bundler: fold code-splitting chunks that always load together, drop dead cross-chunk imports - #40506
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…ting chunks
With --splitting, every distinct set of importers gets its own chunk, so a
large application with many import() boundaries ends up with hundreds of
chunks of a few kilobytes, each costing a module record, a link step and a
file read at startup.
`minChunkSize` (Bun.build) / `--min-chunk-size` (CLI) folds chunks whose
source files add up to fewer than that many bytes into another chunk where
doing so is unobservable:
1. into a chunk loaded under exactly the same conditions. An import() entry
point is redundant in a chunk key when another entry in the key is
guaranteed to be loaded already whenever it loads (greatest fixpoint over
the import() graph; user entry points are process roots); keys equal after
dropping redundant entries describe chunks that are always loaded together.
2. when none of the chunk's live parts run anything at the top level
(declarations only, "sideEffects": false, or a lazily initialized
__esm/__commonJS wrapper that is not an entry point; a static import of a
wrapped or external module counts as running it), into a chunk loaded by
a superset of its entries, provided everything it imports is already
loaded there or side-effect free as well. Each group tracks the entries
under which it is loaded, which grows as folds make a chunk import it, so
a later fold cannot attach it to a chunk with side effects. What an entry
gains this way is capped at 1/64 of what it loaded to begin with.
The pass rewrites File.entry_bits before computeChunks groups files, so chunk
membership and cross-chunk imports follow from the existing machinery.
Supporting fixes this exposed:
- an entry point chunk that other chunks import from reserves its own export
names in the cross-chunk ExportRenamer, so the generated `export {}` does
not duplicate a name the entry already exports
- `/./` segments are collapsed out of `final_rel_path` (a `./[dir]/…`
template with `[dir] == "."` yielded `././x.js`, which importers copied
verbatim)
- under --compile the user entry point's chunk is left alone, since the
executable keys that module at /$bunfs/root/<outfile> after linking
BUN_DEBUG_MergeChunks=1 logs which files kept a chunk from folding.
…ead cross-chunk imports With --splitting, every distinct set of importers becomes a chunk. On top of #40430's opt-in --min-chunk-size this makes the semantics-preserving part unconditional and removes the syntax that splitting emitted for nothing: - Chunks whose files are only ever loaded together with an entry point (code shared between an entry and the import() targets it precedes) fold into that entry's chunk when the entry has no exports of its own, or into one shared chunk otherwise, instead of one chunk per importer set. An entry whose static graph uses top-level await guarantees nothing. - An entry point no longer emits a bare import of a chunk it uses nothing from when loading that chunk (and what it statically imports) runs nothing. - import() of another chunk no longer pulls the runtime's __require into the bundle. - An ESM entry chunk's cross-chunk exports join its export clause instead of adding a second one. - --min-chunk-size keeps gating only the fold of small side-effect-free chunks into a chunk more entry points load; a static import of a wrapped or external module, a top-level require(), top-level await, and being an entry point all count as side effects even under "sideEffects": false.
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Updated 6:51 PM PT - Aug 25th, 2026
❌ @Jarred-Sumner, your commit 4b5c675 has 1 failures in
🧪 To try this PR locally: bunx bun-pr 40506That installs a local version of the PR into your bun-40506 --bun |
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In `@test/bundler/bundler_splitting.test.ts`:
- Around line 972-997: Add an onAfterBundle assertion to
splitting/MinChunkSizeKeepsImportedUserEntryAsRoot that inspects the emitted
chunk layout and verifies the {main, d} chunk remains separate from main.js,
following the neighboring tests’ assertion style; keep the existing runtime
assertion unchanged.
In `@test/bundler/expectBundled.ts`:
- Line 822: Add an esbuild-backend guard in the command-building logic near the
existing bytecode, dotenv, and allowUnresolved checks that rejects any defined
minChunkSize with an explicit cause message. Ensure the guard runs before
invoking esbuild, while leaving the normal Bun bundler path unchanged.
