66ce367703
Two research dossiers under docs/features: - async/: Asyncify currData contention — the clipboard crash and post-idle tool hang as one disease (single global suspension slot, three producers), asyncify internals walkthroughs, prior art, arbiter designs A/B. - wasm-exceptions/: -fexceptions vs -fwasm-exceptions — measured invoke-driven share of the asyncify tax on pcbnew (59% raw / 64% gzip; download 64.5->36 MB if migrated), KiCad dialog-in-catch audit (85 direct / 93 review of 636, catch_audit.py included), binaryen toolchain status (partial EH support since v125; TryTable unsupported), and a catch-arm-hoisting pre-pass design that would obsolete the KiCad-side refactor. Cross-linked with the parked KICAD_WASM_EH end-to-end experiment (docs/wasm-exceptions-experiment.md). Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
93 lines
5.1 KiB
Markdown
93 lines
5.1 KiB
Markdown
# 02 — Measurements: our controlled experiment + published benchmarks
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## §1. Our experiment (pcbnew, 2026-06-10)
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**Question:** how much of the asyncify size tax is invoke-driven (i.e., would vanish under
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wasm-EH)?
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**Method:** take the saved pre-asyncify artifact `output/pcbnew.finalized.wasm` (111 MB,
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post-link, post-finalize, pre-asyncify) and run the production post-link pipeline
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(`apply-asyncify.sh`: `wasm-opt --asyncify` + `wasm-opt -O2`, Binaryen v121) three ways,
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into `output/measure/` (originals untouched):
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1. **O2-only** (no asyncify) — the floor; *not a shippable config* (the app needs asyncify).
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2. **Asyncify, `env.invoke_*` dropped** from the imports list — the wasm-EH proxy.
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3. **Asyncify, full imports** — the shipped configuration (control).
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Control sanity check: variant 3 reproduced the production `pcbnew.wasm` size exactly
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(187 MB), so the pipeline replication is faithful.
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**Results:**
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| Variant | Raw | gzip -9 |
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|---|---|---|
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| O2-only floor | 77.6 MB | 19.8 MB |
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| Asyncify w/o invokes (wasm-EH proxy) | 122.1 MB | 36.0 MB |
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| Asyncify full (shipped) | 187.3 MB | 64.5 MB |
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**Decomposition:**
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- Total asyncify tax today: **+109.7 MB raw = +141% over floor** (+44.7 MB gzip). That is
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~2× the worst-case figures in the published literature — KiCad is an extreme asyncify
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case, and the invoke graph is why.
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- **Invoke-driven share of the tax: 59% raw (65.2 MB), 64% gzip (28.5 MB).** This is the
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part wasm-EH eliminates.
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- The remaining no-invoke tax (+44.5 MB, +57% over floor) is the legitimate cost of the
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real suspend chains (fibers + modal/clipboard sleeps) — in line with the published ~50% —
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and is kept under any design.
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**Headline:** wasm-EH ≈ **download 64.5 → 36.0 MB (−44%), module 187 → 122 MB (−35%)**,
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before counting runtime. The real wasm-EH build differs from the proxy in both directions:
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it also deletes the invoke wrappers + JS-EH glue outright (smaller still) while adding
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wasm-EH's own try/catch instructions (~+4% per published prototype data).
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**Caveats:** (a) the no-invoke proxy is **semantically unsound under `-fexceptions`** —
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with invokes present in JS but uninstrumented in wasm, the first suspension crossing a try
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frame corrupts state; measurement artifact only, never ship. (b) The finalized input is a
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May 26 snapshot while today's apps are newer — irrelevant for the *relative* comparison
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(same input, same tool, only the imports list varies). (c) Runtime gain is not directly
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measurable from these artifacts; the size-tracks-speed heuristic (Zakai) plus the published
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reports below are the basis for the runtime estimate.
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## §2. Published benchmarks and field reports (researched 2026-06-10)
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No clean head-to-head JS-EH vs wasm-EH benchmark of a large C++ app exists; triangulate:
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**Asyncify instrumentation tax (the thing our invoke graph inflates):**
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| Source | Workload | Numbers |
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| Alon Zakai, "Pause and Resume… Asyncify" (2019, canonical) | multiple benchmarks | size ~**+50%** typical, ≤2× worst; speed tracks size; **22%** Fannkuch; **5×** pathological when one huge function dominates (SQLite 150→300 KB) |
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| wa-sqlite (production, measured) | SQLite wasm | default asyncify **+70.6%** → **+35.3%** after narrowing instrumentation to the ~6 actually-async entry points (auto-detection had instrumented 700+ functions) |
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| Wasmer PHP/WordPress (production) | PHP setjmp/longjmp via asyncify | **1.5× size, 2× slower**; replacing with wasm-EH SjLj: **2×** faster cold start (120→60 ms), separately **4×** (100→25 ms) |
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| WordPress Playground | auto-detected asyncify list | ~70,000 functions instrumented, startup +4.5 s, fixed by manual list |
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**JS-EH (`invoke_*`) tax:**
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| Source | Workload | Numbers |
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| OpenCascade.js (CAD kernel — closest cousin to KiCad's code character) | STEP file load | global JS-EH **~10 min → ~2 min (~5×)** with narrowed EH; "performance degradation and file size impact is currently massive" |
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| emscripten discussion #17526 | JS-EH → wasm-EH | "not just 2× faster, but an order of magnitude" (anecdote) |
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| Emscripten docs (Box2D) | `-fno-rtti -fno-exceptions` | −15% size (RTTI+EH combined; only weak isolation) |
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**wasm-EH's own cost (what we'd pay instead):**
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| Source | Numbers |
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| Heejin Ahn, TPAC slides (prototype) | **+4%** whole-module instructions, **+11%** in exception-using functions |
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| V8 / Emscripten docs | non-throwing path **~zero cost** by design |
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**Most transferable to us:** wa-sqlite's 70.6→35.3 (the narrowing effect — ours measured
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59–64%) and OpenCascade's ~5× (exception-dense C++ file loading — our `LoadBoard` path).
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Triangulated runtime expectation for KiCad: **1.5–3× on load/parse/geometry paths**, more
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on exception-dense I/O paths.
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## §3. Reproducing
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```sh
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# control (uses the project script unmodified):
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./scripts/common/apply-asyncify.sh output/pcbnew.finalized.wasm output/measure/pcbnew.asyncify-full.wasm
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# proxy: same script with ASYNCIFY_IMPORTS="env.__asyncjs__*,env.emscripten_fiber_swap"
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# floor: $(scripts/common/get-wasm-opt.sh | tail -1) -O2 in.wasm -o out.wasm
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# then: ls -l + gzip -9c | wc -c
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```
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