Bug 303605 – [JSC] Optimize 2 args `String#concat` call

• Status: RESOLVED
• Resolution: FIXED
• Component: JavaScriptCore

[Sosuke Suzuki]

This patch changes to optimize 2 args `String#concat`.

    Transpilers like Babel transpile template literals such as `${foo}${bar}`
    into a 2 args `String#concat`[1]. In fact, `wasm/tfjs-bundle.js` from JetStream3
    contains hundreds of 2 arguments `String#concat`.

                                            TipOfTree                  Patched

    string-prototype-concat-2-args        7.8642+-0.3652     ^      1.9200+-0.2046        ^ definitely 4.0960x faster

    [1]: https://babeljs.io/repl#?config_lz=N4IgZglgNgpgdgQwLYxALhAJxgBygOgCsBnEAGhB22JgBdS0BtRkeAN3NFoUwHM6GICDAAEARjHkQSAPYATAK6wGYBFBoBfALpaKxGQswBjGABUAnjlQZZi2CA1A&code_lz=AYEg3gZg9lC-4CMCGAnWwDcQ&lineWrap=true&version=7.28.5

[Sosuke Suzuki]

Pull request: https://github.com/WebKit/WebKit/pull/54900

[EWS]

Committed 303971@main (ffad432a5d1a): <https://commits.webkit.org/303971@main>

Reviewed commits have been landed. Closing PR #54900 and removing active labels.

[Radar WebKit Bug Importer]

<rdar://problem/165905684>