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docs/bundler/index.mdxdocs/snippets/cli/build.mdxpackages/bun-types/bun.d.tssrc/bundler/LinkerContext.rssrc/bundler/bundle_v2.rssrc/bundler/lib.rssrc/bundler/linker_context/README.mdsrc/bundler/linker_context/computeChunks.rssrc/bundler/linker_context/computeCrossChunkDependencies.rssrc/bundler/linker_context/generateChunksInParallel.rssrc/bundler/linker_context/mergeSmallChunks.rssrc/bundler/linker_context/postProcessJSChunk.rssrc/bundler/linker_context/scanImportsAndExports.rssrc/bundler/options.rssrc/collections/bit_set.rssrc/js_printer/renamer.rssrc/options_types/context.rssrc/runtime/api/JSBundler.rssrc/runtime/api/js_bundle_completion_task.rssrc/runtime/cli/Arguments.rssrc/runtime/cli/build_command.rstest/bundler/bundler_compile_splitting.test.tstest/bundler/bundler_splitting.test.tstest/bundler/expectBundled.ts
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… emitting __require No-Verification-Needed: snapshot-only update
…; reject minChunkSize under the esbuild backend No-Verification-Needed: test-only change
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… executable (#40509) ### What does this PR do? Follow-up to #40506. Shrinks the serialized `ModuleInfo` (the ES module record — imports, exports, requested modules — that `--compile --bytecode --splitting` embeds per chunk and that the runtime transpiler cache / `--isolate` source-provider cache store per file), and removes the decode step: JSC now reads the wire bytes in place. **Before:** every import/export record was 1 kind byte + 4 × u32 slots, every string had a u32 length prefix, requested modules were three parallel u32/u32/u8 arrays, every chunk carried its own copy of the same chunk specifiers and export names, and loading a record memcpy'd the whole blob into an aligned buffer and built a fresh `new Identifier[]` for all of its strings. **After:** - A record is a **string table** + a **body**. Header bytes pick the integer width — u8, u16 or u32 — for string ids (from the table's count) and for string offsets (from the total string bytes). No varints. - One tag byte per record: `kind | fetch-kind << 3 | same-name << 6`. Slots the kind implies (`*` for a namespace import, `ExportInfoLocal`'s padding, a `None`/JS/Wasm/JSON fetch parameter) are not written; `same-name` drops an import's local name when it equals the imported name. `STAR_NAMESPACE`/`STAR_DEFAULT` are `count`/`count+1`. - `--compile`: **one string table for the whole executable** (`HAS_MODULE_INFO_STRING_TABLE` region in the `StandaloneModuleGraph`, in the startup run next to the bytecode string table); each chunk's body indexes it directly. - **No decoded copy.** `ModuleInfoDeserialized` is a validated view over the body bytes and the table; for a compiled chunk both live in the mapped section, so building the `JSModuleRecord` allocates nothing on the Rust side. `toJSModuleRecord` walks tag bytes + fixed-width ids and range-checks as it goes. - **Per-VM identifier slots.** Ids of a compiled chunk index one `Vector<Identifier>` on `JSVMClientData`, filled on first use, so each name is UTF-8-decoded and atomized once per VM instead of once per chunk that mentions it. Self-contained records (transpiler cache) use a throwaway fastMalloc'd array (was `new Identifier[]`). - Runtime transpiler cache / isolated module cache: one blob = the module's own table + body, same reader. Cache version → 26. ### Numbers `bun build --compile --splitting --bytecode --format=esm` (this branch also has #40506's chunk folding, hence fewer chunks). Every configuration produces the same program output as the unbundled source; debug builds assert every embedded record decodes and diff it against JSC's own `ModuleAnalyzer` result. | fixture | | chunks | module_info | JS | bytecode | |---|---|---:|---:|---:|---:| | 1 entry + 40 `import()` routes, 300 shared modules | main | 218 | 212,887 B | 150,177 | 205,424 | | | **this PR** | 150 | **19,491 B** (10,896 bodies + 8,595 table) | 86,877 | 162,576 | | | main `--minify` | 218 | 217,303 B | 131,654 | 202,192 | | | **this PR `--minify`** | 150 | **20,908 B** | 71,658 | 160,128 | | 1 entry + 100 routes × 40 imports, 600 shared modules (17.7k import/export names) | main | 515 | 1,603,741 B | 1,032,270 | 1,252,992 | | | **this PR** | 358 | **129,266 B** (101,627 + 27,639) | 576,121 | 1,111,664 | | | main `--minify` | 515 | 1,662,600 B | 919,031 | 1,242,784 | | | **this PR `--minify`** | 358 | **154,078 B** | 483,028 | 1,103,488 | −90…91%; ~91 → ~8.7 bytes per import/export name on the larger fixture. Format-only on an identical chunk layout (before rebasing over #40506) was 219,522 → 30,393 B (−86%): after fixed-width packing, 88% of what remained was string bytes repeated across chunks, which the shared table removes. ### How did you verify your code works? - `test/cli/test/isolation.test.ts` (the wide-module test now uses long names so the table's offsets take the u32 path), `test/regression/issue/30887.test.ts` (transpiler-cache round-trip with TLA), `test/cli/run/transpiler-cache.test.ts` (corrupt-record test rewritten for the new layout; out-of-range ids are still rejected with `parseFromSourceCode failed`), `bundler_compile_splitting`, `bundler_compile`, `type-export`, `debugger-buntranspiledmodule`, `bundler_barrel` — green with `bun bd test`. - Compiled both fixtures (plain and `--minify`) plus a zero-import single-file entry with the debug build and ran them, including a driver that loads every lazy route; a 70k-export module round-trips through `--isolate` (u32 ids) locally.
