Keep slice-of-slice and split segments zero-copy#2720
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Two zero-copy String optimizations building on the existing SlicedString view:
- Add JsString.CreateSliced(JsString, int, int) that rebases a slice of an
un-materialized SlicedString directly onto its original backing string,
evaluating the retention policy against that original source so chained views
cannot compound the pinning of a large backing string. A materialized view or
any other receiver falls back to the flat path. Slice/Substring/Substr now
pass the JsString receiver instead of s.ToString(), so a slice-of-slice never
materializes the intermediate view. The copy fallback now routes through
Create so short results still hit the cached single-char / empty instances.
- Split now routes each segment and the tail through CreateSliced (policy
decides copy vs view against the source length), and gains a dedicated
split("") fast path that builds an exact-capacity array from cached single-char
JsStrings, skipping the intermediate segment list. Segment list element type
changed from string to JsString accordingly.
Custom JsString subclasses keep the existing plain path (no new materialization).
Adds a SliceOfSlice benchmark and unit tests for slice-of-slice correctness,
split("") on ASCII/non-ASCII, split tail/segment correctness, and the retention
policy still copying small results.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
This was referenced Jul 21, 2026
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Jul 22, 2026
…tring substrings (#2740) Three review follow-ups to the #2720 zero-copy slices: - JsString.Equals(JsString) gets a length pre-check: comparing a short string against a large unmaterialized view materialized the whole view just to fail the compare. Every Length override answers without materializing. - substring()/substr() gain the whole-string identity shortcut slice() already had: strings are immutable, so s.substring(0) returns the receiver instead of allocating a duplicate wrapper or view. - CreateSliced's whole-string branch routes through Create so 0/1-char sources (e.g. ''.split('x') segments) reuse the cached instances. Claude-Session: https://claude.ai/code/session_0115tQFNyyQqc1HQGPLUgZND Co-authored-by: Claude Fable 5 <noreply@anthropic.com>
This was referenced Jul 22, 2026
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Part of the V8-gap performance campaign (#1775 follow-up). Two increments on the existing
SlicedStringzero-copy machinery.Slice-of-slice stays zero-copy
str.slice(a,b).slice(c,d)materialized the intermediate view:Slice/Substring/Substrpasseds.ToString()intoCreateSliced. A new internalJsString.CreateSliced(JsString source, start, length)overload rebases a not-yet-materializedSlicedStringreceiver directly onto its original backing string — and evaluates the retention policy against that original source length, so chained views cannot compound the pinning of a large backing string. Materialized views and all other receivers (includingCustomStringsubclasses) take today's flat path.Split cheapening
SplitWithStringSeparatorroutes every segment and the tail throughCreateSliced, letting the policy decide copy vs view per segment, andsplit("")gains a fast path building an exact-capacity array straight from the cached single-charJsStrings.Numbers (default BDN job, A/B vs current main, i.e. after #2717)
Gates
Jint.Tests 3661/3598 (net10/net472), PublicInterface 114/114 (CustomString non-materialization tripwire green), Test262 99,431 passed / 0 failed. Rebased on current main (includes #2716–#2718); new tests cover chained-slice value equality across slice/substring/substr, view retention policy against the original source,
split("")for ASCII/BMP/surrogate pairs, and split segment/tail correctness.🤖 Generated with Claude Code