8369045: [lworld] valhalla/valuetypes/WeakReferenceTest.java has an unschedulable graph #1768

dafedafe · 2025-12-02T10:36:04Z

Issue

A few test failed intermittently with -Xcomp on Mac due to an unschedulable graph in C2:

valhalla/valuetypes/WeakReferenceTest.java
valhalla/valuetypes/ProxyTest.java
valhalla/valuetypes/ObjectNewInstance.java
valhalla/valuetypes/ObjectMethodsViaCondy.java

Causes

The origin of the issue seems to be coming from strength reduction (process_late_inline_calls_no_inline) where we replace virtual and MH calls with direct calls.
https://github.com/dafedafe/valhalla/blob/75e2dd95df5d847d7d6e35a23016d22705681cf4/src/hotspot/share/opto/compile.cpp#L3072
If the return type of the methods are not loaded, we add a call to runtime's store_inline_type_fields_to_buf right after the actual method call, to save the scalarized return into a oop. This happens first for the caller at parse time and then for the callee when strength-reducing the virtual call to a direct one. The return projections of the inline fields of the call are added to store_inline_type_fields_to_buf and its oop projection is then added as input to the other store_inline_type_fields_to_buf which fundamentally leaves the graph around it in an awkward state.

If this happens in combination with loop unswitching it can lead to a graph not being schedulable, which is what happens in the failure of this issue, where we have:

a virtual call with a following store_inline_type_fields_to_buf (1) in a loop.
the loop undergoes unswitching, which creates 2 copies of the body (including a copy of the virtual call). All outputs of the new virtual call are phi-d with the one of the other path as input of the store_inline_type_fields_to_buf.
the new copy of the virtual call is later replaced with a direct call: the creation of the new direct call adds a store_inline_type_fields_to_buf (2) right after the direct call. All the inline type return values of the call being inlined are removed, so the phis now only have one input and are removed by a later GVN pass.

this creates an issue later during GCM since the store_inline_type_fields_to_buf (1) call is logically not dominated by any of the 2 sides of the unswitched loop and lands in a separate dominance path of the arguments whose phis have been removed (which are still dominated by the original virtual call).

Solution

The issue happens only when strength-reducing to a direct call and the return type is not loaded. So, there seems to be 2 conceivable fixes: either we avoid strength-reduction if the type is not loaded or we avoid creating the second store_inline_type_fields_to_buf and rewire the output projections of the strength-reduced call to the output of the projections of the original virtual call.
By choosing the first solution we potentially skip a useful optimization (in particular if the return value is always null, which won't trigger the deoptimization). So, the rewiring solution seems to be the best option in this case.

Testing

repeated testing with failing tests (same conditions)
replay of failing compilations
Tier 1-3

Unfortunately creating a regression test that would consistently trigger the specific issue with unloaded return type and strength-reduction to direct call has proven to be unviable.

Progress

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

Error

⚠️ 8369045 is used in problem lists: [test/jdk/ProblemList-enable-preview.txt]

Issue

JDK-8369045: [lworld] valhalla/valuetypes/WeakReferenceTest.java has an unschedulable graph (Bug - P4)

Reviewers

Tobias Hartmann (@TobiHartmann - Committer)

Reviewing

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$ git fetch https://git.openjdk.org/valhalla.git pull/1768/head:pull/1768
$ git checkout pull/1768

Update a local copy of the PR:
$ git checkout pull/1768
$ git pull https://git.openjdk.org/valhalla.git pull/1768/head

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$ git pr checkout 1768

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https://git.openjdk.org/valhalla/pull/1768.diff

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…an unschedulable graph

bridgekeeper · 2025-12-02T10:37:24Z

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

openjdk · 2025-12-02T10:38:56Z

❗ This change is not yet ready to be integrated.
See the Progress checklist in the description for automated requirements.

This reverts commit 696695f.

mlbridge · 2025-12-10T08:31:09Z

Webrevs

TobiHartmann · 2025-12-11T16:01:00Z

src/hotspot/share/opto/graphKit.cpp

-            store_to_buf_call->init_req(i, top());
-            continue;
+        // Don't add store to buffer call if we are strength reducing
+        if (!C->strength_reduction()) {


Can we use _gvn.is_IterGVN() && !C->inlining_incrementally() here instead? Assuming that whenever we call this after parsing and when not incrementally inlining, we are doing post-parse devirtualization of a call.

