[robobun]

Part of #23164

Problem

• Buffer.from(string) costs 957 instructions and 148 ns for a 16-char ASCII string.
• 12 of the 14 arms of constructFromEncoding (src/jsc/bindings/JSBuffer.cpp) fill a heap Vec in Rust and wrap it in an external ArrayBuffer: 3 allocations per call.

Fix

• A string of at most 128 code units encodes straight into the Buffer (utf8, utf16le, latin1): 383 instructions, 27 ns (Node: 41). A result above 128 bytes gets an ArrayBuffer from C++.
• bun:ffi ptr() gives a small view its ArrayBuffer first, so its address stays valid while the view lives (from bun:ffi: materialize a view's ArrayBuffer before ptr() returns its address #32055).
• Verified: test/js/node/buffer.test.js (1 of 25 new tests fails on main), test/js/bun/ffi/ffi.test.js (8 of 10). One sha256 over 76,377 conversions matches Node, bun 1.4.2 and main.
• Self-reviewed: 5 concerns raised, 5 addressed.

Background

• Up to 1000 bytes, a typed array can keep its bytes in GC storage with no ArrayBuffer. The first .buffer read copies them out, so they move.
• That copy sets the limit: at 256 bytes Buffer.from(s).subarray(1) got 3 to 5% slower. Copying the Vec into GC storage keeps an allocation and a free.

Downsides

• The first .buffer or subarray() of such a Buffer costs 610 instructions more. The first ptr() of a small view costs 756 (main: 125), later ones up to 21 more. Longer strings cost 3 more, and the binary grows 1,792 bytes.
• Native code must not keep the pointer of a small Buffer passed directly as a ptr argument: it still moves on .buffer, like new Uint8Array(16).
• ptr(view) of a dropped view goes bad at the next collection (main: when the memory is reused). ffi.test.js:504 did that and failed under BUN_GARBAGE_COLLECTOR_LEVEL=1.

Notes

The change, arm by arm

JSC holds a string as 8-bit (Latin-1) or as 16-bit code units. constructFromEncoding has one arm for each (string width, encoding). It is the function behind Buffer.from(string), new Buffer(string), buf.indexOf(string) and the node:crypto string arguments. On main only 8-bit latin1/ascii and 16-bit utf16le/ucs2 write into the typed array. The other arms call Rust, and JSBuffer__bufferFromPointerAndLengthAndDeinit wraps the Vec. Now, for a string of at most 128 code units:

string	encoding	how
8-bit	utf8, all ASCII	charactersAreAllASCII, then a copy
8-bit	utf8, other	simdutf::utf8_length_from_latin1, then simdutf::convert_latin1_to_utf8 into the Buffer
8-bit	utf16le, ucs2	Bun__encoding__writeLatin1 into the Buffer (at most 64 chars)
16-bit	utf8	simdutf::convert_utf16le_to_utf8_with_errors into a 384-byte stack array, then a copy. A string with a lone surrogate takes simdutf::convert_utf16le_to_utf8_with_replacement
16-bit	latin1, ascii	Bun__encoding__writeUTF16 into the Buffer

Not changed: hex, base64, base64url, every string above 128 code units, and the two arms that wrote into the typed array before (they do so at every size).

Bun__JSValue__materializeArrayBufferViewBuffer (the helper from #37154 that napi_get_buffer_info and napi_get_typedarray_info use) now acts on fast-mode views only. A view above 1000 bytes with no ArrayBuffer keeps its address when it gets one, so ptr() and the two N-API functions leave it as it is.

How the numbers were made

• Release builds of main 4b02e1031d and of this branch, Linux x64, Xeon 6975P-C.
• Instructions: gdb single-steps from the entry of the host function (constructJSBuffer for Buffer.from) to its return. BUN_JSC_useConcurrentJIT=0, one process per row, the lowest of 3 calls after 20,000 warm-up calls. perf and valgrind are not available on this machine.
• Wall clock: ns per call, one process per run, the minimum over 7 to 15 runs that alternate between the binaries. The machine is shared and loaded, so main runs twice (A and B). The difference between A and B is the noise.

