Spider

Spider is a compiler that translates WebAssembly binaries into lua-no-ffi, lua-jit, luau, and json targets.

The current project story is simple:

• lua-no-ffi is now a real working target, not a side experiment.
• Spider is measured on full translated real-world fixtures, not toy snippets.
• The old LuaJIT limits around upvalue, scope, and giant generated closures are no longer treated as "original language limits we must accept"; they are compiler problems that Spider now attacks directly.
• The legacy ffi path still exists, and Windows support for that route was also extended through a machine-instructions-based implementation path.

Short measurement table

Full numbers live in docs/notes/lua-no-ffi-measurements.md. This is the reduced front-page table.

Fixture	Workload	Wasmtime	lua-no-ffi	lua-jit / host note	Size signal
real-world-binjgb	canonical frame_limit = 16	486.18 ms	4091.13 ms	lua-jit: full details in measurements	.wasm: 134,905 B, Lua: 1,576,674 B
real-world-binjgb-host-all	--all-frames 16	188.04 ms	10707.79 ms	host-only row, no lua-jit number	host workflow row in full table
real-world-plmpeg-host-all	--all-frames 12 on canonical sample	66.33 ms	1800.14 ms	compare against plmpeg-stream below	host comparison lives in measurements
real-world-plmpeg-stream-host-all	--all-frames 12 streaming path	40.38 ms	1168.06 ms	sequential stateful path	host comparison lives in measurements
real-world-h264bsd-mp4	canonical MP4 probe set	526.91 ms	7544.49 ms	control target in full table	.wasm: 165,414 B, Lua: 2,184,930 B
real-world-libjpeg-turbo-mjpeg	canonical MJPEG probe set / host-all	102.57 ms	3845.82 ms	host-all 12: 92.18 ms vs 870.81 ms	.wasm: 375,047 B, Lua: 3,678,957 B
self-hosting-luanoffi-builder	generated module snapshot	n/a	current generated lua-no-ffi loads past previous top-level blocker	lua-jit still has a deep upvalue case	.wasm: 165,494 B, generated lua-no-ffi: 2,176,834 B

This is not the full matrix. The complete runtime and size tables, including more fixtures and the detailed comparisons against the old path, are in docs/notes/lua-no-ffi-measurements.md.

What Spider is selling now

Spider now argues a stronger point than before: Wasm can be lowered into plain Lua-family code without centering the whole runtime story on ffi, and that claim is being tested on full translated workloads rather than toy examples.

lua-no-ffi is the main target

lua-no-ffi is now the project's center of gravity because it is benchmarked directly against the older lua-jit route, it is where backend work is paying off, and it gives Spider a more portable path with less dependence on host ffi.

Read these first:

• Notes Index
• Lua No FFI Overview
• Lua No FFI Measurements
• Windows FFI Machine Instructions

Ten strong translated projects / fixtures

Spider is already being pushed on a broad translated workload set. Important examples:

1. tests/manual/real-world-plmpeg
2. tests/manual/real-world-plmpeg-stream
3. tests/manual/real-world-h264bsd-mp4
4. tests/manual/real-world-libjpeg-turbo
5. tests/manual/real-world-libjpeg-turbo-mjpeg
6. tests/manual/real-world-binjgb
7. tests/manual/self-hosting-luanoffi-builder
8. tests/manual/real-world-lodepng
9. tests/manual/real-world-miniz
10. tests/manual/real-world-tinyexpr

This is the point of the repository now: not "can it compile a tiny wasm," but "how far can the backend go on real translated systems?"

What changed versus the old Spider story

The old reading of Spider was "generate Lua, rely on ffi, and treat LuaJIT limits as fixed"; the current reading is "push lua-no-ffi as a first-class target, measure it on real fixtures, and reshape generated code when those limits become a compiler problem instead of an excuse."

Important notes

If you want the current state quickly, read these in order:

1. docs/notes/lua-no-ffi-overview.md
2. docs/notes/lua-no-ffi-measurements.md
3. tests/manual/self-hosting-luanoffi-builder/README.md

If you want current open edges instead of the success story:

• docs/notes/lua-no-ffi-plmpeg-open-problem.md
• docs/notes/lua-no-ffi-smollm2-open-problem.md

Targets

lua-no-ffi

The main target lowers Wasm into plain LuaJIT-compatible code without making host ffi the core dependency.

lua-jit

The legacy target keeps the classic ffi-based fast path as a compatibility and comparison baseline.

luau

The Luau target emits Luau-oriented output for that runtime family.

json

The JSON target emits a structural representation useful for debugging and inspection.

Repository layout

Spider/
├── CLI/                    # Command-line interface
├── Conformance/            # Conformance tests
├── IR/                     # Intermediate Representation
├── Sources/                # Source lifters / frontends
├── Targets/                # Output targets
├── Tools/                  # Helper tooling
├── docs/notes/             # Measurements, status notes, open problems
└── tests/manual/           # Real-world manual fixtures

Build

cargo build --release

Run

./target/release/spider-cli.exe input.wasm -t lua-no-ffi
./target/release/spider-cli.exe input.wasm -t lua-jit
./target/release/spider-cli.exe input.wasm -t luau
./target/release/spider-cli.exe input.wasm -t json

Tests

cargo test
cargo test -p conformance --test luajit
cargo test -p conformance --test luau