8315082: [REDO] Generational ZGC: Tests crash with assert(index == 0 || is_power_of_2(index)) #15589

robcasloz · 2023-09-06T11:54:04Z

This changeset (REDO of JDK-8312749) ensures that the array copy stub underlying the intrinsic implementation of Object.clone only copies its (double-word aligned) payload, excluding the remaining object alignment padding words, when a non-default ObjectAlignmentInBytes value is used. This prevents the specialized ZGC stubs for Object[] array copy from processing undefined object alignment padding words as valid object pointers. For further details about the specific failure, see initial analysis of JDK-8312749 by Erik Österlund and Stefan Karlsson and comments in test/hotspot/jtreg/compiler/gcbarriers/TestArrayCopyWithLargeObjectAlignment.java.

As a side-benefit, the changeset simplifies the array size computation logic in GraphKit::new_array() by decoupling computation of header size and alignment padding size.

Additional changes compared to JDK-8312749

This changeset proposes the exact same solution as JDK-8312749, that is, identical changes to barrierSetC2.cpp, graphKit.cpp, library_call.cpp, and TestArrayCopyWithLargeObjectAlignment.java. On top of that, it relaxes an assertion in the idealization of ArrayCopy nodes violated by JDK-8312749 and reported in JDK-8315029 (new changes in arraycopynode.cpp, new regression test TestCloneArrayWithDifferentLengthConstness.java). The original, stricter assertion checks that, while idealizing an ArrayCopy node, the "constness" of the array copy's word-length (whether it is known by C2 to be constant or not) is equivalent to that of the array copy's element-length. For cases in which the element-length is within a small, fixed range (e.g. for an int array of length 3..4) so that all element-length values lead to the same number of words (2), the assertion used to hold before this changeset only because of weak type propagation in AndL (preventing the constant word-length to be discovered), see the left graph below:

With the proposed changes, the array copy word-length is computed in a more straightforward way that enables C2 to infer the precise number of words in the same scenario (see the right graph above). To accommodate this optimization, this changeset relaxes the assertion to check only one direction of the implication (if the element-length is constant, so is the word-length). The optimization does not affect the remaining idealization code, since it only uses element-length in the context in which the optimization is applied.

The changeset includes an additional regression test (TestCloneArrayWithDifferentLengthConstness.java) that exercises different constant/variable combinations of element-length and word-length.

Testing

Functionality

tier1-5 (windows-x64, linux-x64, linux-aarch64, macosx-x64, and macosx-aarch64)
tier6-7 (linux-x64 only)
compiler-focused stress testing
failing tests reported in JDK-8315029
tier1-3, and a few custom examples, applying JDK-8139457 on top of this changeset

Performance

Tested performance on the following set of OpenJDK micro-benchmarks, on linux-x64 (for both G1 and ZGC, using different ObjectAlignmentInBytes values):

openjdk.bench.java.lang.ArrayClone.byteClone
openjdk.bench.java.lang.ArrayClone.intClone
openjdk.bench.java.lang.ArrayFiddle.simple_clone
openjdk.bench.java.lang.Clone.cloneLarge
openjdk.bench.java.lang.Clone.cloneThreeDifferent

No significant regression was observed.

Progress

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

Issue

JDK-8315082: [REDO] Generational ZGC: Tests crash with assert(index == 0 || is_power_of_2(index)) (Bug - P3)

Reviewers

Albert Mingkun Yang (@albertnetymk - Reviewer)
Tobias Hartmann (@TobiHartmann - Reviewer)
Vladimir Kozlov (@vnkozlov - Reviewer)

Contributors

Stefan Karlsson <stefank@openjdk.org>
Erik Österlund <eosterlund@openjdk.org>

Reviewing

Using git

Checkout this PR locally:
$ git fetch https://git.openjdk.org/jdk.git pull/15589/head:pull/15589
$ git checkout pull/15589