Instructions per Buffer.from(string) call (utf8 unless the row names an encoding)

string	code units	result bytes	main	this PR
8-bit ASCII	1	1	887	375
8-bit ASCII	15	15	954	442
8-bit ASCII	16	16	957	383
8-bit ASCII	64	64	919	396
8-bit ASCII	128	128	929	396
8-bit ASCII	129	129	946	949
8-bit ASCII	256	256	1,301	1,304
8-bit ASCII	1,000	1,000	1,451	1,454
8-bit ASCII	4,096	4,096	2,050	2,053
8-bit, 1 char in 4 is U+00E9	16	20	1,432	524
8-bit, 1 char in 4 is U+00E9	64	80	2,764	506
8-bit, 1 char in 4 is U+00E9	100	125	3,829	633
8-bit, 1 char in 4 is U+00E9	128	160	4,636	1,007
8-bit, 1 char in 4 is U+00E9	129	161	4,737	4,740
8-bit, all U+00E9	16	32	1,531	524
8-bit, all U+00E9	64	128	2,867	506
8-bit, all U+00E9	65	130	2,909	1,006
8-bit, all U+00E9	128	256	4,941	1,353
8-bit, all U+00E9	129	258	4,993	4,996
8-bit ASCII that ends in U+00FF	127	128	1,195	781
8-bit ASCII that ends in U+00FF	128	129	1,189	985
8-bit ASCII that ends in U+00FF	129	130	1,230	1,233
16-bit, all U+3042	1	3	1,148	534
16-bit, all U+3042	16	48	1,433	525
16-bit, all U+3042	42	126	1,227	586
16-bit, all U+3042	43	129	1,227	1,057
16-bit, all U+3042	128	384	1,819	1,594
16-bit, all U+3042	129	387	1,819	1,822
16-bit, all U+3042	1,000	3,000	4,012	4,015
16-bit ASCII	16	16	1,382	476
16-bit ASCII	128	128	1,242	538
16-bit ASCII	129	129	1,242	1,245
16-bit, ASCII with 2 CJK chars in 10	64	88	1,269	596
16-bit, ASCII with 2 CJK chars in 10	128	180	1,473	1,240
16-bit, ASCII with 2 CJK chars in 10	129	181	1,473	1,476
16-bit, half lone surrogates	16	40	2,407	1,021
16-bit, half lone surrogates	128	320	9,732	4,773
16-bit, half lone surrogates	129	323	9,795	9,798
8-bit ASCII as utf16le	16	32	948	507
8-bit ASCII as utf16le	64	128	969	528
8-bit ASCII as utf16le	65	130	986	988
16-bit as utf16le	16	32	467	465
8-bit ASCII as latin1	16	16	460	459
16-bit as latin1	16	16	967	533
16-bit as latin1	128	128	967	533
16-bit as latin1	129	129	976	978
hex	16	8	1,164	1,164
hex	256	128	1,118	1,118
base64	16	12	1,389	1,389
base64	256	192	1,449	1,449

• After 300,000 calls, each row that creates an ArrayBuffer costs 346 more (main: 1,303 for 16 ASCII chars). The rows that use the Buffer's own storage stay the same (383).
• Allocator calls per call (mi_malloc / mi_realloc / mi_free): 3/0/0 on main (3/1/1 for an 8-bit string that is not ASCII). This PR: 0/0/0 for a result of at most 128 bytes, 2/0/0 for a larger result from at most 128 code units. Longer strings, hex and base64 do not change.
• Own instructions of constructFromEncoding: 41 to 41 for hex, 41 to 43 or 44 for a string above the limit.
• The same function serves buf.indexOf("abcd"): 1,131 to 619. And hash.update(16-char string): 1,005 to 431.

