Aliaksandr Kalenik 74e2dac929 LibJS: Avoid ropes for short flat string concatenations ...

Short string concatenations are a common allocation churn pattern in JS
execution. Many of these results are immediately observed as flat
strings, so the intended win is to avoid spending GC and flattening work
on an intermediate representation that does not carry its weight.

Microbenchmark:

    const n = 10_000_000;
    function bench(a, b) {
        let total = 0;
        for (let i = 0; i < n; ++i)
            total += (a + b).length;
        if (total !== n * 2)
            throw new Error(String(total));
    }
    bench("a", "b");

Measured with hyperfine against the same build with the fast path
disabled:

    baseline:  822.7 ms +/- 25.7 ms
    optimized: 385.5 ms +/- 21.5 ms
    speedup:   2.13 +/- 0.14 times faster

2026-04-30 16:25:52 +02:00

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