Test failure: GC/API/GC/Collect0 fails on x86 jitminopts

[JulieLeeMSFT]

Build Information

Build: https://dev.azure.com/dnceng-public/public/_build/results?buildId=1540457
Build error leg or test failing: GC/API/GC/Collect0/Collect0.dll (coreclr windows x86 Checked jitminopts @ Windows.10.Amd64.Open)
Pull request: N/A

Error Message

{
  "ErrorMessage": "Test for GC.Collect(0) failed!",
  "ErrorPattern": "",
  "BuildRetry": false,
  "ExcludeConsoleLog": false
}

Details

Console log

Array is in generation: 0
Object is in generation: 0
Collect(0)
Array is in generation: 0
Object is in generation: 2
Test for GC.Collect(0) failed!

---

Note

This issue was filed with AI assistance on behalf of @JulieLeeMSFT.

Known issue validation

Build: 🔎 https://dev.azure.com/dnceng-public/public/_build/results?buildId=1540457
Error message validated: [Test for GC.Collect(0) failed!]
Result validation: ✅ Known issue matched with the provided build.
Validation performed at: 8/5/2026 5:52:42 PM UTC

Report

Build	Repository	Test	Pull Request
1540457	dotnet/runtime	GC/API/GC/Collect0/Collect0.dll

Summary

24-Hour Hit Count	7-Day Hit Count	1-Month Count
0	1	1

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[JulieLeeMSFT]

(Edited)
DOTNET_GCStress is not set, so the test's existing [SkipOnCoreClr(..., RuntimeTestModes.AnyGCStress)] guard does not apply. Default workstation GC (regions), win-x86, .NET 11 preview.

Analysis

Collect0.cs passes only if GetGeneration(array) == GetGeneration(obj) after two GC.Collect(0) calls. Here array is gen 0 and obj is gen 2.

This looks like the same root cause as #131721, where the sibling test Collect.cs fails on x86 Checked with the identical Expected: 100 / Actual: 1. That test asserts (gen1==2 && gen2==2) || gen2 > gen1 for a live array across a full GC.Collect(), so it fails only when a survivor reports a generation that is not higher than before.

Both tests encode the segment-era invariant that a survivor's reported generation is monotonically non-decreasing. With GC regions that no longer holds: a region's generation is not monotonic, and a region with sparse survivors can be re-labelled younger (demotion). That explains array staying gen 0 here while obj was promoted normally, and it explains the gen2 <= gen1 outcome in #131721.

If so, both are test bugs rather than a GC regression. Collect0.cs's escape hatches (agen1 != 0, agen2 > 1) only inspect array, so demotion slips under the > 1 check. Expect flakiness across legs rather than anything specific to MinOpts — #131721 hit jitstress2_jitstressregs1, this one hit jitminopts.

Worth noting this is inferred from test output, not from a GC trace; an EventPipe trace confirming the condemned generation and whether demotion occurred would settle it.

Proposed fix

int gen2CountBefore = GC.CollectionCount(2);
GC.Collect(0);
GC.Collect(0);
GC.KeepAlive(array);
GC.KeepAlive(obj);

if (GC.CollectionCount(2) != gen2CountBefore)
{
    Console.WriteLine("Collect(0) was escalated to a gen2 GC; skipping.");
    return 100;
}

Collect.cs likely needs an analogous relaxation. cc @jkoritzinsky (assignee on #131721)

---

Note

This comment was written with AI assistance on behalf of @JulieLeeMSFT.

[JulieLeeMSFT]

This time, Collect1 failed in runtime-coreclr pgo/20260806.1

09:29:56.192 Running test: GC/API/GC/Collect1/Collect1.dll
Array is in generation: 0
Object is in generation: 0
Collect(1)
Array is in generation: 0
Object is in generation: 1
Test for GC.Collect(1) failed!
Xunit.Sdk.EqualException: Assert.Equal() Failure: Values differ
Expected: 100
Actual:   1
   at Xunit.Assert.Equal[T](T expected, T actual, IEqualityComparer`1 comparer) in /_/src/arcade/src/Microsoft.DotNet.XUnitAssert/src/EqualityAsserts.cs:line 102
   at Xunit.Assert.Equal[T](T expected, T actual) in /_/src/arcade/src/Microsoft.DotNet.XUnitAssert/src/EqualityAsserts.cs:line 36
   at Program.<<Main>$>g__TestExecutor16|0_17(StreamWriter tempLogSw, StreamWriter statsCsvSw, StreamWriter outOfProcessPlanWriter, <>c__DisplayClass0_0&)
09:29:56.597 Failed test: GC/API/GC/Collect1/Collect1.dll

cc @janvorli.