jun 16, 2023 • edited by openjdk bot Loading

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