8266480: Implicit null check optimization does not update control of hoisted memory operation

[TobiHartmann]

C2 replaces explicit null checks by hoisting a nearby memory operation to the null check and using it as implicit null check. In some cases, control of that memory operation is not updated correctly, leading to assert failures during PhaseCFG::verify() because a use is no longer dominated by its definition.

After matching, the graph looks like this:

64 testP_reg is an explicit null check and 78 loadD, 73 storeD and 77 storeImmI are candidates for an implicit null check because they are operating on the same oop. PhaseCFG::implicit_null_check decides to hoist the 77 storeImmI from the not_null_block B12 to the null check in B11/B13:

Now the problem is that control of 77 storeImmI was not updated and still points into the non-dominating block B15. The following code is supposed to fix this:

jdk/src/hotspot/share/opto/lcm.cpp

Lines 413 to 418 in 9d168e2

	// Move the control dependence if it is pinned to not-null block.
	// Don't change it in other cases: NULL or dominating control.
	if (best->in(0) == not_null_block->head()) {
	// Set it to control edge of null check.
	best->set_req(0, proj->in(0)->in(0));
	}

However, it does not trigger because control is not the not_null_block->head() but 59 MachProj which is the control projection from 60 CallLeafDirect emitted by a drem. The fix is to simply check get_block_for_node(ctrl) instead.

This is an old issue that was only caught by the assert recently introduced by JDK-8263227.

Thanks,
Tobias

---

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Issue

• JDK-8266480: Implicit null check optimization does not update control of hoisted memory operation

Reviewers

• Nils Eliasson (@neliasso - Reviewer)
• Vladimir Kozlov (@vnkozlov - Reviewer)

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[mlbridge]

Webrevs

• 00: Full (d3d02453)

[neliasso]

Does nodes 73, 77 and 78 touch different memory slices? Otherwise 78 should be anti-dependent on 73 and 77.

[TobiHartmann]

Yes, exactly. They are accessing the double and int slices corresponding to dFld and iFld accesses.

[neliasso]

Looks good!

[openjdk]

@TobiHartmann This change now passes all automated pre-integration checks.

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After integration, the commit message for the final commit will be:

8266480: Implicit null check optimization does not update control of hoisted memory operation

Reviewed-by: neliasso, kvn

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At the time when this comment was updated there had been 8 new commits pushed to the master branch:

• 10236e7: 8263242: serviceability/sa/ClhsdbFindPC.java cannot find MaxJNILocalCapacity with ASLR
• e6705c0: 8266949: Check possibility to disable OperationTimedOut on Unix
• b92c5a4: 8265292: [macos_aarch64] java/foreign/TestDowncall.java crashes with SIGBUS
• fadf580: 8262952: [macos_aarch64] os::commit_memory failure
• f8f40ab: 8230486: G1BarrierSetAssembler::g1_write_barrier_post unnecessarily pushes/pops new_val
• 9d168e2: 8266973: Migrate to ClassHierarchyIterator when enumerating subclasses
• 02507bc: 8267166: Remove test file vmTestbase/vm/mlvm/tools/LoadClass.java
• ce88b33: 8266615: C2 incorrectly folds subtype checks involving an interface array

Please see this link for an up-to-date comparison between the source branch of this pull request and the master branch.
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.

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[TobiHartmann]

Thanks, Nils!

[vnkozlov]

Good.

[TobiHartmann]

Thanks, Vladimir!

[TobiHartmann]

/integrate

[openjdk]

@TobiHartmann Since your change was applied there have been 15 commits pushed to the master branch:

• 3f883e8: 8267351: runtime/cds/SharedBaseAddress.java fails on x86_32 due to Unrecognized VM option 'UseCompressedOops'
• 7aa6568: 8256973: Intrinsic creation for VectorMask query (lastTrue,firstTrue,trueCount) APIs
• 65a8bf5: 8265126: [REDO] unified handling for VectorMask object re-materialization during de-optimization
• ff84577: 8267098: AArch64: C1 StubFrames end confusingly
• 0daec49: 8267246: -XX:MaxRAMPercentage=0 is unreasonable for jtreg tests on many-core machines
• 324defe: 8267212: test/jdk/java/util/Collections/FindSubList.java intermittent crash with "no reachable node should have no use"
• bdbe23b: 8265462: Handle multiple slots in the NSS Internal Module from SunPKCS11's Secmod
• 10236e7: 8263242: serviceability/sa/ClhsdbFindPC.java cannot find MaxJNILocalCapacity with ASLR
• e6705c0: 8266949: Check possibility to disable OperationTimedOut on Unix
• b92c5a4: 8265292: [macos_aarch64] java/foreign/TestDowncall.java crashes with SIGBUS
• ... and 5 more: https://git.openjdk.java.net/jdk/compare/894547d2c102dcbe1f9ec8a1edb11c6b31e4270e...master

Your commit was automatically rebased without conflicts.

Pushed as commit c2b50f9.

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