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simdutf selects its kernel by CPUID on first use. When no compiled-in kernel matches the advertised feature set (QEMU's default qemu64 model hides SSE4.2), it installs a stub whose every method returns 0 or false. validate_ascii then reports ASCII input as non-ASCII, and allocate_latin1_into_utf8_with_list never makes progress, so bun spins at 100% CPU on the entry path before it opens the entry file. The scalar fallback kernel is not compiled into WTF's simdutf. At process start, if the active implementation is the stub, select the least demanding compiled-in kernel. On x64 that is westmere, which needs only SSE4.2, the same baseline the whole binary is compiled for.
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Included review availability: Your plan provides up to 10 included reviews per hour; 2 remain after this review. WalkthroughChangesThe runtime initializes simdutf before other startup work. It replaces an unsupported implementation with an available kernel. A regression test covers masked CPU startup. A debug assertion validates Latin-1 conversion state. simdutf startup
Suggested reviewers: Merge Risk: ⚪ Minimal · up to This change initializes simdutf early and selects a fallback kernel when dispatch is unsupported, preventing masked-CPU startup hangs. Regression coverage is included, with no remaining merge-readiness risk identified. 🚥 Pre-merge checks | ✅ 4✅ Passed checks (4 passed)
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In `@test/regression/issue/41361.test.ts`:
- Line 20: Remove the per-test timeout option from the regression test
configuration, including the timeout value shown near the test definition, so
the hang regression uses the runner-managed timeout from the existing test
runner.
- Line 10: Move the “bun starts when simdutf finds no supported kernel” test out
of the issue regression location and into the existing simdutf test module,
preserving its coverage and assertions.
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src/bun_core/lib.rssrc/runtime/bin_entry/mod.rssrc/simdutf_sys/bun-simdutf.cppsrc/simdutf_sys/simdutf.rstest/regression/issue/41361.test.ts
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Ready for review. Across four CI runs the only red lanes were tests my diff does not touch: a chunk-count timing test, a napi test stall on aarch64, a decorator test timeout on alpine, and test-cluster-primary-error.js on x64-asan (passes locally on the ASAN build in 6 s). Each was reported for main triage. |
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LGTM — the hang-guard signalCode assertion I asked for is in, and the shortened comments read well.
What was reviewed:
simdutf__init(): null-guarded, no-op when a real kernel is already active, iteration relies on simdutf's documented priority ordering so the last entry is the least-demanding compiled-in kernel.- Init ordering in
main(): afterbun_crash_handler::init()(which converts no strings) and before anything that can reachto_utf8; thesafe fnextern is sound since the C++ side has no preconditions. - Test: relies on the runner's default timeout for the hang case (matches the fails-on-main evidence), drains both pipes concurrently, asserts stdout/stderr before signalCode/exitCode.
Extended reasoning...
Overview
This PR fixes a startup hang when simdutf's runtime CPUID dispatch selects its "unsupported" stub — reproducible under QEMU's default CPU model (which hides SSE4.2 from CPUID) or via SIMDUTF_FORCE_IMPLEMENTATION=<unknown>. The fix is 59 added lines across five files: a new simdutf__init() in src/simdutf_sys/bun-simdutf.cpp that swaps the stub for the least-demanding compiled-in kernel, a Rust FFI wrapper in src/simdutf_sys/simdutf.rs, a call site in src/runtime/bin_entry/mod.rs right after crash-handler init, a debug_assert! in src/bun_core/lib.rs that turns any future dispatch failure into a debug crash instead of a silent spin, and a regression test under test/regression/issue/41361.test.ts.
Security risks
None. No user input is parsed, no auth/crypto/permission paths are touched, and no new attack surface is exposed. The only environment variable involved (SIMDUTF_FORCE_IMPLEMENTATION) is read by the vendored simdutf library, not by new code; the fix merely recovers when that variable (or CPUID) leads simdutf to install its stub. The safe fn simdutf__init() extern is justified: the C++ function takes no arguments, has no memory-safety preconditions, and is idempotent.
Level of scrutiny
Low-to-moderate. The change is small, mechanical, and well-scoped to a single failure mode with a clear root cause. The C++ logic is defensive (early-returns when a real kernel is active; null-checks least_demanding before assigning). The debug_assert! is release-inert. Cross-platform behavior is safe: on non-x64 or when a real kernel is already selected, simdutf__init() is a no-op on the first branch. The PR's gate evidence shows the test hangs (5s timeout) on both debug-ASAN and release builds without the fix and passes with it, satisfying the fails-without/passes-with requirement.
