pcbjam/docs/features/wasm-exceptions

-fexceptions vs -fwasm-exceptions in KiCad-WASM — research dossier

Status: research / decision record. A parallel session attempted the migration end-to-end and parked it on an emscripten-4.0.2 LLVM codegen bug — see docs/wasm-exceptions-experiment.md (full build-plumbing patch preserved in its appendix; flags -fwasm-exceptions -sSUPPORT_LONGJMP=wasm -sWASM_LEGACY_EXCEPTIONS=0, raw linked pcbnew 92 MB and zero invoke_*/dynCall). This dossier is the research companion: mechanism, measurements, audit, and the asyncify catch-block design. Authored 2026-06-11/12. Companion to docs/features/async/ (the Asyncify currData contention dossier) — this dossier covers the exception-handling axis of the same machine.

Why this exists

The whole build is on -fexceptions (Emscripten's JavaScript-based exception handling). That choice is not cosmetic — it is the single largest driver of our Asyncify cost: it forces env.invoke_* into ASYNCIFY_IMPORTS, which makes nearly the whole call graph "suspension-capable" and therefore instrumented. We measured the consequence on our own binary: 59% of the raw asyncify tax (64% of the gzipped tax) on pcbnew exists only because of the invoke machinery. Migrating to native -fwasm-exceptions would cut the shipped pcbnew download from 64.5 MB to ~36 MB (−44%) and the module from 187 MB to ~122 MB (−35%), plus an unmeasured-but-real runtime win on every try-region hot path.

The migration is currently blocked by one upstream limitation — Binaryen's Asyncify pass cannot handle a suspension inside a catch handler — and KiCad triggers exactly that pattern (modal error dialogs from catch blocks) in 85 audited places. This dossier records the mechanism, the measurements, the toolchain status, the audit, and a concrete fork design (catch-arm hoisting) that would remove the blocker without refactoring KiCad at all.

TL;DR / decision

• Stay on -fexceptions for now. Asyncify stays under any design (fibers + EM_ASYNC_JS have no wasm-EH replacement); this is purely about how much it must instrument.
• The prize is measured, not estimated: −44% download, −35% module size (see 02).
• Binaryen merged partial asyncify+wasm-EH support in v125 (2025-11-19). Our emsdk bundles v121 — we don't even have the partial support locally.
• The remaining hole — unwind-from-catch — is fixable with a bounded new Binaryen pass (catch-arm hoisting, ~400–800 lines, 1–2 weeks, genuinely upstreamable; see 05). It is a pre-pass before stock --asyncify — Asyncify.cpp itself needs zero changes, so "fork" overstates it (one added file; get-wasm-opt.sh already has the build-from-source deployment path). It obsoletes the 85-site KiCad refactor entirely.
• Blocker ordering (learned from the parked experiment): the catch-block limitation is blocker #2. Blocker #1 is an LLVM codegen bug in emscripten 4.0.2 (invalid br_table arity in OpenCASCADE code under wasm-EH) — needs an emsdk bump first. And the experiment had to force the exnref encoding (WASM_LEGACY_EXCEPTIONS=0, because 4.0.2's legacy encoding output doesn't even parse in Binaryen), which collides with the fact that Binaryen's asyncify has zero TryTable support: the encoding choice after the emsdk bump decides which variant of the catch fix applies (see 03 §experiment, 05).
• Trigger to act: when we are ready to invest ~2 weeks of toolchain work, or if upstream lands full support on binaryen #4470 first. Until then the Asyncify arbiter work (docs/features/async/) is the priority — it fixes shipping bugs and is needed either way.

Document index

File	Contents
01-background-two-eh-models.md	How JS-EH (invoke_*) and wasm-EH actually work, and the three concrete couplings into our Asyncify machine.
02-measurements.md	Our controlled size experiment on pcbnew (methodology + numbers) and the published third-party benchmarks.
03-toolchain-status.md	Compatibility matrix: emcc checks, binaryen history (what merged in v125, what didn't), JSPI/fibers, setjmp/longjmp, mixing modes.
04-kicad-audit.md	The brace-matching catch-block audit: 85 direct / 93 review / 458 trivial of 636; libpng/libjpeg setjmp story; refactor effort if done by hand.
05-asyncify-fork-design.md	Asyncify.cpp internals, why catch arms are structurally hard, and the catch-arm-hoisting fork design with limits and effort.
catch_audit.py	The audit tool (re-runnable; suitable as a CI gate on the kicad submodule).
audit-results.txt	Full audit output incl. all 85 direct-suspend sites.

Relationship to docs/features/async/

Independent axes of the same machine. The async dossier is about correctness (one global Asyncify.currData shared by overlapping suspensions → crash/hang); this dossier is about cost (how much code Asyncify instruments). Fixing one does not fix the other. Sequencing: async arbiter first (shipping bugs), wasm-EH migration second (size/speed), and the migration plan below assumes the arbiter exists.

Migration plan (when triggered)

0. Resume the parked experiment (docs/wasm-exceptions-experiment.md): bump emsdk past the 4.0.2 LLVM br_table bug, git apply its appendix patch (KICAD_WASM_EH=1 gated), full clean deps rebuild (stamp-skip gotcha: stale sjlj objects in cairo etc.). Then decide the EH encoding: if newer LLVM emits parseable legacy encoding, the catch-arm-hoisting pre-pass (05) applies; if exnref stays forced, asyncify needs TryTable support + exnref spilling instead (05 §new-EH variant).
1. Newer Binaryen for the post-link step only: scripts/common/get-wasm-opt.sh already abstracts the binary — point it at a ≥ v125 build carrying the hoisting patch (05).
2. Validate partial support first: rebuild one app -fwasm-exceptions + asyncify-asserts, run the e2e suites; the asserts tripwire makes any missed unwind-from-catch a deterministic trap.
3. Uniform flag flip: -fexceptions → -fwasm-exceptions in scripts/build-wxuniversal-wasm.sh:141-142, scripts/kicad/build-kicad-target.sh (lines ~203/207/211/214), tests/apps/Makefile.wasm (all occurrences) — plus -sSUPPORT_LONGJMP=wasm (default with wasm-EH; the emscripten flavor is a hard error).
4. Drop env.invoke_* from ASYNCIFY_IMPORTS in scripts/common/apply-asyncify.sh:33.
5. Expect to delete/shrink shim machinery that exists only for the invoke world (inject-dyncall-shims.sh phases 1–2) — verify, don't assume.
6. Keep catch_audit.py as a CI gate only if shipping without the fork (i.e., the hand-refactor path); with the fork it is informational.