[release/11.0] JIT: Prevent loop hoisting across volatile reads - #134351
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Volatile reads have acquire semantics, but loop optimization could hoist an ordinary load across a prior iteration's acquire. Treat volatile indirections as loop memory havoc and add a regression test for the cross-iteration ordering. Fixes #133579 Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Name the test after issue 133579 and mark it as an outer-loop test so it runs at priority 1 without changing the priority of the merged regression assembly. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com> Copilot-Session: 51ab0cbf-6d32-4701-8e9d-fa19e4d01f6f
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Tagging subscribers to this area: @JulieLeeMSFT, @jakobbotsch |
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@EgorBo ptal |
JulieLeeMSFT
approved these changes
Sep 21, 2026
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Backport of #133825 to release/11.0
/cc @AndyAyersMS
Customer Impact
Loop optimization can hoist an ordinary field load across a
Volatile.Readacquire from a prior loop iteration, violating the managed memory model and allowing stale data to be observed. The reported repro returns0after another thread publishes42. This affects .NET 8, 9, 10, and 11. A community contributor noted that application code may encounter this silently and that the cause would be difficult to diagnose. See #133579.Regression
No recent introducing change has been identified. The issue reproduces across all currently supported runtime versions, including .NET 8 through .NET 11.
Testing
Added a focused cross-iteration ordering regression test that fails without the fix and passes with it. The existing Runtime_40444 volatile-ordering regression also passes. Validation included Checked and Release CoreCLR builds, JIT formatting, and Windows x64 Checked SuperPMI across nine collections. Only 2 of 987,559 successful compilations changed, totaling +4 bytes with effectively zero aggregate PerfScore impact.
Risk
Low. The change treats volatile indirections as loop memory havoc, conservatively preventing unsafe load hoisting across acquire operations. It may inhibit optimization in the affected shape, but SuperPMI measured only two tiny code-generation changes and no meaningful performance impact.