8357619: [JVMCI] Revisit phantom_ref parameter in JVMCINMethodData::get_nmethod_mirror #25488

dougxc · 2025-05-28T10:28:38Z

The point of the phantom_ref parameter (introduced by JDK-8234359) of JVMCINMethodData::get_nmethod_mirror is to avoid the special resurrection semantics of a phantom read when reading the field during GC, which is when JVMCINMethodData::invalidate_nmethod_mirror can be called.
This case can be handled directly in JVMCINMethodData::invalidate_nmethod_mirror and so the phantom_ref parameter can be removed.

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JDK-8357619: [JVMCI] Revisit phantom_ref parameter in JVMCINMethodData::get_nmethod_mirror (Enhancement - P4)

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Erik Österlund (@fisk - Reviewer)
Tom Rodriguez (@tkrodriguez - Reviewer) Review applies to 7456988a

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bridgekeeper · 2025-05-28T10:29:12Z

👋 Welcome back dnsimon! 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 · 2025-05-28T10:29:23Z

@dougxc 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:

8357619: [JVMCI] Revisit phantom_ref parameter in JVMCINMethodData::get_nmethod_mirror

Reviewed-by: eosterlund, never

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At the time when this comment was updated there had been 103 new commits pushed to the master branch:

bce2bd2: 8356308: Assert with -Xlog:class+path when classpath has an empty element
8b6a11f: 8352187: Don't start management agent during AOT cache creation
ab5de45: 8357991: make bootcycle-images is broken after JDK-8349665
... and 100 more: https://git.openjdk.org/jdk/compare/39714b603040f1619f5e0e2a13ea8a90bb993c27...master

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openjdk · 2025-05-28T10:29:51Z

@dougxc The following labels will be automatically applied to this pull request:

graal
hotspot-compiler

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fisk · 2025-05-28T17:15:48Z

src/hotspot/share/jvmci/jvmciCompilerToVM.cpp

          // Only the mirror in the HotSpot heap is accessible
          // through JVMCINMethodData
-          oop nmethod_mirror = data->get_nmethod_mirror(nm, /* phantom_ref */ true);
+          oop nmethod_mirror = data->get_nmethod_mirror(nm);


Is the nmethod guaranteed to be on-stack here? If not it gotta be phantom.

Is the use of JVMCINMethodHandle equivalent to nm being on-stack?

Yes. Great, so that should be fine then.

mlbridge · 2025-05-29T13:04:40Z

Webrevs

fisk

Looks good.

tkrodriguez · 2025-05-30T16:05:23Z

src/hotspot/share/jvmci/jvmciRuntime.cpp

 void JVMCINMethodData::invalidate_nmethod_mirror(nmethod* nm) {
-  oop nmethod_mirror = get_nmethod_mirror(nm, /* phantom_ref */ false);
-  if (nmethod_mirror == nullptr) {
+  if (_nmethod_mirror_index == -1) {


This part is actually wrong as that's the first part of get_nmethod_mirror and we must always check that get_nmethod_mirror doesn't return nullptr. I'd assumed that the mirror was always non-null if _nmethod_mirror_index != -1 but that's not true. The slot is reserved for all non-default nmethods and must stay around so that translate can work.

Fixed: 7456988

tkrodriguez · 2025-06-02T16:37:42Z

src/hotspot/share/jvmci/jvmciRuntime.cpp

-  oop nmethod_mirror = get_nmethod_mirror(nm, /* phantom_ref */ false);
+  oop nmethod_mirror = get_nmethod_mirror(nm);
  if (nmethod_mirror == nullptr) {
-    return;


dougxc · 2025-06-03T06:19:08Z

Thanks for the reviews.

/integrate

openjdk · 2025-06-03T06:20:30Z

Going to push as commit 6cfd405.
Since your change was applied there have been 114 commits pushed to the master branch:

497a182: 8358254: [AOT] runtime/cds/appcds/applications/JavacBench.java#aot crashes with SEGV in ClassLoaderData::holder
c5f235c: 8347826: Introspector shows wrong method list after 8071693
832c5b0: 8350880: (zipfs) Add support for read-only zip file systems
... and 111 more: https://git.openjdk.org/jdk/compare/39714b603040f1619f5e0e2a13ea8a90bb993c27...master

Your commit was automatically rebased without conflicts.

openjdk · 2025-06-03T06:20:37Z

@dougxc Pushed as commit 6cfd405.

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

remove phantom_ref arg from JVMCINMethodData::get_nmethod_mirror

0dcb1c7

openjdk bot added graal graal-dev@openjdk.org hotspot-compiler hotspot-compiler-dev@openjdk.org labels May 28, 2025

dougxc changed the title ~~8357619: [JVMCI] Improve documentation of phantom_ref parameter in JVMCINMethodData::get_nmethod_mirror~~ 8357619: [JVMCI] Clarify phantom_ref parameter in JVMCINMethodData::get_nmethod_mirror May 28, 2025

dougxc changed the title ~~8357619: [JVMCI] Clarify phantom_ref parameter in JVMCINMethodData::get_nmethod_mirror~~ 8357619: [JVMCI] Revisit phantom_ref parameter in JVMCINMethodData::get_nmethod_mirror May 28, 2025

fisk reviewed May 28, 2025

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dougxc marked this pull request as ready for review May 29, 2025 13:00

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tkrodriguez reviewed May 30, 2025

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still rely on get_nmethod_mirror in invalidate_nmethod_mirror

7456988

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tkrodriguez reviewed Jun 2, 2025

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fixed typo

e02be82

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openjdk bot closed this Jun 3, 2025

openjdk bot removed ready Pull request is ready to be integrated rfr Pull request is ready for review labels Jun 3, 2025

8357619: [JVMCI] Revisit phantom_ref parameter in JVMCINMethodData::get_nmethod_mirror #25488

8357619: [JVMCI] Revisit phantom_ref parameter in JVMCINMethodData::get_nmethod_mirror #25488

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