8341127: Extra call to MethodHandle::asType from memory segment var handles fails to inline #21283

mcimadamore · 2024-10-01T10:08:54Z

The fix for JDK-8331865 introduced an accidental performance regression.
The main issue is that now all memory segment var handles go through some round of adaptation.
Adapting a var handle results in a so called indirect var handle.
When an indirect var handle is called through a var handle guard, an extra MethodHandle::asType call is triggered.
In some cases, if asType has already been compiled into a big method, it cannot be inlined into the caller, which then results in a failure to inline through the var handle call, resulting in a big performance regression.

The solution is to make sure that the compiled size of MethodHandle::asType stays small: this is done by making sure that the slowpath (the one which populates the cache used by asType) is not inlined by the JVM. This is done by consolidating the slow path into a separate method, which is annotated with the internal @DontInline annotation.

This problem was originally reported here:
https://mail.openjdk.org/pipermail/panama-dev/2024-September/020643.html

While we did not test this fix directly, we have made sure that running the problematic benchmark with the flags:

-XX:CompileCommand=dontinline,java/lang/invoke/MethodHandle.setAsTypeCache
-XX:CompileCommand=dontinline,java/lang/invoke/MethodHandle.asTypeUncached

Solves the performance regression. The fix in this PR is just a programmatic way to achieve the same results w/o the need of command line flags.

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Issue

JDK-8341127: Extra call to MethodHandle::asType from memory segment var handles fails to inline (Enhancement - P2)

Reviewers

Claes Redestad (@cl4es - Reviewer) Review applies to f644d52d
Jorn Vernee (@JornVernee - Reviewer) Review applies to 964a273f
Paul Sandoz (@PaulSandoz - Reviewer)

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bridgekeeper · 2024-10-01T10:09:54Z

👋 Welcome back mcimadamore! 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.

mcimadamore · 2024-10-01T10:10:04Z

test/micro/org/openjdk/bench/java/lang/foreign/LoopOverNonConstantAsType.java

+
+    static {
+        types = new Class<?>[TYPE_SIZE];
+        ClassLoader customLoader = new URLClassLoader(new URL[0], LoopOverNonConstantAsType.class.getClassLoader());


Technically, using a different class loadrer is not required - but using classes with different class loaders can end up taking even longer code paths when calling MethodHandle::asType, so I've added that here.

openjdk · 2024-10-01T10:10:21Z

@mcimadamore 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:

8341127: Extra call to MethodHandle::asType from memory segment var handles fails to inline

Reviewed-by: psandoz, redestad, jvernee

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 278 new commits pushed to the master branch:

bdfb41f: 8309841: Jarsigner should print a warning if an entry is removed
57c1db5: 8332697: ubsan: shenandoahSimpleBitMap.inline.hpp:68:23: runtime error: signed integer overflow: -9223372036854775808 - 1 cannot be represented in type 'long int'
dc0ce1b: 8341336: Fix -Wzero-as-null-pointer-constant warnings in PRODUCT-only code
c43202b: 8341037: Use standard layouts in DefaultFrameIconTest.java and MenuCrash.java
76283dd: 8341246: Test com/sun/tools/attach/PermissionTest.java fails access denied after JDK-8327114
0bdfe88: 8328313: Archived module graph should allow identical --module-path to be specified during dump time and run time
9fc1c68: 8339850: Restore the interrupt status in FileSystemPreferences.lockFile()
5063494: 8340785: Update description of PassFailJFrame and samples
85f0442: 8317116: Provide layouts for multiple test UI in PassFailJFrame
49501fe: 8341412: Various test failures after JDK-8334305
... and 268 more: https://git.openjdk.org/jdk/compare/0c36177fead8b64a4cee9da3c895e3799f8ba231...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.

➡️ To integrate this PR with the above commit message to the master branch, type /integrate in a new comment.

mcimadamore · 2024-10-01T10:10:58Z

test/micro/org/openjdk/bench/java/lang/foreign/LoopOverNonConstantAsType.java

+@Measurement(iterations = 10, time = 500, timeUnit = TimeUnit.MILLISECONDS)
+@State(org.openjdk.jmh.annotations.Scope.Thread)
+@OutputTimeUnit(TimeUnit.MILLISECONDS)
+@Fork(value = 3, jvmArgsAppend = { "-XX:-TieredCompilation" })


The benchmark only reproduces if tiered compilation is disabled. This is due to the fact that some threshold are updated accordingly, and set to a smaller value, which is enough to exhibit the issue.

