feat: OCCT V8.0.0 + Emscripten 5.0.1 with native WASM exceptions

[rifont]

Note from @rifont : this PR is still under active development and testing, it has thus far been successfully integrated into replicad-opencascadejs but needs further testing to validate the full build.

TLDR:

• 20-30% faster computation across most CAD operations
• Native WebAssembly Exception handling for zero-cost performance exception handling at cost of ~15% larger WASM when comparing non-exceptions to with-exceptions build
• Custom build pipeline with incremental caching to make development iterations substanitally faster, time from clone to .wasm assets on Apple M-series is now ~15 minutes with -O0 compile and -O0 linking. Includes build provenance system for easier tracking of optimization inputs to WASM benchmarking perf.
• Full Doxygen C++ inline documentation parsing to produce jsdoc for Typescript bindings to promote easier OpenCascade.js API consumption+integration for improved developer experience (see img below for jsdoc preview)

Testing & Benchmarking Evidence

This migration was developed and validated in the Tau monorepo, which uses opencascade.js in production via replicad. The full testing and benchmarking harness is available at taucad/tau@b7e12e8:

• Benchmark suite: packages/kernels/src/benchmarks/ — primitives, booleans, fillets, extrusions, complex models, stress tests
• Kernel tests: packages/kernels/src/kernels/replicad/ — 801/801 tests passing with V8 builds
• Experiment configs: scripts/experiments/ — YAML configs for each build variant tested
• Build comparison reports: tarballs/comparisons/ — HTML dashboards comparing size and performance across V7.6.2 vs V8 (single, exceptions, max-perf)
• Experiment results: tarballs/experiments/ — provenance.json + benchmark JSON/HTML for each build variant
• WASM build harness: scripts/wasm-experiment.sh — full lifecycle runner (build → pack → install → benchmark → report)

Summary

Upgrades opencascade.js from OCCT V7.6.2 / Emscripten 3.1.14 to OCCT V8.0.0-RC4 / Emscripten 5.0.1, bringing substantial performance improvements, modern WASM exception handling, and a developer-friendly build system.

Key highlights:

• OCCT V8: 1,085 upstream commits — 22-31% faster boolean operations, 16-19% faster fillets, 23-29% faster complex models
• Native WASM Exceptions: Replaces Emscripten's JavaScript invoke trampolines with the WASM Exception Handling proposal (Phase 4), reducing exception build size overhead from ~80% to ~12% gzipped with zero happy-path performance cost
• Build System: New build-wasm.sh entry point with compilation caching, presets, provenance tracking, and a validate command
• Reproducible Builds: DEPS.json pins all dependencies to exact commit hashes; clone-deps.sh automates setup
• Docker + Native: Updated Dockerfile with pinned base image digest and env var passthrough; native build path documented with 5-command quick start

This PR incorporates the bindings generation speedups from #292 as part of the V8 migration work.

Performance

Benchmarks comparing V7.6.2 to V8 (single-threaded, noLTO):

Category	Improvement
Booleans	22-31% faster
Fillets	16-19% faster
Complex models	23-29% faster
Sketches	9-13% faster

Size (Gzipped) (testing with replicad-opencascadejs build)

Build	V7.6.2	V8	Change
Single (no exceptions)	6.05 MB	5.65 MB	-6.6%
With exceptions	10.42 MB	6.35 MB	-39.1%
Exception overhead	+72.2%	+12.4%

The dramatic exception size reduction comes from replacing Emscripten's JavaScript invoke trampolines (which generated a JS wrapper for every potentially-throwing call site) with native WASM throw/catch instructions. Browser support: 94.5%+ (Chrome 95+, Firefox 100+, Safari 15.2+).

Build System

Quick Start (Native)

git clone https://github.com/donalffons/opencascade.js.git && cd opencascade.js
./scripts/clone-deps.sh          # Clones OCCT, rapidjson, freetype at pinned commits
pip install -r requirements.txt  # Python build deps
OCJS_LTO=0 ./build-wasm.sh full build-configs/full.yml

Docker

docker build -t opencascade-js .
docker run --rm -v $(pwd)/output:/output opencascade-js full build-configs/full.yml

Presets

./build-wasm.sh --preset O2-balanced full build-configs/full.yml   # Recommended
./build-wasm.sh --preset O3-maxperf full build-configs/full.yml    # Max speed
./build-wasm.sh --preset Os-minsize full build-configs/full.yml    # Min size
./build-wasm.sh --preset O0-debug full build-configs/full.yml      # Fast builds

New Features

• ./build-wasm.sh --help — full usage with examples
• ./build-wasm.sh validate <yaml> — check config without building
• ./build-wasm.sh --preset <name> — apply optimization presets
• Compilation cache: skips ~30min compilation on config match
• Build provenance: provenance.json sidecar traces exact source versions
• Build summary: WASM/JS/types sizes with gzip at completion