Other factors
Since my prior review, commit 8346d31 added the expect(proc.signalCode).toBeNull() hang-guard assertion I requested and tightened the comments; fefc6d2 is a CI retrigger with no code change. All inline threads are resolved, several by non-authors, and there are no outstanding CHANGES_REQUESTED reviews. None of the changed paths are covered by CODEOWNERS. The test follows harness conventions (tempDir, bunEnv spread, await using, concurrent pipe drain, stdout/stderr asserted before exit code) and relies on the default runner timeout rather than a custom one, per test/CLAUDE.md. The exit reason for this bug-hunt run was dry_streak, so the hunt completed without being budget-truncated.
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Issue #43683 (NestJS app hangs at 100% CPU on a non-AVX VM) has the same root cause. I merged this branch with current main (2f6284c), built it, and ran the repro from that issue under a CPUID shim that hides SSE4.2 and AVX: the fixed build prints the script output, the v1.4.2 release binary hangs. test/regression/issue/41361.test.ts passes on the merged build. Added fixes #43683 to the PR body. |
) ### Problem - `buffer.transcode()` aborts the process when its output needs 2^31 bytes or more: `panic(main thread): abort() called`, exit code 134, also inside `try`/`catch`. Example: `transcode(new Uint8Array(2 ** 30), "latin1", "ucs2")`. Node v26.3.0 returns 2,147,483,648 bytes. - `jsBufferTranscode` (`src/jsc/modules/NodeBufferModule.cpp:186`) sizes its result and its UTF-16 scratch copies with `WTF::Vector::grow()`. `grow()` calls `CRASH()` past the Vector limit or on a failed allocation (`wtf/Vector.h:228`). ### Fix - Each path measures its output, then converts straight into an uninitialized Buffer. The Vector limit and one copy of the result are gone. The allocation throws `RangeError: Out of memory` when it fails or the result passes the Buffer limit of 2^32 bytes. - The UTF-16 scratch copies use `tryGrow()` and throw the same error. - Correct because every path fills its result exactly. utf8 to ucs2 reads its source twice, so it reports any other count as `U_INVALID_CHAR_FOUND`. - Verified: `test/js/node/buffer.test.js` (three new tests exit 134 on main), `test-icu-transcode.js`, and a 14,000-input differential run against main. Timings are in Notes. Self-reviewed: 8 concerns raised, 7 addressed. Not addressed: a shared memory gate for huge-allocation tests. ### Background - A `WTF::Vector` holds at most `(UINT_MAX >> 1) / sizeof(T)` elements: 2^31 - 1 bytes, or 2^30 - 1 UTF-16 units. `grow()` aborts past that. `tryGrow()` returns false. - `transcode` has three direct paths, for example latin1 to ucs2. Every other pair decodes the source to a UTF-16 copy, then encodes it. - `WebCore::createUninitializedBuffer` allocates a Buffer with no zero fill and throws `RangeError: Out of memory` on failure. #42202 uses that error for a result that cannot exist. <details><summary>Notes</summary> **Where Bun still throws and Node returns a value.** The scratch copies stay `WTF::Vector`s, so they keep the Vector limit: - ucs2 to utf8 with a source of 2^31 bytes or more. The aligned copy of the source needs 2^30 units. - A utf8, latin1 or ascii source of 2^30 bytes or more on a path that decodes to the UTF-16 copy. Node returns a value only when the target is latin1 or ascii and the source is below 2^31 bytes. For the other sizes ICU rejects the call and Node throws `U_ILLEGAL_ARGUMENT_ERROR` (for example latin1 to utf8 at 2^30 bytes, ucs2 to ucs2 at 2^30 bytes). A comment in the code records this difference with a link to Node's source. - Any result above 2^32 bytes, which is the Buffer limit in JSC. Node v26.3.0 on this machine: ``` 2^30 bytes latin1 -> ucs2 returns 2147483648 bytes 2^31 bytes latin1 -> ucs2 returns 4294967296 bytes 2^30 bytes utf8 -> ucs2 returns 2147483648 bytes 2^31 bytes ucs2 -> utf8 returns 3221225472 bytes 2^30 bytes utf8 -> latin1 returns 1073741824 bytes 2^30 bytes latin1 -> utf8 throws U_ILLEGAL_ARGUMENT_ERROR 2^31 bytes utf8 -> latin1 throws U_ILLEGAL_ARGUMENT_ERROR 2^30 bytes ucs2 -> ucs2 throws U_ILLEGAL_ARGUMENT_ERROR ``` **Verified by hand on the debug ASAN build, not in the tests** (each one touches 3 to 7 GiB): ``` 2^31 bytes latin1 -> ucs2 returns 4294967296 bytes (the Buffer limit) 2^30 bytes utf8 -> ucs2 returns 2147483648 bytes 2^31 - 2 bytes ucs2 -> utf8 returns 3221225469 bytes (the largest scratch copy) 2^30 - 1 bytes latin1 -> utf8 returns 2147483646 bytes 2^30 - 1 bytes utf8 -> latin1 returns 1073741823 bytes ``` **The narrow encoder.