JIT: Keep intrinsic-only loops fully interruptible

[Copilot]

Later optimizations can remove a loop’s only intrinsic call after interruptibility is decided, leaving a partially interruptible, safepoint-free loop that hangs GC suspension.

• Safepoint detection: Exclude IsSpecialIntrinsic calls from IsGcSafePoint, conservatively making intrinsic-only loops fully interruptible.
• Regression coverage: Exercise small and zero-length span copies, SequenceEqual, Fill, and Clear during GC, with a large-copy control whose helper call survives.

• Fixes JIT (bug): GC deadlock — method stays partially interruptible after lowering unrolls the loop's only GC safepoint (Span.CopyTo) #133822

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[EgorBo]

Almost no diffs, slight increase in GC info as expected, since full interruptability adds size.

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[EgorBo]

PTAL @AndyAyersMS @dotnet/jit-contrib

[Copilot]

🟡 Changes recommended

Narrow the intrinsic check and add regression tests for the described GC scenarios.

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Pull request overview

This JIT change prevents GC suspension deadlocks in loops whose only intrinsic call is removed during lowering.

Changes:

• Excludes special intrinsics from GC safe-point detection.
• Treats intrinsic-only loops as fully interruptible.

File summaries

File	Summary
src/coreclr/jit/compiler.hpp	Updates IsGcSafePoint; review found the condition may be broader than removable intrinsics and noted missing promised regression coverage.

Review details

Suppressed comments (1)

src/coreclr/jit/compiler.hpp:3892

• IsSpecialIntrinsic is broader than the calls that this PR's lowering can remove: the importer also marks cases such as Thread.get_CurrentThread, Array<T>.GetEnumerator, String.FastAllocateString, and math intrinsics that may be re-materialized as user calls. The relevant unrolling in LowerCall is additionally target-gated to AMD64/ARM64, but this check applies on every target. When one of these calls survives, suppressing its safe-point flag needlessly makes loops fully interruptible and increases GC-info/codegen cost; please narrow the condition to removable intrinsics/targets or justify the broader cost.

    if (!call->IsFastTailCall() && !call->IsSpecialIntrinsic())

• Files reviewed: 1/1 changed files
• Comments generated: 1
• Review effort level: Lite