[Webkit-unassigned] [Bug 239936] New: TypedArray.from is ~60x slower than in V8

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=239936

            Bug ID: 239936
           Summary: TypedArray.from is ~60x slower than in V8
           Product: WebKit
           Version: WebKit Nightly Build
          Hardware: Unspecified
                OS: Unspecified
            Status: NEW
          Severity: Normal
          Priority: P2
         Component: JavaScriptCore
          Assignee: webkit-unassigned at lists.webkit.org
          Reporter: jarred at jarredsumner.com

Created attachment 458644

  --> https://bugs.webkit.org/attachment.cgi?id=458644&action=review

microbenchmark

TypedArray.from(otherTypedArrayView) uses Symbol.iterator when it could use memmove() or possibly memcpy.

A microbenchmark is attached that runs in both JSC shell and node.

On macOS 12.3 aarch64 M1X

For 1 MB:
- JSC: 22ms
- Node : 0.13ms

For 32 MB:
- JSC: 274ms
- Node: 4ms


If TypedArray.prototype.set[0] is used in TypedArrayConstructor.js when !mapFn && @isTypedArrayView(arrayLike), that becomes:

For 1 MB:
- JSC: 22ms
- JSC (modified): 0.4ms
- v8 9.6 (Node 17.7.1): 0.13ms

For 32 MB:
- JSC: 274ms
- JSC (modified): 5ms
- v8 9.6 (Node 17.7.1): 4ms


It isn't precisely correct to use TypedArray.prototype.set, but it shows roughly what the numbers would look like after this is fixed. It does seem that V8 is consistently slightly faster after this change though, which means there is still something more to do here. Maybe it could allocate uninitialized memory because it will copy directly from the other typed array view?

V8's optimization for TypedArray.from: https://github.com/v8/v8/blob/f32335fea75b7bde495e0800d7f7349253f81a7c/src/builtins/typed-array-from.tq#L167

[0]: https://gist.github.com/Jarred-Sumner/543b94142de9f17a9ec86e9dac5cf171#file-typedarrayconstructor-js

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