Wall clock, construct only (ns per call)

string	code units	main A / B	this PR	bun 1.4.2	Node 26.3
8-bit ASCII	16	149.6 / 147.1	26.9	142.6	41.4
8-bit ASCII	64	150.9 / 159.2	31.4	148.7	45.4
8-bit ASCII	128	161.0 / 163.9	37.9	161.4	47.6
8-bit ASCII	129	157.9 / 164.6	158.1	158.9	51.3
8-bit ASCII	1,024	265.6 / 258.3	256.5	239.7	85.2
8-bit ASCII	65,536	5,841 / 5,533	5,520	4,837	7,372
8-bit ASCII	1,048,576	138,578 / 131,731	142,751	113,088	82,670
8-bit, 1 char in 4 is U+00E9	16	165.6 / 171.4	35.5	170.9	62.5
8-bit, 1 char in 4 is U+00E9	64	239.6 / 241.5	36.3	251.2	45.6
8-bit, 1 char in 4 is U+00E9	100	332.1 / 343.1	50.5	325.5	51.1
8-bit, 1 char in 4 is U+00E9	128	433.0 / 444.1	171.4	418.9	52.0
8-bit, all U+00E9	64	277.2 / 258.8	39.7	252.5	46.8
8-bit, all U+00E9	128	349.5 / 386.4	182.2	335.5	54.6
16-bit, all U+3042	16	188.1 / 199.7	41.9	195.7	125.7
16-bit, all U+3042	42	181.9 / 172.6	49.7	205.7	177.0
16-bit, all U+3042	43	203.0 / 174.1	171.6	195.0	184.0
16-bit, all U+3042	128	244.1 / 238.6	218.3	219.5	392.0
16-bit, all U+3042	129	259.1 / 234.9	252.0	243.6	399.3
16-bit, ASCII with 2 CJK chars in 10	64	208.4 / 183.2	49.5	185.6	158.9
16-bit, ASCII with 2 CJK chars in 10	128	236.1 / 209.2	192.0	209.0	264.1
16-bit, half lone surrogates	64	509.1 / 547.9	269.5	518.4	213.5
16-bit, half lone surrogates	128	816.5 / 803.8	376.4	830.0	349.2

• The 64 KiB and 1 MiB rows run the same code on both builds (3 instructions more). The 1 MiB run has this branch 3% over the slower main run. The 64 KiB run has it under both main runs.
• A result above 128 bytes costs about 160 ns or more on every bun build. The cost is the ArrayBuffer, not the encode. Node does not have that step (51 ns at 129 chars).

Why 128 bytes

A Buffer with its own storage is cheap to build, and its first .buffer read, subarray() or ptr() pays for the ArrayBuffer then, plus a copy of the bytes. Construct then use, 15 runs each, ns per call (main A / main B, then this PR):

bytes	.subarray(1)	.buffer	new Uint8Array(b.buffer, b.byteOffset, b.byteLength)
16	238.3 / 222.9, 209.5	226.5 / 222.5, 206.0	256.3 / 247.3, 236.7
64	202.9 / 219.0, 209.0	242.4 / 241.3, 225.2	267.1 / 266.1, 253.6
96	192.7 / 228.5, 208.0	232.2 / 233.8, 202.5	265.2 / 269.6, 270.2
128	207.1 / 194.2, 195.1	232.3 / 234.7, 228.7	273.0 / 275.1, 263.5
129 (not changed)	190.5 / 189.4, 190.2	233.3 / 232.8, 236.1

An earlier build of this branch had a limit of 256 bytes. There .subarray(1) was 211.0 / 209.2, 219.3 at 224 bytes and 202.5 / 201.8, 208.3 at 256 bytes: 3 to 5% slower. The first build had JSC's own limit of 1000 bytes. On main, the latin1 arm has no limit, so it shows what that costs (construct then .buffer, wrapped by the utf8 arm against own storage by the latin1 arm): 244.0 against 232.5 at 128 bytes, 289.6 against 257.5 at 256, 272.7 against 290.1 at 384, 286.1 against 308.0 at 512, 353.0 against 436.4 at 1000.

