[Webkit-unassigned] [Bug 243148] New: TypedArray slice should use copy-on-write

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=243148

            Bug ID: 243148
           Summary: TypedArray slice should use copy-on-write
           Product: WebKit
           Version: Safari 15
          Hardware: Unspecified
                OS: Unspecified
            Status: NEW
          Severity: Normal
          Priority: P2
         Component: JavaScriptCore
          Assignee: webkit-unassigned at lists.webkit.org
          Reporter: dev at sheetjs.com

Parts of the SheetJS library will create many small slices from Uint8Arrays.  A typical use case is performing a shallow parse of a protobuf message (https://github.com/SheetJS/sheetjs/blob/master/modules/83_numbers.ts#L145 for example)

In V8, changing `buf.slice(start, end)` to `buf.subarray(start, end)` results in nearly identical performance.  If we had to speculate, when code only reads from the result, `slice` does not create a new copy of the data.

In Safari, changing `buf.slice(start, end)` to `buf.subarray(start, end)` results in a significant difference in running time. If we had to speculate, Safari is creating new copies with each slice even if the individual slices are not modified.

The performance difference is notable: ~20% savings when testing in the Bun runtime. It shaved nearly 80 seconds (from 453 to 373 seconds) from the browser test suite run time in iOS 11.3.

Given the evolution of `Buffer` in NodeJS and V8's performance profile, many isomorphic libraries use `slice` when `subarray` would have sufficed.

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