openjdk · 2024-10-01T10:11:08Z

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mlbridge · 2024-10-01T10:14:08Z

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mcimadamore · 2024-10-01T10:24:19Z

Benchmark results

Before:

Benchmark                               (asTypeCompiled)  Mode  Cnt  Score   Error  Units
LoopOverNonConstantAsType.segment_loop             false  avgt   30  0.232 ± 0.003  ms/op
LoopOverNonConstantAsType.segment_loop              true  avgt   30  6.943 ± 0.100  ms/op
LoopOverNonConstantAsType.unsafe_loop              false  avgt   30  0.223 ± 0.002  ms/op
LoopOverNonConstantAsType.unsafe_loop               true  avgt   30  0.227 ± 0.002  ms/op

After:

Benchmark                               (asTypeCompiled)  Mode  Cnt  Score   Error  Units
LoopOverNonConstantAsType.segment_loop             false  avgt   30  0.237 ± 0.002  ms/op
LoopOverNonConstantAsType.segment_loop              true  avgt   30  0.233 ± 0.003  ms/op
LoopOverNonConstantAsType.unsafe_loop              false  avgt   30  0.233 ± 0.002  ms/op
LoopOverNonConstantAsType.unsafe_loop               true  avgt   30  0.234 ± 0.002  ms/op

mcimadamore · 2024-10-01T10:27:06Z

Alternatives considered:

don't inline slow path (implemented in this PR)
add @ForceInline to MethodHandle::asType
add extra check on var handle guards to skip redundant asType calls

Of these, (1) was preferred as it fixes the underlying issue (and not just for memory segment var handles), while at the same time avoiding unwanted side-effects (e.g. adding @ForceInline can often result in too much code getting compiled, which can then create issues with other callee-side thresholds).

cl4es

LGTM

In the micro perhaps the unsafe_loop variant is superfluous for this special case test? At least it's distinct from the test in LoopOverNonConstant by summing longs rather than ints.

mcimadamore · 2024-10-01T11:01:58Z

LGTM

In the micro perhaps the unsafe_loop variant is superfluous for this special case test? At least it's distinct from the test in LoopOverNonConstant by summing longs rather than ints.

Yeah - that is a bit redundant - however could be useful to have some sort of baseline, so I left it in there.

JornVernee · 2024-10-01T14:07:57Z

src/java.base/share/classes/java/lang/invoke/MethodHandle.java

@@ -885,7 +886,9 @@ private MethodHandle asTypeCached(MethodType newType) {
        return null;
    }

-    private MethodHandle setAsTypeCache(MethodHandle at) {
+    @DontInline


Could you leave a comment explaining why we put DontInline here?

JornVernee · 2024-10-01T14:10:04Z

test/micro/org/openjdk/bench/java/lang/foreign/LoopOverNonConstantAsType.java

+        }
+    }
+
+    @CompilerControl(CompilerControl.Mode.DONT_INLINE)


I think the intent was to block inlining of asType, so it gets compiled in isolation? That should be done with a CompileCommand though. This annotation just blocks inlining of compileAsType AFAIK.

I see what you mean... I'll do some experiments and see how the current annotation affects the benchmark - if at all.

The issue with using a compile command is that it will then work globally. So the benchmark would not really test much - besides checking that var handle access is slow when asType cannot inline. What I was trying to do here was to avoid asType being inlined into that compileAsType method - but I agree that the annotation I added isn't it.

I believe EXCLUDE is what I was reaching out for

I think EXCLUDE would work, yes. True about CompileCommand having a global effect, so that won't work. (With a CompilerDirective file we could be more precise, not sure if it's worth it though).

In the latest iteration, I removed the annotation. Doesn't seem to be making any difference.

test/micro/org/openjdk/bench/java/lang/foreign/LoopOverNonConstantAsType.java

…tantAsType.java Co-authored-by: Jorn Vernee <JornVernee@users.noreply.github.com>

iwanowww · 2024-10-01T22:51:01Z

I don't have a clear understanding how VarHandleGuards is used, but I assume that MethodHandle::asType calls there are no-ops in scenarios you care about.

It does make sense to annotatesetAsTypeCache with @DontInline, but I'd also add @ForceInline on asType.

mcimadamore · 2024-10-02T13:56:23Z

I don't have a clear understanding how VarHandleGuards is used, but I assume that MethodHandle::asType calls there are no-ops in scenarios you care about.

A var handle guard is a piece of code that detects var handle call of a certain shape, to avoid creation of lambda forms in some common cases. This was done in Java 9, and was unchanged by FFM. In this case, we just happen to hit the var handle guard for (LJ -> J). And yes, the asType adaptation is a no-op here.

