8369646: Detection of redundant conversion patterns in add_users_of_use_to_worklist is too restrictive #27900

benoitmaillard · 2025-10-20T14:08:59Z

This PR addresses a missed optimization opportunity in PhaseIterGVN. The missed optimization is the simplification of redundant conversion patterns of the shape ConvX2Y->ConvY2X->ConvX2Y.

This optimization pattern is implemented as an ideal optimization on ConvX2Y nodes. Because it depends on the input of the input of the node in question, we need to have an appropriate notification mechanism in PhaseIterGVN::add_users_of_use_to_worklist. The notification for this pattern was added in JDK-8359603.

However, that fix was based on the wrong assumption that in PhaseIterGVN::add_users_of_use_to_worklist, argument n is already the optimized node. However in some cases this argument is actually the node that is about to get replaced.

This happens for example in PhaseIterGVN::transform_old. If we find that node k returned by Ideal actually already exists by calling hash_find_insert(k), we call add_users_to_worklist(k).
As we replace node k with i, and i as a different opcode than k, then we cannot use the opcode of k to detect the redundant conversion pattern.

...
  // Global Value Numbering
  i = hash_find_insert(k);      // Check for pre-existing node
  if (i && (i != k)) {
    // Return the pre-existing node if it isn't dead
    NOT_PRODUCT(set_progress();)
    add_users_to_worklist(k);
    subsume_node(k, i);       // Everybody using k now uses i
    return i;
  }
...

The bug was quite intermittent and only showed up in some cases with -XX:+StressIGVN.

Proposed Fix

We make the detection of the pattern less specific by only looking at the opcode of the user of n, and not directly the opcode of n. This is consistent with the detection of other patterns in PhaseIterGVN::add_users_of_use_to_worklist.

Testing

GitHub Actions
tier1-3, plus some internal testing
Added a second run for the existing test with -XX:+StressIGVN and a fixed stress seed

Thank you for reviewing!

Progress

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Commit message must refer to an issue

Issue

JDK-8369646: Detection of redundant conversion patterns in add_users_of_use_to_worklist is too restrictive (Bug - P4)

Reviewers

Christian Hagedorn (@chhagedorn - Reviewer)
Emanuel Peter (@eme64 - Reviewer) Review applies to 16842d01

Reviewing

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$ git fetch https://git.openjdk.org/jdk.git pull/27900/head:pull/27900
$ git checkout pull/27900

Update a local copy of the PR:
$ git checkout pull/27900
$ git pull https://git.openjdk.org/jdk.git pull/27900/head

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Checkout this PR locally:
$ git pr checkout 27900

View PR using the GUI difftool:
$ git pr show -t 27900

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https://git.openjdk.org/jdk/pull/27900.diff

Using Webrev

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…orklist

bridgekeeper · 2025-10-20T14:10:18Z

👋 Welcome back bmaillard! 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 · 2025-10-20T14:11:13Z

@benoitmaillard 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:

8369646: Detection of redundant conversion patterns in add_users_of_use_to_worklist is too restrictive

Reviewed-by: chagedorn, 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 386 new commits pushed to the master branch:

2c3c470: 8354548: Update CLDR to Version 48.0
9bc2360: 8371364: Refactor javax/swing/JFileChooser/FileSizeCheck.java to use Util.findComponent()
a90fc26: 8371421: [AIX] new test ProcessCloseTest fails
... and 383 more: https://git.openjdk.org/jdk/compare/9d11787799718a82087f3c7b0c70abe7dfcead72...master

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openjdk · 2025-10-20T14:12:02Z

@benoitmaillard 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 · 2025-10-20T14:23:40Z

Webrevs

chhagedorn

Otherwise, looks good, thanks!

chhagedorn · 2025-10-27T08:08:41Z

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

 * @run main/othervm -XX:+IgnoreUnrecognizedVMOptions
 *      -XX:CompileCommand=compileonly,compiler.c2.TestEliminateRedundantConversionSequences::test*
- *      -XX:-TieredCompilation -Xbatch -XX:VerifyIterativeGVN=1110 compiler.c2.TestEliminateRedundantConversionSequences
+ *      -XX:-TieredCompilation -Xbatch -XX:VerifyIterativeGVN=1110


You could either add a separate run with -XX:+StressIGVN without a fixed seed or just add -XX:+StressIGVN here. I guess the latter is good enough.

I agree, and I just added -XX:+StressIGVN to the existing run.

Looks good, thanks!

chhagedorn · 2025-10-27T10:32:41Z

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

 * @run main/othervm -XX:+IgnoreUnrecognizedVMOptions
 *      -XX:CompileCommand=compileonly,compiler.c2.TestEliminateRedundantConversionSequences::test*
- *      -XX:-TieredCompilation -Xbatch -XX:VerifyIterativeGVN=1110 compiler.c2.TestEliminateRedundantConversionSequences
+ *      -XX:-TieredCompilation -Xbatch -XX:VerifyIterativeGVN=1110


Looks good, thanks!

eme64

Looks good to me :)

eme64 · 2025-10-30T12:23:18Z

src/hotspot/share/opto/phaseX.cpp

  // ConvF2L->ConvL2F->ConvF2L
  // ConvI2F->ConvF2I->ConvI2F
  // Note: there may be other 3-nodes conversion chains that would require to be added here, but these
  // are the only ones that are known to trigger missed optimizations otherwise


You may want to update the description, and give a bit of extra information. Because you are saying n does not have to be a conversion, but it may be that n is about to be replaced with a conversion, right?

