8255401: Shenandoah: Allow oldval and newval registers to overlap in cmpxchg_oop()

[rkennke]

We encountered a failure in testing:

Internal Error (/home/jenkins/workspace/nightly/jdk-jdk/src/hotspot/share/asm/register.hpp:141), pid=15470, tid=15611
assert(a != b && a != c && a != d && b != c && b != d && c != d) failed: registers must be different: a=0x0000000000000000, b=0x0000000000000000, c=0x000000000000000b, d=0x000000000000000a

in:

Stack: [0x00007fb8fa2e3000,0x00007fb8fa3e4000], sp=0x00007fb8fa3deca0, free space=1007k
Native frames: (J=compiled Java code, A=aot compiled Java code, j=interpreted, Vv=VM code, C=native code)
V [libjvm.so+0x156890e] ShenandoahBarrierSetAssembler::cmpxchg_oop(MacroAssembler*, RegisterImpl*, Address, RegisterImpl*, RegisterImpl*, bool, RegisterImpl*, RegisterImpl*)+0xde
V [libjvm.so+0x3ec1d1] compareAndSwapN_shenandoahNode::emit(CodeBuffer&, PhaseRegAlloc*) const+0x571

It seems to appear very rarely.

The failure is that both newval and oldval are the same register (rax). I believe it is ok for the two registers to overlap:

• It is not expected that newval is preserved across the cmpxchg
• The CAS will override newval, but:
  • The first CAS is unaffected by the overlap
  • The retry-loop is only entered when previous-value == old-value, and thus newval will still hold the same value

For aarch64 it matters even less, because newval is never overridden.

Testing: hotspot_gc_shenandoah (x86 & aarch64).

---

Progress

• Change must not contain extraneous whitespace
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Testing

	Linux x32	Linux x64	Windows x64	macOS x64
Build	✔️ (1/1 passed)	✔️ (5/5 passed)	❌ (1/2 failed)	✔️ (2/2 passed)
Test (tier1)		✔️ (9/9 passed)		✔️ (9/9 passed)

Failed test task

• Windows x64 (build debug)

Issue

• JDK-8255401: Shenandoah: Allow oldval and newval registers to overlap in cmpxchg_oop()

Reviewers

• Roland Westrelin (@rwestrel - Reviewer)

Download

$ git fetch https://git.openjdk.java.net/jdk pull/871/head:pull/871
$ git checkout pull/871

[bridgekeeper]

👋 Welcome back rkennke! 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]

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

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[mlbridge]

Webrevs

• 00: Full (1548bd9)

[rwestrel]

Looks good to me

[openjdk]

@rkennke 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:

8255401: Shenandoah: Allow oldval and newval registers to overlap in cmpxchg_oop()

Reviewed-by: roland

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 112 new commits pushed to the master branch:

• 1769c48: 8255471: ZGC: Rework root iterators and closures
• b028074: 8255673: Wrong version in docs bundles
• 4c66b15: 8255235: ZGC: Allocate and initialize forwarding data structures in parallel
• 1019581: 8255691: Shenandoah: Invoke native-LRB only on non-strong refs
• d2812f7: 8254072: AArch64: Get rid of --disable-warnings-as-errors on Windows+ARM64 build
• 120aec7: 8255720: Optimize bci_to_dp/-data by enabling iteration over raw DataLayouts
• 4b775e6: 8255721: Remove no-op clean_weak_method_links methods
• 3302d3a: 8255544: Create a checked cast
• 54c8813: 8255734: VM should ignore SIGXFSZ on ppc64, s390 too
• ceab9f3: 6816284: Notepad class should be public
• ... and 102 more: https://git.openjdk.java.net/jdk/compare/83a91bfaed4194064e3a5550b1b25980c9adcb8c...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.

[rkennke]

/integrate

[openjdk]

@rkennke Since your change was applied there have been 113 commits pushed to the master branch:

• a3aad11: 8255400: Shenandoah: C2 failures after JDK-8255000
• 1769c48: 8255471: ZGC: Rework root iterators and closures
• b028074: 8255673: Wrong version in docs bundles
• 4c66b15: 8255235: ZGC: Allocate and initialize forwarding data structures in parallel
• 1019581: 8255691: Shenandoah: Invoke native-LRB only on non-strong refs
• d2812f7: 8254072: AArch64: Get rid of --disable-warnings-as-errors on Windows+ARM64 build
• 120aec7: 8255720: Optimize bci_to_dp/-data by enabling iteration over raw DataLayouts
• 4b775e6: 8255721: Remove no-op clean_weak_method_links methods
• 3302d3a: 8255544: Create a checked cast
• 54c8813: 8255734: VM should ignore SIGXFSZ on ppc64, s390 too
• ... and 103 more: https://git.openjdk.java.net/jdk/compare/83a91bfaed4194064e3a5550b1b25980c9adcb8c...master

Your commit was automatically rebased without conflicts.

Pushed as commit 0e19ded.

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