Fix O(n²) complexity for String.prototype.slice() on rope strings

[robobun]

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

[coderabbitai]

Walkthrough

Reworked tryJSSubstringImpl to extract substrings crossing multiple rope fibers by slicing each involved fiber and constructing new ropes from the resulting parts, replacing previous NULL-return fallback behavior.

Changes

Cohort / File(s)	Summary
Rope Substring Extraction Source/JavaScriptCore/runtime/JSString.h	Enhanced tryJSSubstringImpl to handle multi-fiber substring extraction without full rope resolution. When substrings span fiber0 to fiber1 (and optionally fiber2), the function now slices each fiber and constructs a new rope from parts instead of returning NULL. Added logic for two-fiber and three-fiber crossing scenarios with offset adjustments.

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title 'Fix O(n²) complexity for String.prototype.slice() on rope strings' directly and clearly describes the main change—optimizing string slicing performance by handling cross-fiber rope strings efficiently.
Description check	✅ Passed	The description is well-structured with Summary, Changes, Performance Impact, and Reproduction sections explaining the fix clearly, though it does not follow the WebKit template format with Bugzilla link and commit message structure.
Linked Issues check	✅ Passed	The PR successfully addresses issue #26682 by implementing multi-fiber substring extraction to avoid resolving entire ropes, transforming O(n²) slice operations into linear time operations.
Out of Scope Changes check	✅ Passed	All changes in JSString.h are focused on the tryJSSubstringImpl function to handle cross-fiber rope slices, directly addressing the performance issue identified in #26682 with no extraneous modifications.

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