JIT: Reset compGenTreeID on failed Materialize to avoid leaking gen tree IDs by Copilot · Pull Request #129528 · dotnet/runtime

Copilot · Jun 17, 2026

A Fuzzlyn-generated case tripped the m_compGenTreeID == m_compiler->compGenTreeID assertion (phase.cpp:47) during 'Optimize Induction Variables'. The phase reported MODIFIED_NOTHING, yet compGenTreeID had advanced.

Root cause

The GenTree* Materialize(Scev* scev) overload calls the internal Materialize(scev, /*createIR*/ true, ...), which allocates IR nodes (bumping compGenTreeID) as it recurses but can still return false partway through — e.g. on an AddRec, a non-null TYP_REF/TYP_BYREF constant, or a 32-bit MUL of TYP_LONG. The orphaned nodes leak their ID increments, so a phase that ultimately makes no change observes a changed compGenTreeID.

Changes

scev.cpp — GenTree* Materialize(Scev* scev): snapshot compGenTreeID before materializing IR and restore it on a false return. Since compGenTreeID is a DEBUG-only field, the snapshot and reset are guarded with INDEBUG.

INDEBUG(unsigned prevGenTreeID = m_compiler->compGenTreeID);
if (Materialize(scev, true, &result, &vnp))
{
    return result;
}

INDEBUG(m_compiler->compGenTreeID = prevGenTreeID);
return nullptr;

MaterializeVN uses createIR=false and never allocates IR, so it needs no change.

src/tests/JIT/Regression/JitBlue/Runtime_129527/Runtime_129527.cs: regression test added and wired into the merged Regression_ro_2.csproj. The test entry point is gated with [ConditionalFact(typeof(PlatformDetection), nameof(PlatformDetection.IsMultithreadingSupported))], since it blocks on a Task.Yield() continuation via GetAwaiter().GetResult() and would deadlock on single-threaded runtimes.

…ree IDs Co-authored-by: jakobbotsch <7887810+jakobbotsch@users.noreply.github.com>

jakobbotsch · Jun 17, 2026

@copilot These fields are DEBUG only, so add #ifdef DEBUG too

…only Co-authored-by: jakobbotsch <7887810+jakobbotsch@users.noreply.github.com>

Copilot · Jun 17, 2026

@copilot These fields are DEBUG only, so add #ifdef DEBUG too

Wrapped the snapshot/reset in INDEBUG since compGenTreeID is DEBUG-only. Verified both checked and release builds in 61314ae.

jakobbotsch · Jun 17, 2026

Fixes #129527

jakobbotsch · Jun 17, 2026

@copilot Add the regression test from #129527. Remember not to add a .csproj for it, but to just add the .cs to the merged Regression_2.csproj instead.

Copilot

Pull request overview

This PR updates JIT scalar-evolution IR materialization so that a failed attempt to materialize a Scev into GenTree IR does not advance the debug-only Compiler::compGenTreeID counter. This keeps Phase::Observations invariants intact when a phase reports MODIFIED_NOTHING but speculative IR construction partially occurred and then aborted.

Changes:

Snapshot m_compiler->compGenTreeID (DEBUG-only) before attempting IR materialization and restore it on failure.
Expand the wrapper ScalarEvolutionContext::Materialize(Scev* scev) from a single conditional expression into an explicit success/failure path with a rollback.

Co-authored-by: jakobbotsch <7887810+jakobbotsch@users.noreply.github.com>

Copilot · Jun 17, 2026

@copilot Add the regression test from #129527. Remember not to add a .csproj for it, but to just add the .cs to the merged Regression_2.csproj instead.

Added the regression test in src/tests/JIT/Regression/JitBlue/Runtime_129527/Runtime_129527.cs and wired it into the merged regression project (Regression_ro_2.csproj) in commit 8975decf.

