buffer: optimize Buffer.prototype.copy

[ronag]

Route the native backing of Buffer.prototype.copy (CopyImpl, the _copy binding) through V8's new v8::ArrayBuffer::CopyArrayBufferBytes API instead of materializing an ArrayBufferViewContents and doing a manual memmove. This speeds up partial copies (sourceStart > 0). All copies now go through this binding: the previous %TypedArray%.prototype.set fast-path for whole-buffer copies is dropped, since it would throw on a detached or immutable target rather than report a 0-byte no-op, and the benchmarks below show the native path is comparable for that case.

V8 clamps the byte range and handles detached and immutable buffers. When both sides are backed by a SharedArrayBuffer the relaxed-atomic overload is used, which honors the SharedArrayBuffer memory model. Mixed copies (one SharedArrayBuffer, one ArrayBuffer) use the regular overload.

copy() now reports the number of bytes actually copied, as returned by V8: 0 when the target is backed by a detached or immutable ArrayBuffer. The copy is then a no-op rather than a write to read-only memory.

buffer-copy.js vs node v26.3.0 (x64, 30 runs):
partial=false bytes=1024: +7.91% ()
partial=false bytes=128: +0.17%
partial=false bytes=8: -0.89%
partial=true bytes=1024: +22.33% ()
partial=true bytes=128: +19.58% ()
partial=true bytes=8: +18.70% ()

The V8 API is carried as a floating patch cherry-picked from:

https://chromium-review.googlesource.com/c/v8/v8/+/7735151

That CL is authored by a Node.js collaborator (Ben Noordhuis) and is expected to land upstream, so the floating patch should be easy to maintain: it touches nothing but the public ArrayBuffer API and can be dropped wholesale on the next V8 upgrade that includes it. v8_embedder_string is bumped to -node.21 accordingly.

This supersedes the prototype in
#62491, which added a bespoke ArrayBufferView::FastCopy instead of using the upstream-friendly CopyArrayBufferBytes API.

Adds SharedArrayBuffer and immutable-ArrayBuffer coverage to the buffer copy tests.

Assisted-by: Claude Opus 4.8 (1M context) noreply@anthropic.com

[nodejs-github-bot]

Review requested:

• @nodejs/gyp
• @nodejs/security-wg
• @nodejs/v8-update

[ronag]

https://ci.nodejs.org/view/Node.js%20benchmark/job/benchmark-node-micro-benchmarks/1868/

[Copilot]

Pull request overview

This PR updates Node’s Buffer.prototype.copy() implementation to use V8’s new ArrayBuffer::CopyArrayBufferBytes / SharedArrayBuffer::CopyArrayBufferBytes APIs, improving performance for partial copies and ensuring detached/immutable targets become no-op copies that report 0 bytes copied.

Changes:

• Route the native _copy binding through V8’s new CopyArrayBufferBytes API (including relaxed-atomic handling when both sides are SharedArrayBuffer-backed).
• Update JS-side copy() to return the actual number of bytes copied (as reported by the native binding), including 0 for immutable/detached targets.
• Add test coverage for SharedArrayBuffer copies and immutable ArrayBuffer targets, plus V8 cctests and API header/docs updates.

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
test/parallel/test-buffer-copy.js	Adds SharedArrayBuffer copy coverage (including overlap + byteOffset cases).
test/parallel/test-buffer-copy-immutable.js	Adds coverage for copying into immutable ArrayBuffer-backed targets (expects 0 bytes copied).
src/node_buffer.cc	Reworks native copy to call V8 CopyArrayBufferBytes and return actual copied byte count.
lib/buffer.js	Simplifies JS copy path to rely on native _copy return value for bytes-copied semantics.
deps/v8/test/cctest/test-api-array-buffer.cc	Adds V8 cctests for the new CopyArrayBufferBytes APIs (detached + immutable cases).
deps/v8/src/api/api.cc	Implements ArrayBuffer::CopyArrayBufferBytes and SharedArrayBuffer::CopyArrayBufferBytes.
deps/v8/include/v8-array-buffer.h	Exposes the new public V8 API methods and documents their behavior.
common.gypi	Bumps v8_embedder_string to reflect the added floating V8 patch.

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[Copilot]

Pull request overview

Copilot reviewed 8 out of 8 changed files in this pull request and generated no new comments.

[marco-ippolito]

can you split the v8 patch commit and the implementation commit?

[bnoordhuis]

That CL is authored by a Node.js collaborator (Ben Noordhuis) and is expected to land upstream

For posterity, said CL was merged upstream earlier today.