Yes, in some cases add_users_of_use_to_worklist is called with the node about to be replaced as argument n. The point is that we might replace n with a node that already has other uses, and we only want to notify the uses for which there is a potential change.
But this is in no way specific to this one optimization, so I think adding something here would cause more confusion than anything else. Perhaps we should update the description of add_users_of_use_to_worklist then?

Right, this is not specific to this optimization here. Why not add something at the level of add_users_of_use_to_worklist.

Sorry for the delay, I ended up adding comments to the definition of both add_users_to_worklist and add_users_of_use_to_worklist. I think this might help avoid some confusion in the future.

…e_to_worklist

chhagedorn · 2025-11-07T07:48:34Z

src/hotspot/share/opto/phaseX.hpp

+  // known optimization could be applied to those users. Certain
+  // optimizations have dependencies that extend beyond a node's direct
+  // inputs, so it is necessary to ensure the appropriate notifications
+  // are made here.


Maybe also add what 'n' is. Could it be named 'parent'?

I would keep the name n for consistency, parent sounds a bit confusing from the point of view of the caller imho (as it is the node we are modifying in the first place). But I described more explicitly what n is.

chhagedorn · 2025-11-07T07:51:44Z

src/hotspot/share/opto/phaseX.hpp

+  // Add users of 'n', and any other nodes that could be directly
+  // affected by changes to 'n', to the worklist.
+  // This function may be called with a node that is about to be
+  // replaced as argument 'n'. In this case, 'n' should not be considered


By another node?

Suggested change

// replaced as argument 'n'. In this case, 'n' should not be considered

// replaced by another node. In this case, 'n' should not be considered

That's not what I meant, but yes it was confusing. I have changed the comment a bit.

chhagedorn

Looks good, thanks for the updated comments!

benoitmaillard · 2025-11-10T08:38:48Z

Thank you for the reviews @chhagedorn @eme64!
/integrate

openjdk · 2025-11-10T08:39:22Z

Going to push as commit 5e8bf7a.
Since your change was applied there have been 396 commits pushed to the master branch:

79fee60: 8371473: Problem list TestEmergencyDumpAtOOM.java on ppc64 platforms related to JDK-8371014
d570765: 8367149: Add convenient construction for creating ad-hoc VMErrorCallback
f77a511: 8350468: x86: Improve implementation of vectorized numberOfLeadingZeros for int and long
... and 393 more: https://git.openjdk.org/jdk/compare/9d11787799718a82087f3c7b0c70abe7dfcead72...master

Your commit was automatically rebased without conflicts.

openjdk · 2025-11-10T08:39:29Z

@benoitmaillard Pushed as commit 5e8bf7a.

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

benoitmaillard added 2 commits October 20, 2025 15:51

8369646: Add second test run with -XX:+StressIGVN and stress seed

c63e308

8369646: Fix excissively restrictive pattern in add_users_of_use_to_w…

402def2

…orklist

openjdk bot changed the title ~~8369646~~ 8369646: Test compiler/c2/TestEliminateRedundantConversionSequences.java fails with "Missed optimization opportunity in PhaseIterGVN" Oct 20, 2025

openjdk bot added the hotspot-compiler hotspot-compiler-dev@openjdk.org label Oct 20, 2025

benoitmaillard changed the title ~~8369646: Test compiler/c2/TestEliminateRedundantConversionSequences.java fails with "Missed optimization opportunity in PhaseIterGVN"~~ 8369646: Detection of redundant conversion patterns in add_users_of_use_to_worklist is too restrictive Oct 20, 2025

benoitmaillard marked this pull request as ready for review October 20, 2025 14:18

openjdk bot added the rfr Pull request is ready for review label Oct 20, 2025

benoitmaillard added 2 commits October 20, 2025 16:37

Add new issue name to jtreg headers

86ed466

Missing -XX:+UnlockDiagnosticVMOptions

1270663

chhagedorn approved these changes Oct 27, 2025

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Add -XX:+StressIGVN to run without fixed seed

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eme64 approved these changes Oct 30, 2025

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Add comments to clarify how add_users_to_worklist and add_users_of_us…

103cc58

…e_to_worklist

openjdk bot removed the ready Pull request is ready to be integrated label Nov 6, 2025

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eme64 mentioned this pull request Nov 10, 2025

8371528: C2: disable VerifyIterativeGVN Identity verification for ConvF2L etc #28212

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	// replaced as argument 'n'. In this case, 'n' should not be considered
	// replaced by another node. In this case, 'n' should not be considered

8369646: Detection of redundant conversion patterns in add_users_of_use_to_worklist is too restrictive #27900

8369646: Detection of redundant conversion patterns in add_users_of_use_to_worklist is too restrictive #27900

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