Enable relaxed-simd-deterministic for Viceroy.

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

Author: dgohman-fastly

src/execute.rs

@@ -1025,6 +1025,15 @@ fn configure_wasmtime(
         config.wasm_component_model(true);
     }
 
+    // Wasm permits the "relaxed" instructions to be nondeterministic
+    // between runs, but requires them to be deterministic within runs.
+    // Snapshotting a program's execution to avoid redundantly running
+    // initialization code on each request is an important optimization,
+    // so we enable deterministic lowerings for relaxed SIMD to ensure
+    // that it works consistently even if the initialization runs on a
+    // different host architecture.
+    c.wasm_relaxed_simd_deterministic(true);
+
     config
 }