Update a local copy of the PR:
$ git checkout pull/15589
$ git pull https://git.openjdk.org/jdk.git pull/15589/head

Using Skara CLI tools

Checkout this PR locally:
$ git pr checkout 15589

View PR using the GUI difftool:
$ git pr show -t 15589

Using diff file

Download this PR as a diff file:
https://git.openjdk.org/jdk/pull/15589.diff

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bridgekeeper · 2023-09-06T11:55:50Z

👋 Welcome back rcastanedalo! 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 · 2023-09-06T11:57:49Z

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

hotspot

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.

robcasloz · 2023-09-07T07:36:08Z

/contributor add @stefank

robcasloz · 2023-09-07T07:36:17Z

/contributor add @fisk

robcasloz · 2023-09-07T07:36:31Z

/label add hotspot-compiler

robcasloz · 2023-09-07T07:36:40Z

/label add hotspot-gc

openjdk · 2023-09-07T07:38:39Z

@robcasloz
Contributor Stefan Karlsson <stefank@openjdk.org> successfully added.

openjdk · 2023-09-07T07:39:30Z

@robcasloz
Contributor Erik Österlund <eosterlund@openjdk.org> successfully added.

openjdk · 2023-09-07T07:40:10Z

@robcasloz
The hotspot-compiler label was successfully added.

openjdk · 2023-09-07T07:40:58Z

@robcasloz
The hotspot-gc label was successfully added.

…jAlignmentInBytes)

…ayCopy

This reverts commit 5b73ae4.

This reverts commit ddbc110.

This reverts commit eda6cf0.

mlbridge · 2023-09-11T07:08:03Z

Webrevs

00: Full (b3109e30)

robcasloz · 2023-09-11T07:11:30Z

@albertnetymk @vnkozlov @TobiHartmann could you please review this revised version of JDK-8312749? The core changes remain identical, the only additional changes are an assertion relaxation and a second regression test (see description for details). Thanks!

openjdk · 2023-09-11T11:33:24Z

@robcasloz 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:

8315082: [REDO] Generational ZGC: Tests crash with assert(index == 0 || is_power_of_2(index))

Co-authored-by: Stefan Karlsson <stefank@openjdk.org>
Co-authored-by: Erik Österlund <eosterlund@openjdk.org>
Reviewed-by: ayang, thartmann, kvn

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 205 new commits pushed to the master branch:

54028e7: 8316562: serviceability/sa/jmap-hprof/JMapHProfLargeHeapTest.java times out after JDK-8314829
7c991cc: 8296246: Update Unicode Data Files to Version 15.1.0
a021dbc: 8316149: Open source several Swing JTree JViewport KeyboardManager tests
455cfae: 8315880: change LockingMode default from LM_LEGACY to LM_LIGHTWEIGHT
9e00949: 8316427: Duplicated code for {obj,type}ArrayKlass::array_klass
c43ebd3: 8315981: Opensource five more random Swing tests
e30e356: 8316461: Fix: make test outputs TEST SUCCESS after unsuccessful exit
242eeae: 8286757: adlc tries to build with /pathmap but without /experimental:deterministic
b275bdd: 8308995: Update Network IO JFR events to be static mirror events
e1870d3: 8316411: compiler/compilercontrol/TestConflictInlineCommands.java fails intermittent with force inline by CompileCommand missing
... and 195 more: https://git.openjdk.org/jdk/compare/024133b089d911dcc3ea70dfdaa6b150b14a9eb4...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.

robcasloz · 2023-09-11T11:38:08Z

Thanks for reviewing, Albert!

