[Wasm RyuJit] repair multi-entry try regions

[AndyAyersMS]

Runtime-async resumption and Wasm EH flow can add edges into the middle of a try region. For try/catch at least we need to transform these back to single-entry regions to match Wasm structured control flow.

Previously we were leveraging the SCC transform for this, by (effectively) adding additional edges to the control flow graph between try region entries and side entries. But this has become increasingly cumbersome over time, and also cannot handle cases where the SCC entry headers span more than one try region, since the SCC transform introduces just one dispatch block.

So instead, we now transform all try regions back into single entry regions, after the async and EH transforms, via a new phase fgWasmRepairTryEntries. We do this for all try regions instead of just try/catch for simplicity and consistency during layout; the issue we're fixing was a layout problem with an isolated resumption block in a try/fault.

Each try region with a side entry gets a dispatcher: the header is split so it holds nothing but a switch on a control variable, with normal entry as the default case. Side entry preds set the control variable and branch to the outermost region header they enter, and the dispatchers cascade inward until the target is reached, where a landing pad resets the control variable so a later normal entry is not misdirected by a stale control variable value. Wasm try/catch headers are already split by fgWasmEhFlow, so there the dispatcher hangs off the GT_WASM_JEXCEPT false edge.

We also remove the mechanisms introduced for the SCC-based solution, including the side entry accommodations in FlowGraphTryRegions, the pseudo-successor hack in VisitWasmSuccs, and the multiple-entry region edge helpers. FlowGraphTryRegions::Build now records whether a side entry exists. If so, consumers now defensively bail out with NYI_WASM since side entries are no longer expected.

Fixes #131393

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[AndyAyersMS]

@adamperlin PTAL
fyi @dotnet/wasm-contrib

As noted in the issue the old approach was increasingly creaky. This one should be much more robust.

Each dispatcher switches over the entire range of control values; while this might seem wasteful, the number of these needed (at least in SPC) is quite small: 71 methods, at most 3 dispatchers/method (average is 1.5ish), and at most 10 cases/dispatcher (average is maybe 2.5). Optimizing this to a contiguous range per switch does not yet seem worthwhile.

[Copilot]

Pull request overview

Adds a new wasm-specific JIT phase to ensure all EH try regions are single-entry (entered only via ebdTryBeg), which is required to reliably express structured Wasm control flow after async/EH transforms can introduce mid-try edges.

Changes:

• Introduces Compiler::fgWasmRepairTryEntries (new PHASE_WASM_REPAIR_TRY_ENTRIES) that rewrites side entries into try regions via cascading per-try dispatchers driven by a control local.
• Removes the prior SCC-based “pseudo-successor / temporary edge” machinery used to coerce multi-entry try/catch regions into single-entry form.
• Refactors FlowGraphTryRegions tracking to record the existence of side entries and make wasm codegen defensively NYI if any survive past the repair phase.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
src/coreclr/jit/flowgraph.cpp	Stops special-casing async/catch resumption edges; instead records any non-header try entry as a “side entry” for wasm to reject if it remains.
src/coreclr/jit/fgwasm.h	Removes the wasm DFS pseudo-successor hack now that try regions are expected to be single-entry earlier.
src/coreclr/jit/fgwasm.cpp	Implements fgWasmRepairTryEntries; updates SCC/control-flow phases to NYI if HasSideEntry() is still true.
src/coreclr/jit/compphases.h	Registers the new wasm phase.
src/coreclr/jit/compiler.h	Renames/remodels try-region “multi-entry” tracking to m_hasSideEntry; declares fgWasmRepairTryEntries.
src/coreclr/jit/compiler.cpp	Wires the new phase into the wasm pipeline after wasm EH-flow + unreachable-block removal, before SCC transform.

[Copilot]

Pull request overview

Copilot reviewed 6 out of 6 changed files in this pull request and generated no new comments.

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[AndyAyersMS]

Was going to hold this until after testing is on, but am hitting this assert, so will merge this first.