Bug 294247 – [JSC] Skip index-type update in `tryCloneArrayFromFast` when `fillMode` is `Empty`

• Component: JavaScriptCore
• Status: RESOLVED FIXED

[Sosuke Suzuki]

`tryCloneArrayFromFast` adjusts the result’s index type
    if the source array contains holes, on the assumption
    that the holes will be filled with `undefined`.
    
    This adjustment is valid only when `fillMode` is `Undefined`,
    but the current implementation also performs it when
    `fillMode` is `Empty`.
    
    This patch changes to prevents the index-type update when
    `fillMode` is `Empty`.
    
                                            TipOfTree                  Patched
    
    array-prototype-concat-empty            7.7375+-0.1340     ^      5.3688+-0.1247        ^ definitely 1.4412x faster

[Sosuke Suzuki]

Pull request: https://github.com/WebKit/WebKit/pull/46536

[EWS]

Committed 296082@main (14ddbfed34e8): <https://commits.webkit.org/296082@main>

Reviewed commits have been landed. Closing PR #46536 and removing active labels.

[Radar WebKit Bug Importer]

<rdar://problem/153064115>