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…t {x as y}` / `import {y as z}` between chunks) (#40518)
### What does this PR do?
Third code-splitting overhead PR after #40506 / #40509: **every binding
that crosses a chunk boundary gets one bundle-wide name**, so
cross-chunk `export {}` / `import {}` clauses print bare names instead
of `export { x as Qc }` … `import { Qc as ur }`.
Today (esbuild's structure) three namers disagree:
`computeCrossChunkDependencies` picks an export alias with its own
`ExportRenamer` before any chunk is renamed, then the exporting chunk's
renamer and each importing chunk's renamer name the same ref
independently, and the printer bridges them with `as` on both ends.
Under `--minify` that is essentially every item (and the alias counter
was never reset per chunk, so aliases were `Qc`/`Th` while the locals
were 1 char).
Now:
- `computeCrossChunkDependencies` builds the clauses with empty aliases;
import items are ordered by stable ref instead of by alias.
- The per-chunk renamer pass stays parallel but the minifier is split
into *accumulate* and *finish*. Between the two,
`cross_chunk_names::assign_minified` sums each cross-chunk binding's use
count over every chunk that sees it, hands out the shortest names in
that order from one bundle-wide `NameMinifier` (skipping every module
scope's reserved/unbound names), and **pins** them in each chunk's
`MinifyRenamer` (`pin` names the slot and reserves the name; `finish`
names everything else around it). Without `--minify-identifiers`,
`assign_unminified` runs before the renamers: each binding keeps its own
name, numbered only against the other cross-chunk bindings, and
`NumberRenamer::pin_top_level_symbol` seeds it.
- Exporter local == alias == importer local by construction, so the
printer's existing "elide `as` when equal" path fires everywhere; the
only `as` left are an entry point's own public exports (`export { m as
default, X as load3 }`).
- Names only need to be unique within a chunk, so a chunk that doesn't
see a given shared binding can still use its short name locally — the
bundle-wide namespace costs each chunk only the handful of shared refs
it actually touches.
### Numbers
1 entry + 100 `import()` routes + 600 shared modules (17.7k
import/export items), `bun build --splitting`, main (with #40506/#40509)
vs this PR, identical 359-file layout:
| | main | this PR |
|---|---:|---:|
| `--minify` total JS | 421,089 B | **340,416 B (−19.2%)** |
| `--minify` total JS, gzip | 98,382 B | **79,945 B (−18.7%)** |
| `--minify` export items with `as` | 2,286 | 496 (entry points' public
names only) |
| `--minify` import items with `as` | 14,096 | **0** |
| no minify, total JS | 517,157 B | 517,157 B (gzip 102,861 → 98,493) |
40-route fixture `--minify`: 59,677 → 54,474 B (−8.7%). All variants
(plain / `--minify` / `--compile --bytecode --minify`) produce identical
program output, including every lazy route.
### How did you verify your code works?
`bun bd test` on `bundler_splitting`, `esbuild/splitting`,
`esbuild/default`, `bundler_minify`, `bundler_edgecase`,
`bundler_naming`, `esbuild/importstar`, `bundler_html`,
`bundler_compile_splitting` — green; two expectations updated
(`splitting/FoldsSharedIntoEntry` import order now follows stable ref
order; `splitting/MinifyIdentifiersCrashESBuildIssue437`'s shared `foo`
now prints under its bundle-wide minified name). Built and ran both
fixtures split / minified / compiled+bytecode with the debug build.