The difference is that in the normal code path (lambda form), the call to asType is guarded by a check:

https://github.com/openjdk/jdk/blob/master/src/java.base/share/classes/java/lang/invoke/Invokers.java#L513

Normally, var handle guards don't need an asType adaptation:

https://github.com/openjdk/jdk/blob/master/src/java.base/share/classes/java/lang/invoke/VarHandleGuards.java#L230

But if a var handle guard is triggered on an "indirect" var handle (read: a var handle that has been adapted with some combinator - this part is new since FFM), then we go through a different path in the guard code which always calls asType:

https://github.com/openjdk/jdk/blob/master/src/java.base/share/classes/java/lang/invoke/VarHandleGuards.java#L233

(note: this "slow path" inside var handle guards predates FFM, but var handle adaptation has increased the number of cases in which we hit this slow path).

The PR that introduced this regression made all FFM var handles undergo some adaptation. Which is why now we're seeing this issue - as we're now hitting a (no-op) asType call which was never made before.

mcimadamore · 2024-10-04T09:44:04Z

/integrate

openjdk · 2024-10-04T09:45:51Z

Going to push as commit 7fa2f22.
Since your change was applied there have been 306 commits pushed to the master branch:

1bdd79e: 8341261: Tests assume UnlockExperimentalVMOptions is disabled by default
ec020f3: 8340426: ZGC: Move defragment out of the allocation path
a63ac5a: 8340792: -XX:+PrintInterpreter: instructions should only be printed if printing all InterpreterCodelets
3f420fa: 8341451: Remove C2HandleAnonOMOwnerStub
d3139b4: 8341000: Open source some of the AWT Window tests
4ded283: 8338136: Hotspot should support multiple large page sizes on Windows
10402b4: 8341489: ProblemList runtime/cds/appcds/DumpRuntimeClassesTest.java in Xcomp mode
6bc3971: 8341316: [macos] javax/swing/ProgressMonitor/ProgressMonitorEscapeKeyPress.java fails sometimes in macos
e89fd1d: 8341128: open source some 2d graphics tests
6f459af: 8340077: Open source few Checkbox tests - Set2
... and 296 more: https://git.openjdk.org/jdk/compare/0c36177fead8b64a4cee9da3c895e3799f8ba231...master

Your commit was automatically rebased without conflicts.

openjdk · 2024-10-04T09:46:03Z

@mcimadamore Pushed as commit 7fa2f22.

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

mcimadamore · 2024-10-04T09:47:05Z

/backport :jdk23

openjdk · 2024-10-04T09:48:45Z

@mcimadamore the backport was successfully created on the branch backport-mcimadamore-7fa2f229-jdk23 in my personal fork of openjdk/jdk. To create a pull request with this backport targeting openjdk/jdk:jdk23, just click the following link:

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This pull request contains a backport of commit 7fa2f229 from the openjdk/jdk repository.

The commit being backported was authored by Maurizio Cimadamore on 4 Oct 2024 and was reviewed by Paul Sandoz, Claes Redestad and Jorn Vernee.

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mcimadamore · 2024-10-04T10:39:38Z

/backport jdk23u

openjdk · 2024-10-04T10:41:40Z

@mcimadamore the backport was successfully created on the branch backport-mcimadamore-7fa2f229-master in my personal fork of openjdk/jdk23u. To create a pull request with this backport targeting openjdk/jdk23u:master, just click the following link:

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This pull request contains a backport of commit 7fa2f229 from the openjdk/jdk repository.

The commit being backported was authored by Maurizio Cimadamore on 4 Oct 2024 and was reviewed by Paul Sandoz, Claes Redestad and Jorn Vernee.

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mcimadamore added 5 commits September 27, 2024 22:05

Add benchmark

08cd53c

Disable tiered compilation

12809eb

Use different classloaders

2cc2045

Fix bench

c9c2f19

DontInline slow path in MethodHandle::asType

8c2f978

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Update test/micro/org/openjdk/bench/java/lang/foreign/LoopOverNonCons…

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…tantAsType.java Co-authored-by: Jorn Vernee <JornVernee@users.noreply.github.com>

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Add @ForceInline to MethodHandle::asType, as per review comment

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8341127: Extra call to MethodHandle::asType from memory segment var handles fails to inline #21283

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8341127: Extra call to MethodHandle::asType from memory segment var handles fails to inline #21283

8341127: Extra call to MethodHandle::asType from memory segment var handles fails to inline #21283

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