8364191: [lworld] Accesses to atomic flat fields prevent scalar replacement

[merykitty]

Hi,

Flat accesses prevent scalar replacement because they are mismatched accesses. It is also generally not possible to look through them, because the payload may contain an oop in the form of raw bits. As a result, this PR adds LoadFlatNode and StoreFlatNode, which act as high-level abstractions for atomic accesses on flat fields. When it is determined that there is no racing access on the flat field (e.g. because the holder object does not escape), these flat access nodes can be expanded into multiple accesses to each flattened fields, otherwise, they will be expanded into a sequence of inferring a payload and accessing memory with that payload.

I also fix an issue with deoptimization reallocation where we miss assigning the null marker of elements in a nullable flat array.

Please take a look and leave your reviews, thanks a lot.

---

Progress

• Change must not contain extraneous whitespace

Issue

• JDK-8364191: [lworld] Accesses to atomic flat fields prevent scalar replacement (Bug - P4)

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

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8364191: [lworld] Accesses to atomic flat fields prevent scalar replacement

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At the time when this comment was updated there had been 4 new commits pushed to the lworld branch:

• 67e6bcc: 8360653: [lworld] LayoutIterationTest.java fails with -XX:ForceNonTearable=*
• 4f9deef: 8364321: Add JDI tests for value objects
• 79618d4: Remove ensure_objArray and copying code and write a test
• ... and 1 more: https://git.openjdk.org/valhalla/compare/b2e518b7cf85527b90f34ba63bf5a1b775430cd0...lworld

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[mlbridge]

Webrevs

• 02: Full - Incremental (eb23f4e5)
• 01: Full - Incremental (428b656c)
• 00: Full (23828cc7)

[TobiHartmann]

Thanks for working on this @merykitty! I haven't reviewed this in yet but noticed a build failure:

[2025-08-04T17:23:31,905Z] /opt/mach5/mesos/work_dir/slaves/f7f8bd65-a387-4a2b-b519-702f2fefaf87-S170287/frameworks/1735e8a2-a1db-478c-8104-60c8b0af87dd-0196/executors/8d44f2d4-5a02-4ac8-95c1-3a37a1d82705/runs/1b653695-e82f-4430-a41f-d099d041486e/workspace/open/src/hotspot/share/opto/escape.cpp: In member function 'void ConnectionGraph::add_final_edges(Node*)':
[2025-08-04T17:23:31,905Z] /opt/mach5/mesos/work_dir/slaves/f7f8bd65-a387-4a2b-b519-702f2fefaf87-S170287/frameworks/1735e8a2-a1db-478c-8104-60c8b0af87dd-0196/executors/8d44f2d4-5a02-4ac8-95c1-3a37a1d82705/runs/1b653695-e82f-4430-a41f-d099d041486e/workspace/open/src/hotspot/share/opto/escape.cpp:1874:25: error: no matching function for call to 'ConnectionGraph::set_escape_state(PointsToNode*&, PointsToNode::EscapeState, const char [24])'
[2025-08-04T17:23:31,905Z]  1874 |         set_escape_state(field_value_ptn, PointsToNode::GlobalEscape, "store into a flat field");
[2025-08-04T17:23:31,905Z]       |         ~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
[2025-08-04T17:23:31,905Z] In file included from /opt/mach5/mesos/work_dir/slaves/f7f8bd65-a387-4a2b-b519-702f2fefaf87-S170287/frameworks/1735e8a2-a1db-478c-8104-60c8b0af87dd-0196/executors/8d44f2d4-5a02-4ac8-95c1-3a37a1d82705/runs/1b653695-e82f-4430-a41f-d099d041486e/workspace/open/src/hotspot/share/opto/escape.cpp:38:
[2025-08-04T17:23:31,905Z] /opt/mach5/mesos/work_dir/slaves/f7f8bd65-a387-4a2b-b519-702f2fefaf87-S170287/frameworks/1735e8a2-a1db-478c-8104-60c8b0af87dd-0196/executors/8d44f2d4-5a02-4ac8-95c1-3a37a1d82705/runs/1b653695-e82f-4430-a41f-d099d041486e/workspace/open/src/hotspot/share/opto/escape.hpp:434:8: note: candidate: 'void ConnectionGraph::set_escape_state(PointsToNode*, PointsToNode::EscapeState)'
[2025-08-04T17:23:31,905Z]   434 |   void set_escape_state(PointsToNode* ptn, PointsToNode::EscapeState esc
[2025-08-04T17:23:31,905Z]       |        ^~~~~~~~~~~~~~~~
[2025-08-04T17:23:31,905Z] /opt/mach5/mesos/work_dir/slaves/f7f8bd65-a387-4a2b-b519-702f2fefaf87-S170287/frameworks/1735e8a2-a1db-478c-8104-60c8b0af87dd-0196/executors/8d44f2d4-5a02-4ac8-95c1-3a37a1d82705/runs/1b653695-e82f-4430-a41f-d099d041486e/workspace/open/src/hotspot/share/opto/escape.hpp:434:8: note:   candidate expects 2 arguments, 3 provided
[2025-08-04T17:23:32,241Z] lib/CompileJvm.gmk:172: recipe for target '/opt/mach5/mesos/work_dir/slaves/f7f8bd65-a387-4a2b-b519-702f2fefaf87-S170287/frameworks/1735e8a2-a1db-478c-8104-60c8b0af87dd-0196/executors/8d44f2d4-5a02-4ac8-95c1-3a37a1d82705/runs/1b653695-e82f-4430-a41f-d099d041486e/workspace/build/linux-x64/hotspot/variant-server/libjvm/objs/static/escape.o' failed
[2025-08-04T17:23:32,241Z] make[3]: *** [/opt/mach5/mesos/work_dir/slaves/f7f8bd65-a387-4a2b-b519-702f2fefaf87-S170287/frameworks/1735e8a2-a1db-478c-8104-60c8b0af87dd-0196/executors/8d44f2d4-5a02-4ac8-95c1-3a37a1d82705/runs/1b653695-e82f-4430-a41f-d099d041486e/workspace/build/linux-x64/hotspot/variant-server/libjvm/objs/static/escape.o] Error 1

[merykitty]

@TobiHartmann Ah dummy mistake, I missed the NOT_PRODUCT guard, fixed it now.