Branch: refs/heads/main
Home: https://github.com/WebKit/WebKit
Commit: f6d1b6eb589dcf909f1770238f5a682260990f73
https://github.com/WebKit/WebKit/commit/f6d1b6eb589dcf909f1770238f5a682260990f73
Author: Sosuke Suzuki <[email protected]>
Date: 2026-04-24 (Fri, 24 Apr 2026)
Changed paths:
A JSTests/microbenchmarks/string-pad-end-consume.js
A JSTests/microbenchmarks/string-pad-end-indexof.js
A JSTests/stress/string-pad-flat-result.js
M Source/JavaScriptCore/runtime/StringPrototype.cpp
Log Message:
-----------
[JSC] Produce flat string from `padStart` / `padEnd` for short results
https://bugs.webkit.org/show_bug.cgi?id=312740
Reviewed by Yusuke Suzuki.
Currently padStart / padEnd always return a 2-fiber rope by concatenating
the filler and `this` via jsString(). When the result is consumed (e.g.
charCodeAt, indexOf, or as an element passed to Array#join), the rope must
be resolved, allocating yet another buffer.
This patch produces a flat string directly when the result length is
<= 1024, matching the existing threshold used by String.prototype.repeat.
The filler is written via doubling memcpy and `this` is copied into the
same buffer in one allocation. The 8 / 1024 thresholds are now shared
between repeat and padStart/padEnd as maxPatternLengthForFlatRepeat /
maxResultLengthForFlatRepeat.
TipOfTree Patched
string-pad-end-indexof 94.1830+-15.9586 ^ 58.3430+-2.8945 ^
definitely 1.6143x faster
string-pad-end-consume 69.5300+-4.4738 ^ 49.9553+-4.2149 ^
definitely 1.3918x faster
Tests: JSTests/microbenchmarks/string-pad-end-consume.js
JSTests/microbenchmarks/string-pad-end-indexof.js
JSTests/stress/string-pad-flat-result.js
* JSTests/microbenchmarks/string-pad-end-consume.js: Added.
(bench):
* JSTests/microbenchmarks/string-pad-end-indexof.js: Added.
(bench):
* JSTests/stress/string-pad-flat-result.js: Added.
(shouldBe):
(shouldBe.string_appeared_here.padStart):
* Source/JavaScriptCore/runtime/StringPrototype.cpp:
(JSC::fillBufferWithPattern):
(JSC::createFillerString):
(JSC::createPaddedString):
(JSC::padString):
(JSC::JSC_DEFINE_HOST_FUNCTION):
Canonical link: https://commits.webkit.org/312007@main
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