That's a possibility. I've also considered checking for !C->inlining_incrementally() && C->late_inline_count() > 0 but I'm a bit torn between being 100% sure we are doing strength reduction but using a compile "flags" (a bit 🫤) and checking for other, already available, conditions (like the one you suggest), which make me a bit unsure and could potentially change in the future...
But I guess introducing a new field to Compile just for this should be avoided. So, let's go for _gvn.is_IterGVN() && !C->inlining_incrementally() 🙂

I spoke too soon: _gvn is actually set when the GraphKit is created (when generating the call) and is not using the context of the call being strength-reduced. So, unfortunately it seems that the least-invasive way to figure out if we are strength-reducing is to have a compile flag. I reverted the change.

src/hotspot/share/opto/graphKit.cpp

TobiHartmann · 2025-12-16T08:51:59Z

src/hotspot/share/opto/graphKit.cpp

-          if (domain->field_at(i) == Type::HALF) {
-            store_to_buf_call->init_req(i, top());
-            continue;
+        // Don't add store to buffer call if we are strength reducing


Why do we need to add the above runtime check of the result again when doing strength reduction?

We actually don't 🙂
I moved it inside the not-strength-reducing block.

TobiHartmann · 2025-12-17T06:42:09Z

You also need to remove the tests from the problem list again (see #1793).

Co-authored-by: Tobias Hartmann <tobias.hartmann@oracle.com>

This reverts commit ccf5812.

…to JDK-8369045

openjdk · 2025-12-17T13:37:46Z

⚠️ @dafedafe This pull request contains merges that bring in commits not present in the target repository. Since this is not a "merge style" pull request, these changes will be squashed when this pull request in integrated. If this is your intention, then please ignore this message. If you want to preserve the commit structure, you must change the title of this pull request to Merge <project>:<branch> where <project> is the name of another project in the OpenJDK organization (for example Merge jdk:master).

openjdk · 2025-12-17T14:58:54Z

@dafedafe this pull request can not be integrated into lworld due to one or more merge conflicts. To resolve these merge conflicts and update this pull request you can run the following commands in the local repository for your personal fork:

git checkout JDK-8369045
git fetch https://git.openjdk.org/valhalla.git lworld
git merge FETCH_HEAD
# resolve conflicts and follow the instructions given by git merge
git commit -m "Merge lworld"
git push

dafedafe · 2025-12-17T15:08:46Z

You also need to remove the tests from the problem list again (see #1793).

Removed.

TobiHartmann

Looks good to me!

JDK-8369045: [lworld] valhalla/valuetypes/WeakReferenceTest.java has …

75e2dd9

…an unschedulable graph

openjdk bot changed the title ~~JDK-8369045: [lworld] valhalla/valuetypes/WeakReferenceTest.java has an unschedulable graph~~ 8369045: [lworld] valhalla/valuetypes/WeakReferenceTest.java has an unschedulable graph Dec 2, 2025

dafedafe added 5 commits December 4, 2025 14:59

JDK-8369045: replace result projections instead of no strength reduction

4616774

JDK-8369045: add bool for rt_loaded

696695f

JDK-8369045: add comment

92d1965

Revert "JDK-8369045: add bool for rt_loaded"

f59ecef

This reverts commit 696695f.

JDK-8369045: remove new line

76f2e96

dafedafe marked this pull request as ready for review December 10, 2025 08:23

openjdk bot added the rfr Pull request is ready for review label Dec 10, 2025

TobiHartmann reviewed Dec 11, 2025

View reviewed changes

JDK-8369045: remove strength reduction flag

ccf5812

TobiHartmann reviewed Dec 16, 2025

View reviewed changes

dafedafe and others added 4 commits December 17, 2025 10:10

Update src/hotspot/share/opto/graphKit.cpp

60aef93

Co-authored-by: Tobias Hartmann <tobias.hartmann@oracle.com>

Revert "JDK-8369045: remove strength reduction flag"

b384c0a

This reverts commit ccf5812.

JDK-8369045: don't re-add condition if stress-reducing

0685e19

Merge branch 'JDK-8369045' of https://github.com/dafedafe/valhalla in…

e1fe634

…to JDK-8369045

dafedafe added 3 commits December 17, 2025 14:48

Merge branch 'lworld' into JDK-8369045

10b8f79

JDK-8369045 remove tests from problemlist

32fae20

JDK-8369045 remove another test from problemlist

7790bef

openjdk bot added the merge-conflict Pull request has merge conflict with target branch label Dec 17, 2025

Merge branch 'lworld' into JDK-8369045

b2c399f

openjdk bot removed the merge-conflict Pull request has merge conflict with target branch label Dec 17, 2025

TobiHartmann approved these changes Dec 19, 2025

View reviewed changes

openjdk bot added the merge-conflict Pull request has merge conflict with target branch label Dec 19, 2025

8369045: [lworld] valhalla/valuetypes/WeakReferenceTest.java has an unschedulable graph #1768

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8369045: [lworld] valhalla/valuetypes/WeakReferenceTest.java has an unschedulable graph #1768

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