TobiHartmann

Thanks for the detailed analysis and explanation. The fix looks good.

vnkozlov

Few comments.

vnkozlov · 2023-09-14T17:41:51Z

src/hotspot/share/gc/shared/c2/barrierSetC2.cpp

+  if (is_array) {
+    // Ensure the array payload size is rounded up to the next BytesPerLong
+    // multiple when converting to double-words. This is necessary because array
+    // size does not include object alignment padding, so it might not be a
+    // multiple of BytesPerLong for sub-long element types.
+    payload_size = kit->gvn().transform(new AddXNode(payload_size, kit->MakeConX(BytesPerLong - 1)));
+  }


Back to this change. Why rounding is only arrays? Do we have a check that object's alignment >= 8 bytes? If it less you may access beyond array.
Can Add result overflow in 32-bit VM?

Why rounding is only arrays?

Class instance (bytewise) size is already rounded to MinObjAlignmentInBytes (i.e. it is a multiple of BytesPerLong, assuming minimum object alignment of 8 bytes), so rounding it again would be a no-op.

Do we have a check that object's alignment >= 8 bytes? If it less you may access beyond array.

This code simply assumes that the minimum object alignment in HotSpot is 8 bytes, as per src/hotspot/share/runtime/globals.hpp:

#ifdef _LP64 ... product(int, ObjectAlignmentInBytes, 8, \ "Default object alignment in bytes, 8 is minimum") \ range(8, 256) \ constraint(ObjectAlignmentInBytesConstraintFunc, AtParse) ... #else ... const int ObjectAlignmentInBytes = 8; ... #endif

Is there any VM configuration where this might not hold? Or do I misunderstand your question?

Can Add result overflow in 32-bit VM?

payload_size + (BytesPerLong - 1) cannot overflow because it is performed after a subtraction of a larger quantity (base_off). Would it make sense to add an assertion like this?:

assert(base_off >= BytesPerLong - 1, "array payload size computation should not overflow");

vnkozlov · 2023-09-14T17:46:47Z

src/hotspot/share/opto/graphKit.cpp

  }
-  // else if round_mask == 0, the size computation is self-rounding
+  Node* non_rounded_size = _gvn.transform(new AddXNode(headerx, abody));


headerx + abody cannot overflow because of the array length limit imposed by TypeAryPtr::max_array_length(), which is designed to fit the total bytes of an array header and payload (including external alignment) in 32 bits (on 32-bits platforms). I checked this using the length limits for all basic types. Exceeding this limit causes the array allocation to throw a java.lang.OutOfMemoryError: Requested array size exceeds VM limit exception).

vnkozlov · 2023-09-14T17:48:18Z

src/hotspot/share/opto/graphKit.cpp

+  Node* size = non_rounded_size;
+  if (round_mask != 0) {
+    Node* mask1 = MakeConX(round_mask);
+    size = _gvn.transform(new AddXNode(size, mask1));


and here again question about overflow in 32-bit VM.
Do we generate compare with FastAllocateSizeLimit before this code is executed?

size + mask1 cannot overflow because, as you suggest, it is only computed in the fast allocation path (i.e. for small values of size restricted by FastAllocateSizeLimit). The only way to bypass this check and indeed trigger the overflow is to set -XX:FastAllocateSizeLimit=2147483647 (debug-only flag), this causes indeed a segmentation fault, reported in JDK-8316512.

In general, the behavior of size computation within GraphKit::new_array w.r.t. overflow is not altered by this changeset, as far as I can see, because it essentially only changes whether round_mask is added to the header size (as done before) or to the total, non-rounded size (as proposed here).

robcasloz · 2023-09-19T09:21:35Z

Thanks for the detailed analysis and explanation. The fix looks good.

Thanks for reviewing, Tobias!

robcasloz · 2023-09-19T09:22:29Z

Few comments.

