Avoid Wasm unwindable-frame identity for call-free funclets #134597

Problem

CoreCLR's Wasm JIT stores each unwindable frame's function identity in the shadow-stack frame. Funclets that make no calls currently receive the same identity prolog even though they cannot have a callee that needs to walk through that frame.

In a recent retail System.Private.CoreLib.wasm measurement, 162 call-free funclets had this prolog. The function-index immediate also prevents otherwise identical funclet bodies from being folded: 58 funclets formed 21 identical groups after normalizing only that immediate.

Proposed change

Teach the JIT to omit unwindable-frame identity for funclets that make no calls, while retaining it for:

• Root functions.
• Funclets containing direct or indirect calls.
• Any funclet for which GC, EH, resume, or stack-walking behavior requires the identity.

The decision should use JIT IR/control-flow information rather than scanning emitted Wasm bytes.

Validation

• Build CoreCLR browser/Wasm in Release.
• Verify leaf funclets omit the tableBase + functionIndex store.
• Verify calling funclets and root methods retain it.
• Run focused Wasm startup/tests and compare CoreLib Code-section size.