8361140: Missing OptimizePtrCompare check in ConnectionGraph::reduce_phi_on_cmp #26125

hgqxjj · 2025-07-04T02:49:27Z

When running with -XX:-OptimizePtrCompare (which disables pointer comparison optimization), the compiler may hit an assertion failure in debug builds because optimize_ptr_compare is still being called. This violates the intended usage of the flag and leads to unexpected crashes.

This patch adds an early return to reduce_phi_on_cmp when OptimizePtrCompare is false. Since the optimization relies on optimize_ptr_compare for static reasoning about comparisons, there's no benefit in proceeding with reduce_phi_on_cmp when this flag is disabled.

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Issue

JDK-8361140: Missing OptimizePtrCompare check in ConnectionGraph::reduce_phi_on_cmp (Bug - P4)

Reviewers

Christian Hagedorn (@chhagedorn - Reviewer)
Cesar Soares Lucas (@JohnTortugo - Committer) Review applies to fd6f90f5

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…phi_on_cmp When running with `-XX:-OptimizePtrCompare` (which disables pointer comparison optimization), the compiler may hit an assertion failure in debug builds because `optimize_ptr_compare` is still being called. This violates the intended usage of the flag and leads to unexpected crashes. This patch adds an early return to `reduce_phi_on_cmp` when `OptimizePtrCompare` is false. Since the optimization relies on `optimize_ptr_compare` for static reasoning about comparisons, there's no benefit in proceeding with `reduce_phi_on_cmp` when this support is disabled.

bridgekeeper · 2025-07-04T02:50:15Z

👋 Welcome back hgqxjj! 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 · 2025-07-04T02:51:03Z

@hgqxjj 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:

8361140: Missing OptimizePtrCompare check in ConnectionGraph::reduce_phi_on_cmp

Reviewed-by: chagedorn, cslucas

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 4 new commits pushed to the master branch:

3ffc5b9: 8359344: C2: Malformed control flow after intrinsic bailout
529049b: 8356137: GifImageDecode can produce opaque image when disposal method changes
eddfc64: 8361383: LogFileStreamOutput::write_decorations uses wrong type for format precisions
... and 1 more: https://git.openjdk.org/jdk/compare/ee0d309bbd33302d8c6f35155e975db77aaea785...master

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openjdk · 2025-07-04T02:51:37Z

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

hotspot-compiler

When this pull request is ready to be reviewed, an "RFR" email will be sent to the corresponding mailing list. If you would like to change these labels, use the /label pull request command.

mlbridge · 2025-07-04T02:55:27Z

Webrevs

chhagedorn · 2025-07-07T12:11:46Z

src/hotspot/share/opto/escape.cpp

+  if (!OptimizePtrCompare) {
+    return;
+  }


Thanks for working on this! IIUC, having the bailout here will fail to reduce the phi which could be unexpected. Shouldn't we just return UNKNOWN from within ConnectionGraph::optimize_ptr_compare() when we run without OptimizePtrCompare?

On a separate note, can you also add a regression test? Maybe you can also just add a run with -XX:-OptimizePtrCompare - maybe together with -XX:+VerifyReduceAllocationMerges for more verification - to compiler/c2/irTests/scalarReplacement/AllocationMergesTests.java.

@JohnTortugo you might also want to have a look at this.

Thanks a lot for your suggestion!
I took a closer look at the code, and I now fully agree that your approach is the better one. Returning UNKNOWN from optimize_ptr_compare() when OptimizePtrCompare is disabled makes the behavior more consistent and avoids skipping reduce_phi_on_cmp() entirely, which could lead to unexpected results or missed optimization opportunities. I appreciate your feedback and will move forward with this approach. Thanks again!

Thanks for the ping @chhagedorn and I fully agree with your comment. Actually, that's the correct way to do this.
Thank you for fixing this @hgqxjj .

hi @chhagedorn , I already update PR and add regression test. Please take another look when you have time .
Thanks a lot.

chhagedorn

Thanks for the update! I have some follow-up comments.

test/hotspot/jtreg/compiler/c2/TestReducePhiOnCmpWithNoOptPtrCompare.java

src/hotspot/share/opto/escape.cpp

hgqxjj · 2025-07-10T14:38:01Z

hi @chhagedorn ，thanks a lot for your detailed and thoughtful review. I'm still learning more about this area, and your feedback has been a great help. I've updated the PR based on your suggestions . Please feel free to let me know if anything else needs improvement.

test/hotspot/jtreg/compiler/c2/TestReducePhiOnCmpWithNoOptPtrCompare.java

chhagedorn

Looks good, thanks! I'll give this a spinning in our testing and report back again.

openjdk · 2025-07-11T07:23:20Z

⚠️ @hgqxjj the full name on your profile does not match the author name in this pull requests' HEAD commit. If this pull request gets integrated then the author name from this pull requests' HEAD commit will be used for the resulting commit. If you wish to push a new commit with a different author name, then please run the following commands in a local repository of your personal fork:

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hgqxjj · 2025-07-11T07:32:34Z

/integrate

openjdk · 2025-07-11T07:33:17Z

@hgqxjj
Your change (at version 0e9aa95) is now ready to be sponsored by a Committer.

hgqxjj · 2025-07-11T07:37:09Z

@chhagedorn @JohnTortugo Thanks again for all your feedback. It’s been very helpful! If possible, could you please run /sponsor？

chhagedorn · 2025-07-14T07:38:08Z

Testing was clean 👍

/sponsor

openjdk · 2025-07-14T07:39:04Z

Going to push as commit 14c79be.
Since your change was applied there have been 21 commits pushed to the master branch:

7c34bdf: 8362122: Problem list TestStressBailout until JDK-8361752 is fixed
0029554: 8360647: [XWayland] [OL10] NumPad keys are not triggered
77bd417: 8350177: C2 SuperWord: Integer.numberOfLeadingZeros, numberOfTrailingZeros, reverse and bitCount have input types wrongly truncated for byte and short
... and 18 more: https://git.openjdk.org/jdk/compare/ee0d309bbd33302d8c6f35155e975db77aaea785...master

Your commit was automatically rebased without conflicts.

openjdk · 2025-07-14T07:39:14Z

@chhagedorn @hgqxjj Pushed as commit 14c79be.

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

openjdk bot added the rfr Pull request is ready for review label Jul 4, 2025

openjdk bot added the hotspot-compiler hotspot-compiler-dev@openjdk.org label Jul 4, 2025

chhagedorn suggested changes Jul 7, 2025

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hgqxjj added 2 commits July 10, 2025 01:54

Merge remote-tracking branch 'upstream/master' into 8361140

787eb0c

update modification and add regression test

2feca6a

chhagedorn reviewed Jul 10, 2025

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hgqxjj added 2 commits July 10, 2025 14:16

Merge remote-tracking branch 'upstream/master' into 8361140

6dda1e9

update regression test

fd6f90f

JohnTortugo approved these changes Jul 10, 2025

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test/hotspot/jtreg/compiler/c2/TestReducePhiOnCmpWithNoOptPtrCompare.java Outdated Show resolved Hide resolved

hgqxjj added 2 commits July 11, 2025 00:29

Merge remote-tracking branch 'upstream/master' into 8361140

4ef7ed3

Remove the unused variable

0e9aa95

chhagedorn approved these changes Jul 11, 2025

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8361140: Missing OptimizePtrCompare check in ConnectionGraph::reduce_phi_on_cmp #26125

8361140: Missing OptimizePtrCompare check in ConnectionGraph::reduce_phi_on_cmp #26125

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