WebAssembly optimized code caching?

[Michael Hagar]

Is the description here : https://v8.dev/docs/wasm-compilation-pipeline#code-caching still accurate?

I was trying to confirm that using WebAssembly.compileStreaming would cache the optimized code, but I couldn't find the v8.wasm.cachedModule logs in the trace, nor the v8.wasm.moduleCacheHit logs.

Is there any case where Chrome will cache optimized WASM code in the case that WebAssembly.instantiate is used? Or does it strictly require that WebAssembly.compileStreaming or WebAssembly.instantiateStreaming be used?

[Andreas Haas]

Hi,

The description should still be accurate. Did you make sure that enough code gets optimized to trigger caching?

Code does not get cached for WebAssembly.instantiate because we use the url of the wasm module as the cache key, and with WebAssembly.instantiate we don't know the url where the module came from.

Cheers, Andreas

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[Michael Hagar]

Thanks Andreas - what confused me is this comment on the Firefox bug tracker: https://bugzilla.mozilla.org/show_bug.cgi?id=1763375#c1 . I took that to mean that Chrome would cache compiled code for regular WebAssembly.instantiate, but I guess that's not the case, and it's only for the streaming functions.

When you say "enough code", what are the current thresholds for caching the module?

[Andreas Haas]

Hi,

The threshold is defined in [1]. The Firefox issue is talking about a different caching, that I assume you are not talking about. There is an in-process cache of wasm modules, so if you compile the same module twice, then the second compilation will just be a cache lookup. But when you restart Chrome, or open a different web page, then this cache is not available.

Cheers, Andreas

[1] https://source.chromium.org/chromium/chromium/src/+/main:v8/src/flags/flag-definitions.h;l=1330;drc=7972c3d25cbbd57b38bb275c36c105a7e03bcacb

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