### Also in this PR: direct `eval` at module scope + `--minify`
Writing the collision tests turned up a pre-existing bug (same behaviour
in esbuild): `pop_scope` pins the names of every scope that contains a
direct eval, but the module scope never pops, so `var x = 1; eval('x')`
at the *top level* of a bundled file lost `x` to the minifier —
`ReferenceError` at runtime, splitting or not. When the bundler wraps
such a file in a CommonJS closure (which a top-level direct eval in a
file without ESM exports forces), its top-level names are private to
that closure, so they are now pinned as well; a flat ESM file's
top-level names still share the chunk scope with other files and stay
renameable, as in esbuild. Tests:
`minify/DirectEvalKeepsTopLevelNamesOfWrappedFile`,
`splitting/CrossChunkNamesWithDirectEval{,Minified}`,
`splitting/CrossChunkNameCollidesWithLocal{,Minified}`.
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…a chunk boundary (#40519) ### What does this PR do? With `--splitting` and `--target bun`, a `require()` of a bundled ES module now becomes a chunk of its own, exactly like `import()`, and the call is printed as `import.meta.require("./chunk-<hash>.js")`. On by default for that target; `splitRequire: false` / `--no-split-require` keeps the old inlined form. Other targets are unchanged (they cannot call `import.meta.require`). Today, with `--splitting`, a `require()` of a bundled module is inlined into the calling chunk behind a lazy `__esm` wrapper, because the call has to return synchronously. For a compiled binary that means every function-scoped `require()` still ships inside the entry chunk, so the bytecode of a module that is never required is decoded (and its `__esm`/`__export` closures created) on every launch. Split this way the call stays synchronous — Bun evaluates the chunk when the call runs — but a `require()` inside a function that never runs keeps its module out of the startup working set entirely. - `find_reachable_files` records an ESM `require()` target as a dynamic-import entry point and marks the record `CROSS_CHUNK_REQUIRE`; that flag is the single signal the linker (`is_external_dynamic_import`: wrapper skipped, chunk path substituted, tree shaking marks the target live from the live part as for `import()`, entry bits stop at the boundary) and the printer key on. CJS targets keep the in-chunk wrapper so `require()` keeps returning `module.exports`; browser-side files of a server build (an imported HTML page's scripts) are left alone since they cannot call `import.meta.require`. - The printer emits `import.meta.require` rather than the runtime's `__require` because `__require` would resolve the relative chunk path against the runtime's chunk, not the calling one. Printing it sets the module-info `contains_import_meta` flag so a bytecode build's module record provides `import.meta`. - Chunk folding (#40506) treats a `require()` edge differently from `import()`: the call runs while its importer is still evaluating, so no importer is guaranteed to precede the target and code shared by the entry and the required chunk stays in its own chunk instead of folding into the entry chunk after the call site. - The metafile and `cross_chunk_imports` carry the edge as `require-call`; the `--compile` load order treats it as a lazy edge (same as `import()`), so such chunks sit after the entry's static closure and are not part of the cold-start prefetch. - Runtime: a `require()` of an ES module that is still Evaluating — the call sits inside that module's own evaluation, a require cycle — now returns the live namespace (hoisted functions callable, later bindings in TDZ) instead of the empty CommonJS placeholder. `functionEsmLoadSync` answers from the registry before calling `loadModuleSync`, which would `link()`/`evaluate()` a record whose status both reject (a debug-build assertion today); an Evaluating record with top-level await gets the existing "async module" error instead of the same assertion. `overridableRequire` / `requireESMFromHijackedExtension` use the namespace `$requireESM` returned instead of re-probing `$esmNamespaceForCjs`, which only answers for an Evaluated record, and `requireESM` drops the pre-check that `$esmLoadSync` already performs. Observable differences, for reference: - A require cycle *between two split chunks* goes through the CommonJS module map and sees the `{}` placeholder, the