OCCT V8 Breaking Changes

10 systemic API changes that affect downstream consumers:

1. TopoDS_Shape::HashCode removed → use OCJS_ShapeHasher wrapper
2. TopoDS namespace not directly bindable → use TopoDS_Cast wrapper
3. BRepMesh_IncrementalMesh constructor changed → use BRepMesh_IncrementalMeshWrapper
4. Handle_* types need explicit typedef in additionalCppCode
5. Bnd_Box::Get() removed → CornerMin()/CornerMax()
6. Poly_Triangulation normals API changed
7. Poly_PolygonOnTriangulation::Nodes() removed → NbNodes() + Node(i)
8. Constructor renumbering (e.g. gp_Ax2_3 → _4)
9. Method overload suffix changes (e.g. SetValue → SetValue_1)
10. Bnd_Box2d::Get() removed → individual accessors

Full migration guide: docs/occt-v8-migration.md

New Files

File	Purpose
DEPS.json	Pinned dependency commits for reproducible builds
requirements.txt	Python build dependencies
build-wasm.sh	Unified build entry point
scripts/clone-deps.sh	Automated dependency cloning
scripts/docker-e2e-validate.sh	Docker E2E validation
src/build-cache.py	Config-keyed compilation cache
src/provenance.py	Build metadata tracking
src/patches/patch_standard_dump.py	Optional OCCT DumpJson stub
build-configs/full.yml	233 symbols, no exceptions
build-configs/full-exceptions.yml	235 symbols, native WASM exceptions
build-configs/presets/*.yml	Optimization presets
docs/occt-v8-migration.md	V8 migration guide
docs/optimization-guide.md	Build optimization reference
docs/build-config-reference.md	YAML schema documentation

Test Plan

• All replicad kernel tests passing (801/801) with V8 WASM builds
• All 8 example models passing with single-exceptions WASM variant (tray, birdhouse, bottle, gridfinity-box, vase, wavy-vase, cycloidal-gear, projection-test)
• Full benchmark suite (18 benchmarks) passing for both single and exceptions variants
• Both full.yml (no exceptions) and full-exceptions.yml (WASM exceptions) produce valid WASM/JS/DTS output
• Full native dev flow validated from scratch: clone-deps.sh → emsdk activation → Python deps → -O0 build → -O3 max-perf build
• Build cache correctly identifies hits/misses across optimization levels
• ./build-wasm.sh validate correctly validates and rejects YAML configs
• Docker E2E validation (scripts/docker-e2e-validate.sh) — script provided, requires ~1hr build time

Risks

• Constructor renumbering: OCCT V8 changes constructor overload numbering in Emscripten bindings. Downstream consumers using hardcoded _N suffixes will need to update. The .d.ts file provides the correct suffixes.
• Emscripten 5.x: Some Emscripten flags were removed or changed. The migration guide documents all flag changes.
• WASM EH browser support: 94.5%+ coverage. Consumers targeting older browsers can still use the no-exceptions build config.

Related Issues & PRs

• Closes Update opencascade to 7.8.0 #270 Update to a newer version of Opencascade #299 — updates OCCT from 7.6.2 to V8.0.0-RC4
• Closes Make bindings generation dramatically faster #292 — bindings generation speedups by @jwatzman
• Closes Project Maintenance Status? #273 — demonstrates active maintenance with major upgrade
• Closes how to build my own opencascade.js docker image and get opencascade.wasm? #278 — updated Dockerfile with documentation for building custom WASM
• Closes Few errors during custom build #272 — modernized build system with validate command and clear error messages
• Closes How to build only a few specific modules of occ #265 — documentation for building with specific modules via custom YAML configs

AI Disclosure

• AI assistance used: yes
• Model: Claude claude-4.6-opus (Anthropic)
• Scope of AI assistance: implementation of build system changes (DEPS.json, clone-deps.sh, build-wasm.sh UX, Docker E2E script, presets), documentation (migration guide, optimization guide, config reference, README), and PR description drafting
• Human verification: all WASM builds validated manually, benchmark results verified, kernel tests run, build cache behavior confirmed

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[rifont]

OCJS v8 API design discussions are underway in CodeCAD Discord server, under the replicad channel here.

[zalo]

Oh yeah, nice, I have the same upgrade here: master...zalo:opencascade.js:cascadestudio-v2
Deployed to: https://github.com/zalo/CascadeStudio

It looks like you had to deal with the same pain with the RC4 upgrade that I did 😄

That said, I suspect my build may be smaller because I trim out a lot of unnecessary files 🤔

This guy also has an entry for OpenCascade 8.0.0: https://github.com/andymai/brepjs