Thanks for reviewing, Vladimir! See my replies inlined for each comment/question.

vnkozlov

Thank you for answering all my questions.

robcasloz · 2023-09-21T05:47:35Z

/integrate

openjdk · 2023-09-21T05:49:13Z

Going to push as commit ceff47b.
Since your change was applied there have been 211 commits pushed to the master branch:

df4a25b: 8308762: Metaspace leak with Instrumentation.retransform
8412479: 8316229: Enhance class initialization logging
c04c9ea: 8316627: JViewport Test headless failure
5cacf21: 8316156: ByteArrayInputStream.transferTo causes MaxDirectMemorySize overflow
3461c7b: 8316532: Native library copying in BuildMicrobenchmark.gmk cause dups on macOS
3301fb1: 8315869: UseHeavyMonitors not used
54028e7: 8316562: serviceability/sa/jmap-hprof/JMapHProfLargeHeapTest.java times out after JDK-8314829
7c991cc: 8296246: Update Unicode Data Files to Version 15.1.0
a021dbc: 8316149: Open source several Swing JTree JViewport KeyboardManager tests
455cfae: 8315880: change LockingMode default from LM_LEGACY to LM_LIGHTWEIGHT
... and 201 more: https://git.openjdk.org/jdk/compare/024133b089d911dcc3ea70dfdaa6b150b14a9eb4...master

Your commit was automatically rebased without conflicts.

openjdk · 2023-09-21T05:49:24Z

@robcasloz Pushed as commit ceff47b.

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

openjdk bot added the hotspot hotspot-dev@openjdk.org label Sep 6, 2023

openjdk bot added the hotspot-compiler hotspot-compiler-dev@openjdk.org label Sep 7, 2023

openjdk bot added the hotspot-gc hotspot-gc-dev@openjdk.org label Sep 7, 2023

robcasloz added 17 commits September 8, 2023 09:27

Clarify that the currently computed size is rounded (aligned to MinOb…

ce68606

…jAlignmentInBytes)

Compute both sizes: rounded for AllocateArray and non-rounded for Arr…

0719214

…ayCopy

Round up number of 8-byte words

99b66d6

Rounding up payload size is only necessary for arrays

fa05b44

TEMPORARY: add additional macro-assembly comments

2d600a6

TEMPORARY: set UseNewCode to true by default

0128d79

TEMPORARY: print only 'oop_disjoint_arraycopy_uninit' stub code

8b98bbd

Use more accurate names

89d4e0c

Remove unused variable

05e72cc

Add a regression test

9766c41

Comment and rename for clarity

57e6865

Round up object size at the end of the computation

93bf598

Require GenZGC in the test

2675dd5

Revert "TEMPORARY: print only 'oop_disjoint_arraycopy_uninit' stub code"

b62d01f

This reverts commit 5b73ae4.

Revert "TEMPORARY: set UseNewCode to true by default"

25f4151

This reverts commit ddbc110.

Revert "TEMPORARY: add additional macro-assembly comments"

bce8cb1

This reverts commit eda6cf0.

Revert use of UseNewCode

68443da

robcasloz added 4 commits September 8, 2023 09:27

Remove extra whitespace

7528dd5

Remove extra whitespace

a5188cf

Relaxed assertion

ae16d2f

Add regression test that exercises the relaxed assertion

b3109e3

robcasloz force-pushed the JDK-8315082 branch from b703e12 to b3109e3 Compare September 8, 2023 13:18

robcasloz marked this pull request as ready for review September 11, 2023 07:02

openjdk bot added the rfr Pull request is ready for review label Sep 11, 2023

albertnetymk approved these changes Sep 11, 2023

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openjdk bot added the ready Pull request is ready to be integrated label Sep 11, 2023

TobiHartmann approved these changes Sep 13, 2023

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vnkozlov reviewed Sep 14, 2023

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vnkozlov approved these changes Sep 20, 2023

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openjdk bot added the integrated Pull request has been integrated label Sep 21, 2023

openjdk bot closed this Sep 21, 2023

openjdk bot removed ready Pull request is ready to be integrated rfr Pull request is ready for review labels Sep 21, 2023

8315082: [REDO] Generational ZGC: Tests crash with assert(index == 0 || is_power_of_2(index)) #15589

8315082: [REDO] Generational ZGC: Tests crash with assert(index == 0 || is_power_of_2(index)) #15589

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Additional changes compared to JDK-8312749

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