** A latin1 or ascii target writes one byte per code point, and a surrogate pair becomes one `?`. For a latin1 target the simdutf bulk conversion runs first, into a Buffer of one byte per unit, as on main. When it fails, and for an ascii target, the substitution path counts the units that are not a trail surrogate, and its loop skips trail surrogates: the count and the writes share one predicate (`writesByte`). The substitution path reuses the Buffer of the bulk attempt unless the source has surrogate pairs, which need a shorter result. The first revision took that count from `simdutf::count_utf16le`. The self-review found that this made the bounds of the writes depend on a simdutf return value. simdutf selects its implementation at run time, and every function of its `unsupported` stub returns 0 (`SIMDUTF_FORCE_IMPLEMENTATION=nonsense` selects the stub). The result was then 0 bytes long and the loop wrote past it. The count is now a plain loop in the same function. The bulk attempt writes at most one byte per unit into a Buffer of that size, and the stub reports an error and writes nothing. With the stub forced, the debug ASAN build reports no error for the narrow paths. #41374 and #30642 own the stub itself. This PR does not change what the other paths return when the stub is active: old and new both return whatever the stub left in the buffer. **Timings.** Release builds with ThinLTO of three trees on one base (`367d939d9`): main, this PR before `6c9b2ed2c8`, and this PR now. One pinned core, 5 alternating runs for each binary, 15 rounds in each run, minimum ns per call: ``` main before now ucs2 -> latin1 64 KiB in-range 5,324 14,371 4,352 ucs2 -> latin1 1 MiB in-range 166,633 291,345 133,865 ucs2 -> latin1 64 KiB U+0100 last 64,605 35,067 35,009 ucs2 -> latin1 1 MiB U+0100 last 1,145,194 622,295 628,289 ucs2 -> latin1 64 KiB pair last 64,555 33,686 35,695 ucs2 -> latin1 1 MiB pair last 1,149,060 588,495 636,765 ucs2 -> ascii 64 KiB ASCII 65,132 33,680 33,735 utf8 -> latin1 64 KiB ASCII 11,580 29,820 9,912 ucs2 -> latin1 16 units 127 130 118 ``` "before" counted the output bytes with a scalar loop ahead of the bulk conversion, so an in-range latin1 result was 2.7 times slower than main at 64 KiB. A measurement before the merge found this. The bulk conversion now runs first, and the in-range result is 18 to 20 % faster than main. A source with a surrogate pair pays for the failed bulk attempt and for a second allocation: 6 to 8 % over "before", and 45 % under main. The substitution paths are faster than main. They write through an index, and main called `append()` for each byte. **The trailing odd byte of a ucs2 source** was an `append()` after the `grow()`, so it reallocated the whole copy. The copy is now sized once. **Tests.** - `transcode to latin1 and ascii writes one byte for each code point`: 330 comparisons in process, at lengths on both sides of the simdutf block sizes and of the 1000 bytes above which a typed array is allocated with malloc. It passes on main too. It pins the output of the rewritten encoder. - `throws when the UTF-16 copy of the source cannot be allocated` (debug builds): `BUN_JSC_maxSingleAllocationSize` fails each of the five scratch allocations with a source of 3 or 10 MiB. On main the infallible allocation asserts. - `throws past the size limit of a Buffer or a Vector`: the child reserves 2 GiB and never writes to it. It takes 0.4 s in a debug ASAN build and its RSS stays at the baseline. It skips below 4 GiB of total memory, because a small host can refuse the reservation. - `returns a result of 2 GiB`: the child writes the whole result (2.4 GB RSS, 2.4 s in a debug ASAN build). It skips below 10 GiB of total memory, the gate its neighbor uses. - The result allocation has one failing case in the tests (latin1 to ucs2 past the Buffer limit). The debug cap and the ASAN allocation cap do not reach typed array storage, and utf8 to ucs2 past the limit needs a 2.6 s pass over 2 GiB in a debug build. **Other checks.** The full `buffer.test.js` passes (683 pass, 1 skip that is also skipped on main). The transcode matrix and the new cases pass under `BUN_JSC_validateExceptionChecks=1`. The differential run compares a release and a debug ASAN build of this branch with a release build of main. It covers all 16 encoding pairs with random bytes, ASCII, UTF-8 text of every width, UTF-16 with and without lone surrogates, Latin-1 range UTF-16 with and without one unit above U+00FF at a random place, odd lengths, lengths around the SIMD block sizes, and views at an odd `byteOffset`. **Not changed here.