The first .buffer read of a string-born Buffer costs 492 instructions on main and 1,102 (16 bytes) to 1,142 (128 bytes) here. subarray(1): 425 on main, 1,038 to 1,060 here. Above 128 bytes both are as on main.

bun:ffi

• On main, ptr() of a view of at most 1000 bytes with no ArrayBuffer returns an address in GC storage. The bytes move when the view gets its ArrayBuffer: a .buffer read, subarray(), DFG tier-up of a function that uses the view (Typed-array reads after a bun:ffi call return stale values once the calling function is JIT-compiled (DFG load elimination across native calls) #32054). new Uint8Array(16), Buffer.alloc(16), Buffer.from(array), Buffer.from(string, "latin1") and new TextEncoder().encode(string) all do this today. Buffer.from(string) did not, because it always had an ArrayBuffer. docs/runtime/ffi.mdx shows ptr(Buffer.from(string)), so this PR needs the guard.
• bun:ffi: materialize a view's ArrayBuffer before ptr() returns its address #32055 has the same guard with a helper of its own and has merge conflicts. napi: fix dropped writes through data pointers for small typed arrays #37154 merged an equal helper later. The commit here uses that helper and the tier-up test of bun:ffi: materialize a view's ArrayBuffer before ptr() returns its address #32055. When one of the two PRs merges, the other one drops that part.
• The rule, now in docs/runtime/ffi.mdx and in the JSDoc of ptr(): the address from ptr(view) is valid while the view is referenced and its ArrayBuffer is not transferred, and a view passed directly as a ptr argument is valid for the call. A transfer detaches the view and gives the same memory to the new ArrayBuffer. resize() of a resizable ArrayBuffer does not move it.
• Code that breaks the first half: the address from ptr() of a view that nothing references. On main it points into GC storage, which stays readable until something reuses it. Here it points into the view's ArrayBuffer, which the GC frees with the view. ffi.test.js:504 and :505 read through ptr(Buffer.from([...])) of a temporary. CI runs with BUN_GARBAGE_COLLECTOR_LEVEL=1, and there line 505 read one garbage character on five Linux lanes. BUN_GARBAGE_COLLECTOR_LEVEL=1 bun test test/js/bun/ffi/ffi.test.js -t primitives is the command. The test now keeps the Buffer in a constant.
• Code that breaks the second half: native code that keeps the pointer of a view passed directly. The argument conversion is in JSC (FFIConversions.cpp, the IC stub and the DFG read the vector at the call) and does not go through ptr(). A check there makes every call that passes a new small typed array pay an allocation and a copy. For Buffer.from(string) of at most 128 bytes this is new: the bytes move on the first .buffer read, subarray() or ptr(), as for every other small typed array.
• Instructions of the ptr() host function: first ptr() of a 16-byte view 125 to 756, of a 1000-byte view 125 to 840. A later ptr() of the same view 125 to 146. A view above 1000 bytes 125 to 138. An ArrayBuffer 113 to 124. A view over an ArrayBuffer 133 to 146.

Memory

• process.memoryUsage().arrayBuffers per live 16-byte result: 112 to 0. Resident memory per live result: 250 to 76 bytes (16 bytes), 361 to 221 (128 bytes).
• Eden collections for 1,000,000 Buffer.from(16-char string): 20 to 7.

Same bytes

One script hashes every result of Buffer.from(string, encoding) for 8 kinds of 8-bit string and 13 kinds of 16-bit string (ASCII, Latin-1, CJK, surrogate pairs, lone surrogates in every order, hex and base64 text with characters outside the alphabet), lengths 0 to 300 and 11 larger ones, 11 encoding arguments, plus new Buffer, indexOf, lastIndexOf, includes, createHash and createHmac with string input. The sha256 over the 76,377 conversions is af4d89005e63bcd216ffc2d60c8f31d6ef98b4bdcb700b703bab05aa285faf84 on Node 26.3, bun 1.4.2, main and this branch. With byteOffset in the hash it is equal on the three bun builds. The same script passes on the ASAN build, and on an ASAN build that sends every such result to an exact-size heap allocation, with the simdutf kernel forced to icelake, haswell and westmere.

Issue #23164

Its benchmark is Buffer.from('abcnajnbainbika').length against textEncoder.encode(...).length. ns per call: main 143.8 / 160.3 and 24.2, this PR 29.8 and 24.6, Node 68.5 and 585.1. Buffer.from is now faster than in Node, and still slower than TextEncoder.encode in Bun. The two host functions cost about the same (442 and 412 instructions). Buffer.from is a JS builtin that calls the native constructor, and that path costs about 90 instructions more per call. That is a different change.