same as unbundled `require()` of an ES module in Bun today (Node throws `ERR_REQUIRE_CYCLE_MODULE`); the in-chunk wrapper gave live getters there. Documented. - In a require cycle the wrapper hoists side-effect-free constant initializers out of the lazy init, so a module re-entered mid-evaluation could already see `export const x = "x"`; a chunk sees `undefined` (bundled `var`), and unbundled ESM throws a TDZ `ReferenceError`. Neither bundled form matches ESM here; unchanged by this PR. ### Numbers `bench/split-require` (`bun bench/split-require/gen.ts app 400 K && bun bench/split-require/run.ts <bun> app`; the `split-require` variants are the default, the others pass `--no-split-require`): an entry that reaches 400 ~14 KB "tool" modules only through function-scoped `require()` in a registry and invokes K of them at startup, built with `--compile --splitting --format=esm --minify`, release build, macOS arm64, medians of 15 interleaved runs. `footprint` is `Bun.unsafe.memoryFootprint()` (phys_footprint). | K used | variant | exe bytes | wall ms | entry done ms | footprint MB | |---|---|---:|---:|---:|---:| | 8 | source | 62,897,842 | 81.4 | 68.5 | 50.4 | | 8 | source + split-require | 62,848,306 | 9.2 | 3.7 | 6.7 | | 8 | bytecode | 72,838,066 | 23.2 | 15.8 | 34.6 | | 8 | bytecode + split-require | 72,227,122 | 11.2 | 2.5 | 6.9 | | 100 | bytecode | 72,838,066 | 28.4 | 18.3 | 39.7 | | 100 | bytecode + split-require | 72,227,122 | 10.9 | 5.0 | 13.9 | | 400 (all) | bytecode | 72,854,578 | 31.5 | 23.3 | 48.0 | | 400 (all) | bytecode + split-require | 72,243,634 | 22.0 | 14.3 | 27.0 | Even with every tool required at startup the split build is ahead, because the wrapper form pays for `__esm`/`__export` getter closures per module (this synthetic app has 40 exports per module, which exaggerates that part). On a real ~2400-module CLI built with `--compile --bytecode`, the entry chunk went from 14.3 MB / 2383 modules to 6.8 MB / 1602 modules and idle footprint at the prompt from 246 MB to 229 MB, with first-render time unchanged. Cost per call: a split `require()` goes through the CommonJS require path (resolve + cache lookup, ~0.5 µs on a cached chunk) where the wrapper is a flag check — the same cost as a runtime `require()` of an already-loaded module. A per-chunk memo in the printer could remove it; not done here. ### How did you verify your code works? - `bun bd test test/bundler/bundler_splitting.test.ts` — new `SplitRequireEmitsChunk-{cli,api}`, `SplitRequireOffKeepsWrapper`, `SplitRequireDeadTargetGetsNoChunk`, `SplitRequireLeavesCommonJSTargetInline`, `SplitRequireCycleDuringEvaluation`, `SplitRequireKeepsSharedCodeOutOfRequirer`, `SplitRequireLeavesBrowserFilesOfServerBuildAlone`, `SplitRequireSharesChunkWithDynamicImport`, `SplitRequireIsBunTargetOnly`. - `bun bd test test/bundler/bundler_compile_splitting.test.ts` — new `SplitRequireLoadsChunkSynchronously-{source,bytecode}` (the chunk is embedded under `/$bunfs/root` and loaded from a call made while the entry is still evaluating). - `bun bd test test/js/bun/resolve/require-esm-evaluating-cycle.test.ts` — new; fails on `USE_SYSTEM_BUN=1` (`selfHoisted: "undefined"`) and hit the `CyclicModuleRecord::link` status assertion on a debug build before the `functionEsmLoadSync` change. - `esbuild/splitting`, `bundler_compile`, `bundler_html_server`, `bundler_edgecase`, `bundler_cjs2esm`, `bundler_bun`, `bundler_naming`, `esbuild/default`, `esbuild/importstar`, `esbuild/dce`, `bake/dev-and-prod`, `bake/dev/{bundle,esm}`, `resolve/require*`, `resolve/esModule`, `node/module` green locally with the default on. - Drove the debug binary by hand: non-compile and `--compile --bytecode` builds of a registry/eager/lazy/CJS fixture, metafile `require-call` edges, the default / `--no-split-require` / `--target node` and `Bun.build` equivalents, require cycles across chunks, the shared-module folding case with and without `--minify` / `--min-chunk-size`, and an HTML-importing server build. --------- Co-authored-by: autofix-ci[bot] <114827586+autofix-ci[bot]@users.noreply.github.com> Co-authored-by: Jarred Sumner <jarred@jarredsumner.com>