** The same differential run against Node shows an older difference in the substitution paths: ICU drops default-ignorable code points such as U+00AD, U+200B and U+FEFF when the target cannot encode them, and Bun writes `?`. For example `transcode(Buffer.from([0x41, 0xad, 0x42]), "latin1", "ascii")` is `[65, 66]` in Node and `[65, 63, 66]` in Bun. This PR keeps that output as it was. **Related open PRs.** #42300 changes the ascii fix-up loop in `transcodeDecodeToUtf16`. #41574 copies a `SharedArrayBuffer` source before the conversion. Neither touches the `grow()` calls. Expect a small textual conflict with each. </details> <!-- robobun:evidence:begin --> --- **no test proof** · iteration 5 · platform-specific test(s) that do not run on this machine, deferring to CI, which covers all platforms: test/js/node/buffer.test.js <!-- robobun:evidence:end -->
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Problem
bun <file>spins at 100% CPU before it opens the entry file. Top frame:allocate_latin1_into_utf8_with_list(src/bun_core/lib.rs:1595).-march=nehalem, so WTF's simdutf drops its scalar fallback kernel. At first use simdutf reads CPUID, matches no kernel, and installs itsunsupportedstub. Every stub method returns 0 or false.first_non_ascii_usizethen returnsSome(0)for an ASCII byte and the loop never advances.Fix
simdutf__init(src/simdutf_sys/bun-simdutf.cpp) runs once inmain(). If the active implementation is the stub, it selects the least demanding compiled-in kernel. On x64 that is westmere, which needs only SSE4.2, the baseline the whole binary already assumes.SIMDUTF_FORCE_IMPLEMENTATION=<unknown name>installs the same stub, so the test reproduces on any machine.test/regression/issue/41361.test.tshangs on the unfixed debug build and passes on the fixed one. The reporter's 80 MB bundle loads under a CPUID-masking shim. Also ran the encoding andbuffersuites.Background
qemu64model does this for SSE4.2.Fixes #41361, fixes #43683
Notes
-marchflags.debug_assert!inallocate_latin1_into_utf8_with_listturns a future dispatch failure into a crash in debug builds instead of a silent spin.init()runs afterbun_crash_handler::init(). That init converts no strings, so the order is safe, and it keeps the crash handler first as the existing comment asks.LD_PRELOADshim that callsarch_prctl(ARCH_SET_CPUID, 0)(Intelcpuid_fault) and answers everycpuidfrom a SIGSEGV handler with a qemu64-like feature set. Under it, the unfixed 1.4.0 baseline binary and the unfixed debug build both hang on any entry path longer than 32 bytes. The fixed debug build prints the script output. With the reporter's 80 MBserver.mjs, the fixed build opens the file and reads 171 MB within 40 s (debug ASAN); the unfixed baseline reads 23 KB and sits at 11 MB RSS.nmon the 1.4.0 profile build: 103simdutf::westmere::symbols, 0simdutf::fallback::. The debug build here also has haswell and icelake. The list is in priority order, so the last entry is the least demanding. westmere requiresinstruction_set::SSE42at runtime and is compiled withSIMDUTF_TARGET_REGION("sse4.2,popcnt").to_utf8, not the file size.is_all_asciihas a scalar fast path up to 32 bytes, which is whybun --versionand a short path work.test/js/web/encoding/text-encoder.test.js,test/js/web/encoding/text-decoder.test.js,test/js/bun/util/toUTF16Alloc.test.ts,test/js/node/buffer.test.js,test/js/bun/util/highway-strings.test.ts(one highway test exceeds the 5 s default under ASAN in this container, unrelated to simdutf).[human-review] gate passed · iteration 1 · 5 files touched
fails on main (without fix)
passes on PR (with fix)
diff hotspot
gate history · 1 passed · 0 rejected · iteration 1
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root cause · written by the author bot
simdutf selects its SIMD kernel by CPUID at first use, and on CPUs that hide SSE4.2 (such as QEMU's default CPU model) no compiled-in kernel matched, so it installed a stub whose every call returned 0 or false, causing Bun to spin forever in the Latin-1 to UTF-8 conversion loop on the first ASCII string longer than 32 bytes before the entry file was even opened. The fix initializes simdutf early in runtime startup and, when the active implementation is the unsupported stub, replaces it with the least-demanding compiled kernel so conversions make progress. A regression test forces an unsuppo…