Not done here

• hex, base64 and base64url still fill a Vec. Their result size is not known before the decode. A stack array for short inputs is a follow-up.
• Strings above 128 code units still fill a Vec. Use simdutf for Latin-1 to UTF-8 encoding in TextEncoder and Buffer.from #41595 makes the Rust encoder for 8-bit text that is not ASCII use simdutf, which is what this path needs (4,996 instructions at 129 chars).
• Other producers that wrap a Vec: TextEncoderStream chunks, bytes() of a string body, other callers of JSValue::create_buffer.
• Found while measuring, not caused by this PR: Buffer.from of a long ASCII string is slower on main than on bun 1.4.2 (5.5 to 5.8 µs against 4.8 µs at 64 KiB, 132 to 139 µs against 113 µs at 1 MiB).

Suites

• Debug (ASAN) build: test/js/node/buffer.test.js 708 pass. test/js/bun/ffi/ffi.test.js 157 pass and 1 timeout (ptr argument: ArrayBuffer cells through an FTL-compiled call site needs 11 s on this machine's debug build, with and without this PR). The new tests also pass with BUN_JSC_validateExceptionChecks=1. Node test-buffer-*, test-string-decoder*, test-crypto-hash*, test-crypto-hmac*: 68 of 69 pass. test-buffer-resizable.js needs the node:test runner and fails the same way on main.
• Release build: test/js/bun/ffi/ffi.test.js 148 pass on main and on this branch without BUN_GARBAGE_COLLECTOR_LEVEL.
• test/integration/bun-types/bun-types.test.ts: 22 pass.
• bench/snippets/buffer-from-string.mjs is new. It has rows for construct only and for construct then .buffer, view and subarray.

---

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Walkthrough

The change adds short-string conversion paths for Buffer.from(string), tests and benchmarks their byte and storage behavior, and updates ArrayBufferView materialization used by FFI pointers. FFI documentation and tests describe and check pointer lifetime and address stability.

Changes

Buffer String Encoding

Layer / File(s)	Summary
Short-string conversion paths src/jsc/bindings/JSBuffer.cpp	Short Latin-1 and UTF-16 conversions write directly to Buffer storage. Short UTF-8 paths handle Latin-1 input and replace lone UTF-16 surrogates.
Encoding tests and benchmarks test/js/node/buffer.test.js, bench/snippets/buffer-from-string.mjs	Tests check output bytes, storage boundaries, independent results, transfer behavior, and empty inputs. Benchmarks cover string shapes, lengths, and encodings.

FFI Pointer Stability

Layer / File(s)	Summary
View materialization and pointer stability src/jsc/bindings/bindings.cpp, src/runtime/ffi/FFIObject.rs, docs/runtime/ffi.mdx, packages/bun-types/ffi.d.ts, test/js/bun/ffi/ffi.test.js	Non-fast typed-array views skip buffer materialization in the binding. FFI pointer access materializes the view buffer and reports allocation failure. Documentation and tests cover pointer lifetime and address stability.

Suggested reviewers: jarred-sumner

Priority: ➖ Normal

Merge Risk: 🟡 Moderate · up to dc236

Code retaining a ptr() while its resizable buffer shrinks may use invalid storage. Clarify this lifetime limit before merging.

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Title check	✅ Passed	The title clearly and concisely identifies the main change: direct encoding of short strings into Buffer storage.
Description check	✅ Passed	The description explains the problem, implementation, performance impact, limitations, risks, and verification results. It does not use the exact template headings, but it provides the required inform…

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Review comments at @test/js/bun/ffi/ffi-ptr.test.ts:
- Line 5: Move the pointer-stability regression tests from the separate test
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[claude]

Beyond the inline findings, I also checked exception propagation in the new constructFromEncoding arms (the existing RETURN_IF_EXCEPTION after the switch covers every new helper that can throw OOM), the [[fallthrough]] reordering (over-limit inputs reach the same Rust entry point with the same encoding byte as before), and the 384-byte stack scratch in constructShortUTF16AsUTF8 (3 bytes per code unit is the worst case, including U+FFFD replacement) — none of these turned up a problem.