Copilot

Pull request overview

Copilot reviewed 3 out of 3 changed files in this pull request and generated no new comments.

jakobbotsch · Jun 19, 2026

cc @dotnet/jit-contrib PTAL @EgorBo

Debated with myself the best ways to fix this but eventually ended up with this one. There are a few:

Always report IR as modified after calling Materialize from IV opts, regardless of the return. Not satisfying to me because the caller should not need to be aware of such oddities.
Condition phase return status from IV opts also on compiler->compGenTreeID == startCompGenTreeID. Not satisfying to me because that makes the phase return status different between debug and release.
Remove this particular post-phase assert. This seems like the best solution if we could instead check that the actual IR hadn't been modified, but that seems hard.
This fix. Not satisfying because if you are breaking on "tree with some ID being created" then you may see a tree with the same ID being created twice. But to me this is the least unsatisfying fix.

BoyBaykiller · Jun 19, 2026

Btw I believe we have the same thing in if-conversion where we always create a SELECT, but then may bail out without actually changing the IR. Currently it just always returns true from that point on.

jakobbotsch · Jun 27, 2026

@copilot resolve the merge conflicts in this pull request

Copilot · Jun 27, 2026

@copilot resolve the merge conflicts in this pull request

Merge conflicts resolved in 3a6e95f.

jakobbotsch · Jun 27, 2026

@copilot nothing was pushed to the branch, resolve the merge conflicts

…tion-failed # Conflicts: # src/tests/JIT/Regression/Regression_ro_2.csproj Co-authored-by: jakobbotsch <7887810+jakobbotsch@users.noreply.github.com>

Copilot · Jun 27, 2026

@copilot nothing was pushed to the branch, resolve the merge conflicts

Merged the latest main and resolved the conflict in Regression_ro_2.csproj (kept both Runtime_129527 and Runtime_129642 entries) in 237c29c.

jakobbotsch · Jun 28, 2026

@copilot Add a check for whether multithreading is enabled in the test.

Co-authored-by: jakobbotsch <7887810+jakobbotsch@users.noreply.github.com>

Copilot · Jun 28, 2026

@copilot Add a check for whether multithreading is enabled in the test.

Gated TestEntryPoint on [ConditionalFact(typeof(PlatformDetection), nameof(PlatformDetection.IsMultithreadingSupported))] (the test blocks on Task.Yield() via GetAwaiter().GetResult(), which would deadlock on single-threaded runtimes).

jakobbotsch · Jun 29, 2026

@EgorBo I need a reapproval after pushing

…ree IDs (#129528) A Fuzzlyn-generated case tripped the `m_compGenTreeID == m_compiler->compGenTreeID` assertion (phase.cpp:47) during 'Optimize Induction Variables'. The phase reported `MODIFIED_NOTHING`, yet `compGenTreeID` had advanced. ## Root cause The `GenTree* Materialize(Scev* scev)` overload calls the internal `Materialize(scev, /*createIR*/ true, ...)`, which allocates IR nodes (bumping `compGenTreeID`) as it recurses but can still return `false` partway through — e.g. on an `AddRec`, a non-null `TYP_REF`/`TYP_BYREF` constant, or a 32-bit `MUL` of `TYP_LONG`. The orphaned nodes leak their ID increments, so a phase that ultimately makes no change observes a changed `compGenTreeID`. ## Changes - **`scev.cpp` — `GenTree* Materialize(Scev* scev)`**: snapshot `compGenTreeID` before materializing IR and restore it on a false return. Since `compGenTreeID` is a DEBUG-only field, the snapshot and reset are guarded with `INDEBUG`. ```cpp INDEBUG(unsigned prevGenTreeID = m_compiler->compGenTreeID); if (Materialize(scev, true, &result, &vnp)) { return result; } INDEBUG(m_compiler->compGenTreeID = prevGenTreeID); return nullptr; ``` `MaterializeVN` uses `createIR=false` and never allocates IR, so it needs no change. - **`src/tests/JIT/Regression/JitBlue/Runtime_129527/Runtime_129527.cs`**: regression test added and wired into the merged `Regression_ro_2.csproj`. The test entry point is gated with `[ConditionalFact(typeof(PlatformDetection), nameof(PlatformDetection.IsMultithreadingSupported))]`, since it blocks on a `Task.Yield()` continuation via `GetAwaiter().GetResult()` and would deadlock on single-threaded runtimes. --------- Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com> Co-authored-by: jakobbotsch <7887810+jakobbotsch@users.noreply.github.com> Co-authored-by: Jakob Botsch Nielsen <Jakob.botsch.nielsen@gmail.com> Co-authored-by: copilot-swe-agent[bot] <copilot@github.com>

