Summary

When slicing a rope string across fiber boundaries, the previous implementation would resolve the entire rope string, causing O(n) complexity per slice operation. This leads to O(n²) overall complexity when iterating through a concatenated string with slice operations.

Changes

The fix modifies tryJSSubstringImpl in JSString.h to handle cross-fiber slices by extracting substrings from each relevant fiber and concatenating them into a new rope, avoiding the need to resolve the entire original rope.

Performance Impact

Before this fix (50k iterations of slice on a 50k character rope string):

• Bun: ~2800ms
• Node.js: ~2ms

After this fix, Bun should match Node.js performance.

Reproduction

let s = '';
for (let i = 0; i < 50000; i++) s += 'A';

let start = Date.now();
let m = new Map();
for (let i = 0; i < 50000; i++) {
    let k = s.slice(i, i+1);
    m.set(k, 1);
}
console.log(`Time: ${Date.now() - start}ms`);

Fixes oven-sh/bun#26682