Extended reasoning...

The change adds an in-place fast path for Buffer.from(string) up to 128 code units in src/jsc/bindings/JSBuffer.cpp, narrows Bun__JSValue__materializeArrayBufferViewBuffer to fast-mode views, and makes bun:ffi ptr() materialize a view's ArrayBuffer first. It touches no auth or injection surface but does change pointer-stability guarantees for native consumers, which is where the inline findings sit, so a human should weigh those.

Findings marked 🟡 are optional suggestions and need no follow-up push.

[robobun]

Status: ready for review. The tests of this PR pass on every CI lane. CI is red on tests that this PR does not touch.

The runs so far:

build	commit	what failed
122604	ff10ea6bf0	ffi.test.js:505. This PR caused it and fixed it in 1f1c3c2d55 (second bullet below).
122715	df706d4ac9	The macOS aarch64 test job could not git clone a vendor repository.
122745	f21f0296b4	resolve-dns.test.ts on macOS aarch64: getaddrinfo ETIMEOUT.
122766	dc2363d034	serve.test.ts, s3.test.ts, fetch-leak.test.ts on macOS aarch64: timeouts and Failed to start server.
122798	be7d6bd1c7	worker-terminate-lifetime.test.ts on macOS x64: a worker exits with code 3 when no fetch answers in 150 ms.

Every other failure in these runs passed on a retry.

Two things need a maintainer's decision. Both are in Downsides:

• A small Buffer from Buffer.from(string) that is passed directly as a ptr argument now moves on the first .buffer read, as new Uint8Array(16) does. Native code must not keep that pointer. ptr(buffer) gives an address that stays valid.
• The address from ptr(view) is valid while the view is referenced. For a view that nothing references, it now goes bad at the next collection. ffi.test.js:504 read through such an address and now keeps its Buffer.

How the cost was reproduced, on a release build of main:

1. Run bun under gdb with BUN_JSC_useConcurrentJIT=0 and a script that calls Buffer.from("abcdefghijklmnop") in a loop.
2. After 20,000 calls, break at the entry of constructJSBuffer and single-step to its return.
3. Count the steps: 957 on main, 383 on this branch.

How the tests show it:

• bun bd test test/js/node/buffer.test.js -t "Buffer.from\(string, encoding\)": on main, keeps a result of at most 128 bytes in the Buffer's own storage fails with 112 bytes of ArrayBuffer per 16-byte result. All 25 pass on this branch.
• bun bd test test/js/bun/ffi/ffi.test.js -t "ptr\(\) returns an address": 8 of 10 fail on main, because ptr() of a small view returns an address that moves on the first .buffer read. All 10 pass on this branch.
• BUN_GARBAGE_COLLECTOR_LEVEL=1 bun test test/js/bun/ffi/ffi.test.js -t primitives is the run that failed in build 122604.

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Inline comments:
Review comments at @docs/runtime/ffi.mdx:
- Line 401: Qualify the ptr() lifetime guidance in the FFI documentation and
retained-pointer guidance in the type declarations: a referenced TypedArray does
not guarantee pointer stability if its resizable backing ArrayBuffer is resized,
so state that callers must not resize it while native code uses the pointer.
Preserve the existing warning to retain the TypedArray reference.

Review comments at @test/js/bun/ffi/ffi.test.js:
- Line 691: Change the parameterized test beginning with “keeps its address
after a .buffer read” to use describe.each(Object.keys(views)), placing the
per-view it inside that block and preserving the existing test behavior.

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Reviewing files that changed from the base of the PR and between ff10ea6 and 1f1c3c2.

📒 Files selected for processing (3)

• docs/runtime/ffi.mdx
• packages/bun-types/ffi.d.ts
• test/js/bun/ffi/ffi.test.js

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[claude]

Code review found no issues

No high-confidence issues detected in this change.

