JIT: Reset compGenTreeID on failed Materialize to avoid leaking gen tree IDs

[Copilot]

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.

[jakobbotsch]

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

[Copilot]

@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]

Fixes #129527

[jakobbotsch]

@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.

[Copilot]

@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.

[dotnet-policy-service]

Tagging subscribers to this area: @JulieLeeMSFT, @jakobbotsch
See info in area-owners.md if you want to be subscribed.