Enable relaxed-simd-deterministic for Viceroy. #539
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The Wasm spec defines several [relaxed SIMD instructions], which are permitted to have nondeterministic results, in order to accommodate differences between hardware CPU architectures. However, rather than being fully nondeterministic, it requires them to be deterministic within a run of a program, so that programs can assume that whatever behavior they see the first time they use a relaxed SIMD instruction will continue to be the behavior for the rest of the run of the program. Fastly's Compute platform makes use of snapshotting using [wizer] to optimize program startup times. This involves running the program initialization on one machine, snapshotting the program, and then resuming the program on another. To ensure that the relaxed SIMD instructions don't change behavior across the snapshot and resume, enable the deterministic lowerings. [relaxed SIMD instructions]: https://webassembly.github.io/spec/core/exec/numerics.html#relaxed-operations [wizer]: https://github.com/bytecodealliance/wizer
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chenyan2002
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Oct 16, 2025
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The Wasm spec defines several relaxed SIMD instructions, which are permitted to have nondeterministic results, in order to accommodate differences between hardware CPU architectures. However, rather than being fully nondeterministic, it requires them to be deterministic within a run of a program, so that programs can assume that whatever behavior they see the first time they use a relaxed SIMD instruction will continue to be the behavior for the rest of the run of the program.
Fastly's Compute platform makes use of snapshotting using wizer to optimize program startup times. This involves running the program initialization on one machine, snapshotting the program, and then resuming the program on another. To ensure that the relaxed SIMD instructions don't change behavior across the snapshot and resume, enable the deterministic lowerings.