[robobun]

^{Updated 3:23 AM PT - Oct 2nd, 2026}

❌ @robobun, your commit be7d6bd has 1 failures in Build #122798 (All Failures):

• test/js/web/workers/worker-terminate-lifetime.test.ts - code 1 on 🍎 any x64

---

🧪   To try this PR locally:

bunx bun-pr 44404

That installs a local version of the PR into your bun-44404 executable, so you can run:

bun-44404 --bun

[claude]

Nothing blocking. The comments below are optional suggestions. There is no need to push a fix for them before merging.

Beyond the inline nit, I also checked a few things on this push and ruled them out: the nullptr returns from the new short-string helpers in src/jsc/bindings/JSBuffer.cpp are covered by the RETURN_IF_EXCEPTION that follows the switch, so OOM propagates before the "error occurred while decoding" TypeError; Bun__encoding__writeLatin1 (utf16le) and Bun__encoding__writeUTF16 (latin1) write exactly the byte counts the in-place buffers are sized for and match the construct_from_* slow paths in src/runtime/webcore/encoding.rs; and the two NAPI callers of Bun__JSValue__materializeArrayBufferViewBuffer only need address stability, which non-fast-mode views already have, so the early return does not change their behavior.

Extended reasoning...

The PR adds short-string in-place encoding fast paths to constructFromEncoding in src/jsc/bindings/JSBuffer.cpp, makes bun:ffi ptr() materialize fast-mode views in src/runtime/ffi/FFIObject.rs, and narrows the shared materialize helper in src/jsc/bindings/bindings.cpp, with new tests in test/js/node/buffer.test.js and test/js/bun/ffi/ffi.test.js. It touches no auth or injection surface but does touch native memory handling and FFI pointer lifetime. One nit finding is posted inline; the latest commit only shortened comments and regrouped tests, so this note records the additional checks performed rather than a new verdict.

[claude]

The latest push addresses the subprocess-test feedback from my previous pass, and I found no new bugs in this version; a maintainer should still look before merging, since the Buffer.from fast path changes where short Buffers keep their bytes and with it the bun:ffi pointer-stability contract for Buffers passed directly as arguments (the inline thread the author resolved is a design tradeoff, not a mechanical fix).

What was reviewed:

• Each new in-place arm against the Rust path it bypasses (8-bit to utf16le, 16-bit to latin1 low-byte narrowing, 16-bit to utf8 with lone surrogates): same encoder helpers or equivalent replacement semantics, so bytes match the slow path on both sides of the 128 limit.
• OOM paths: a null Buffer from createUninitialized* is caught by the RETURN_IF_EXCEPTION after the switch; the 384-byte stack scratch is only reachable for spans of at most 128 code units.
• The FastTypedArray-only early return in Bun__JSValue__materializeArrayBufferViewBuffer against its two napi callers: they only need address stability, and an oversize view adopts its vector in place, so they are unaffected.

Extended reasoning...

The change adds an in-place encoding fast path to constructFromEncoding in src/jsc/bindings/JSBuffer.cpp for strings of at most 128 code units (utf8, utf16le, latin1 arms for both 8-bit and 16-bit strings), makes bun:ffi ptr() materialize the ArrayBuffer of fast-mode views in src/runtime/ffi/FFIObject.rs, narrows Bun__JSValue__materializeArrayBufferViewBuffer to FastTypedArray views, and adds docs, types, a benchmark and tests. It touches no auth, crypto or injection surface, but it is a hot-path memory-layout change plus an FFI semantics change: short Buffer.from(string) results become GC-movable fast-mode views, so an address taken through the automatic typed-array-to-pointer argument conversion (not ptr()) is only valid for the duration of the call, which the author documents and accepted as a downside after self-resolving my earlier inline objection. The single-commit since my last review only reworks the test to pipe stderr and assert a combined object, which is what I asked for. The combination of a self-resolved objection and a user-visible tradeoff in a widely used API is what makes a human look worthwhile rather than approval.

[claude]

Code review found no issues

No high-confidence issues detected in this change.

[bompus]

I measured Buffer.from of a 28-character string at 42 ns in Node 26.12.0 and 146 ns in Bun 1.4.3 (Node faster in 6 of 6 alternating pairs), and @smithy/signature-v4 calls it about 7 times per signature. Two other small costs from the same signer benchmark are reported in #45860 and #45861.