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… preceded by the key (#40660) ### What does this PR do? Widens rule 1 of `mergeSmallChunks` (#40506), the always-on fold of code-splitting chunks that are loaded under the same conditions. Rule 1 drops an `import()` entry `D` from a chunk key when `D` can never be the first thing loaded among the key's entries. #40506 required a single entry to precede `D` on every path (`guaranteed[D]`, the intersection over `D`'s importers). That never fires when the same module is `import()`ed from two places that don't have a common ancestor: with two entries `main` and `repl` that both `import("./cmd")`, the `{main, repl, cmd}` chunk is loaded exactly when the `{main, repl}` chunk is (whichever way `cmd` loads, `main` or `repl` came first), but `guaranteed[cmd] = {main} ∩ {repl} = ∅` kept the two chunks apart. The new rule is per key: `D` is redundant when no importer of `D` can be reached from a process root through `import()`s without passing through the key. Roots are the entries nothing is known to precede: user entries, `require()` targets (#40519), entries with an importer that may be mid-evaluation at a top-level await, and entries nothing imports. `load_class` walks the `import()` graph from the roots outside the key, stopping at the key's entries, and drops each key entry none of whose importers was reached. The guarantor may differ per path, which is the only change in what is folded; importer cycles behind the key (lazy pages that `import()` each other) fold as before, and a key whose entries only `import()` each other is left alone. The walk is a single pass per distinct chunk key over an epoch-stamped scratch array, so its cost is proportional to what the roots reach, not to the number of entries. A first cut that rescanned every entry to a fixpoint per key took 448 s on a 3000-entry `import()` chain that `main` bundles in 1.2 s; this version takes 0.85 s on it. A self-`import()` is dropped from an entry's importers up front, so an entry that also imports itself is treated like any other. ### Measurements A real ~2400-module CLI compiled with `--compile --splitting --bytecode` (macOS arm64, release builds; `main` is 65362b5): | | main | this PR | |---|---|---| | chunks | 2,368 | 2,148 | | cross-chunk `import` statements | 208,916 | 163,302 | | chunks statically reachable from the main screen's `import()` | 692 | 530 | | executable size | 311.5 MB | 309.1 MB | Startup (interleaved runs, medians): the `import()` of the main screen 45.0 → 42.0 ms in the interactive path (15 runs) and 79.2 → 69.7 ms in the headless path (20 runs); time to first render 388 → 372 ms (hyperfine, 20 runs). Peak `phys_footprint` at first render 247 → 242 MB, within run-to-run noise. Bundling the CLI takes the same ~5.8 s. Synthetic graphs (3000 `import()` entries + 3000 shared modules, `bun build --splitting --target=bun`, hyperfine 5 runs): random importers 327 → 326 ms, a chain of entries 1.16 → 0.85 s. ### How did you verify your code works? - `test/bundler/bundler_splitting.test.ts`: new `FoldsChunkWhoseImportersTheKeyCovers` (`cmd` is `import()`ed from `main` and from a lazy module only `repl` loads; fails on `main` with 6 outputs, passes with 5) and `FoldsChunkBehindDynamicImportCycle` (`main → x ⇄ y`, `x → d`; the `{main, d}` chunk folds into `main.js` as on `main`), plus the existing 57 tests. - `test/bundler/bundler_compile_splitting.test.ts` (19) and `test/bundler/esbuild/splitting.test.ts` (26) pass. - Ran the compiled CLI above from both builds and compared chunk layout from the metafile and startup timings. --------- Co-authored-by: Jarred Sumner <jarred@jarredsumner.com> Co-authored-by: autofix-ci[bot] <114827586+autofix-ci[bot]@users.noreply.github.com>
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Sep 2, 2026
… the static import cycle it could create (#41170) ### What does this PR do? `--min-chunk-size` (#40430, #40506) folds small side-effect-free code-splitting chunks into a chunk that a superset of their entry points already loads, within a per-entry byte budget. This makes it actually fire on React apps, fixes the crash it caused on the Medusa admin dashboard after #41145, and makes the pass cheap enough to run by default later. The default stays 0 (off) in this PR; the option is now `Option`-plumbed with a per-target default hook so a later release can turn it on for `--target=browser` (16 KiB is the value the numbers below use). **Component files were never candidates.