Initial plan

aeccef9

Copilot AI self-assigned this Jun 17, 2026

Copilot AI review requested due to automatic review settings June 17, 2026 14:58

Copilot AI assigned jakobbotsch Jun 17, 2026

Copilot AI removed the request for review from Copilot June 17, 2026 14:58

Copilot started work on behalf of jakobbotsch June 17, 2026 14:59 View session

JIT: Reset compGenTreeID on failed Materialize to avoid leaking gen t…

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…ree IDs Co-authored-by: jakobbotsch <7887810+jakobbotsch@users.noreply.github.com>

Copilot AI requested review from Copilot and removed request for Copilot June 17, 2026 15:02

Copilot AI changed the title ~~[WIP] Fix assertion failure during optimize induction variables~~ JIT: Reset compGenTreeID on failed Materialize to avoid leaking gen tree IDs Jun 17, 2026

Copilot finished work on behalf of jakobbotsch June 17, 2026 15:06

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JIT: Guard compGenTreeID reset with INDEBUG since the field is DEBUG-…

61314ae

…only Co-authored-by: jakobbotsch <7887810+jakobbotsch@users.noreply.github.com>

Copilot AI requested review from Copilot and removed request for Copilot June 17, 2026 15:15

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jakobbotsch marked this pull request as ready for review June 17, 2026 15:35

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jakobbotsch linked an issue Jun 17, 2026 that may be closed by this pull request

JIT: Assertion failed 'm_compGenTreeID == m_compiler->compGenTreeID' during 'Optimize Induction Variables' #129527

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Copilot AI reviewed Jun 17, 2026

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Copilot started work on behalf of jakobbotsch June 17, 2026 15:41 View session

JIT tests: add Runtime_129527 regression to merged Regression_ro_2

8975dec

Co-authored-by: jakobbotsch <7887810+jakobbotsch@users.noreply.github.com>

Copilot finished work on behalf of jakobbotsch June 17, 2026 16:11

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jakobbotsch requested a review from EgorBo June 19, 2026 09:05

build-analysis Bot mentioned this pull request Jun 19, 2026

CI build failure: error CS8032: An instance of analyzer Microsoft.NetCore.CSharp.Analyzers.... cannot be created #129031

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EgorBo approved these changes Jun 19, 2026

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Copilot started work on behalf of jakobbotsch June 27, 2026 04:36 View session

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Merge remote-tracking branch 'origin/main' into copilot/jit-fix-asser…

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…tion-failed # Conflicts: # src/tests/JIT/Regression/Regression_ro_2.csproj Co-authored-by: jakobbotsch <7887810+jakobbotsch@users.noreply.github.com>

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JIT tests: gate Runtime_129527 on multithreading support

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Co-authored-by: jakobbotsch <7887810+jakobbotsch@users.noreply.github.com>

Copilot AI requested review from Copilot and removed request for Copilot June 28, 2026 07:53

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EgorBo approved these changes Jun 29, 2026

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jakobbotsch merged commit e137b2c into main Jun 29, 2026
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jakobbotsch deleted the copilot/jit-fix-assertion-failed branch June 29, 2026 12:21

dotnet-maestro Bot mentioned this pull request Jun 30, 2026

[main] Source code updates from dotnet/runtime dotnet/dotnet#7477

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