The install steps' comment said every WAI workflow that runs wasmtime pins one
version and passes both determinism settings, but nothing enforced it: a step
that ran a stock or differently pinned wasmtime would have merged green.
scripts/check-wasmtime-pin.py now fails, in the repository checks, any workflow
that
1. downloads a wasmtime release other than 50.0.1, or checks it against any
SHA-256 but that archive's;
2. installs wasmtime another way (cargo install, the install script, an action);
3. runs wasmtime (run, serve, compile, or a module given to it) or names it as
a CARGO_TARGET_*_RUNNER without -W nan-canonicalization=y and
-W relaxed-simd-deterministic=y.
It also fails when it finds no download or no run, so a check that looked in
the wrong place cannot pass. On this tree it finds the downloads and runs it
should. Its self-test (scripts/check-wasmtime-pin-selftest.sh) passes a scratch
workflow shaped like these and fails each way of breaking it that the
self-test lists, matching the reason in the output.
The comment's relaxed_madd claim was measured on arm64 only. It now rests on
both architectures, measured on an arm64 macOS host and an x86_64 Linux host
with a small probe (wasmtime 50.0.1's release for the host, checked against
the digest the release lists):
host Darwin arm64; wasmtime 50.0.1 (c307ec81d 2026-10-29)
f32.add(bits 0x7fa00001, 1.0) [no flags]: 0x7fe00001
f32x4.relaxed_madd(1+2^-12, 1+2^-12, -(1+2^-11)) [no flags]: 0x33800000
f32.add(bits 0x7fa00001, 1.0) [both flags]: 0x7fc00000
f32x4.relaxed_madd(1+2^-12, 1+2^-12, -(1+2^-11)) [both flags]: 0x33800000
an unknown -W option is refused: error: failed to parse -W / --wasm option:
unknown option: no-such-option
host Linux x86_64; wasmtime 50.0.1 (c307ec81d 2026-10-29)
(the same five lines, bit for bit)
0x33800000 is 2^-24, the fused result; the unfused fadd(fmul) gives 0. So the
NaN setting takes effect on both, and relaxed_madd is fused on both with and
without the relaxed setting. That matches 50.0.1's source (the
wasmtime-internal-cranelift crate, read from the registry): the translator
lowers F32x4/F64x2RelaxedMadd through fma_f32x4/fma_f64x2
(src/translate/code_translator.rs L2530-2539), which emit a native fma on a
host that has one, a fused library call under relaxed-simd-deterministic on a
host that has none, and the unfused fmul and fadd only on a host with no FMA
and the setting off (src/func_environ.rs L5907-5925). The setting is what
makes the result fused on every host. And since an unknown -W option stops
wasmtime, a run that exits 0 with the two flags has parsed them.
Co-Authored-By: Claude Opus 5.5 <[email protected]>
888cc3ba20