** `import React from "react"` prints a top-level `require_react()` in the importer, which counted as a side effect. Such a call is now permitted when the wrapped module lives in another chunk whose own top level makes the same call: the moved code imports `require_x` from that chunk, so that chunk has run to completion first and the call is a no-op. A bare `import "x"` still counts as a side effect (it exists for the effect), a dependency that starts being loaded by more entries has to pass the same test, and a file moved this way no longer anchors where its new chunk sits in the importer's evaluation order (test: `MinChunkSizeHoistedRequireKeepsChunkOrder`). **The Medusa cycle.** The pass built its dependency graph from every live import record; since #41145 chunk assignment skips `import … from` records into `sideEffects: false` files. A live barrel in the entry chunk (`react-query/index.js`) re-exporting a route-only module therefore looked like "the entry loads that module", a small chunk importing it was allowed into the entry chunk, and the entry chunk imported a chunk that imports `require_react` back from it: `TypeError: i is not a function`, blank page. `file_loaded_by_import` is now the one predicate used by the entry-bit walk, this pass and `inert_chunks`; the entry chunk's re-export edges are added too; and a reachability check over topologically numbered groups rejects any fold that would close a static import cycle. Supersedes #41164 (same root cause; this keeps those chunks foldable instead of leaving them per-route, and the cycle check is pruned rather than a full walk per attempt). **`loaded` and dominators.** An `import()` entry that can only load after entry `e` (its dominator in the `import()` graph rule 1 already builds) now counts `e`'s chunks as loaded, so a lazy route's budget and "already loaded wherever the target is" reflect the page's entry chunk. **Cost.** Byte budgets are mirrored into per-power-of-two bitsets so "can every entry in `target.loaded & ~candidate.loaded` afford this" is a word-wide mask test with exact arithmetic only near the boundary; a target's `loaded` size is bounded by how many entries can afford the candidate, and the per-entry lists are sorted by it; groups are sorted once per pass; a candidate is re-examined only when something that could give it a target happened (a superset started loading under one of its entries, an importer's dependency moved, or it failed on `require_*` coverage and something folded). ### Numbers (release builds, same commit as baseline, `--min-chunk-size=16384`) | | main | this PR | |---|---|---| | Medusa admin v2.8.4, `bun build ./index.html --production --splitting`: JS files | 349 (no flag) / blank page (flag) | 245 | | route navigation, requests median / max | 13 / 75 | 8 / 30 | | startup requests / bytes | 1 / 4.37 MB | 1 / 4.37 MB | | static import cycles with the flag | 5 | 0 | | build time | 272 ms (no flag) | 265 ms (flag) | | 50-route synthetic React SPA: JS files | 242 (no flag) / 209 (flag) | 123 | | requests per route navigation | 54 / 25 | 15 | | route navigation, headless Chrome, LTE / 4G emulation | 535 / 676 ms (no flag) | 412 / 616 ms | | adversarial 1000-route / 8000-module synthetic where nothing can fold: build | 293 ms (no flag), 380 ms (flag) | 306 ms (flag) | The 50-route app's self-test output is byte-identical to the unfolded build; Medusa `/login`, `/orders`, `/products` render in headless Chrome. ### How did you verify your code works? `test/bundler/bundler_splitting.test.ts`: - `MinChunkSizeBarrelRecordIsNotAnImport` — the Medusa shape (live `sideEffects:false` barrel in the entry chunk re-exporting route-only modules that `require()` a CommonJS module from the entry chunk); on main the output throws `require_lib is not a function`. - `MinChunkSizeFoldsImporterOfInitializedWrappedModule` / `MinChunkSizeKeepsFirstInitializerOfWrappedModule` — a `require_lib()` importer moves only when `lib`'s own chunk already initializes it. - `MinChunkSizeHoistedRequireKeepsChunkOrder` — a moved `require_lib()` does not pull its new chunk's side effects ahead of a sibling import. - `MinChunkSizeDefault{Browser,BrowserZero,BrowserOn,Bun,BunOn}` — off unless asked for on every target. - Two existing expectations updated for folds the dominator change enables (commented in place). Also ran `esbuild/splitting`, `bundler_compile_splitting`, `bundler_html`, `bundler_edgecase`, `bundler_barrel`, `bundler_cjs`, `bundler_cjs2esm`, `bundler_dynamic_import_dce`, `esbuild/default`, `esbuild/importstar`, `esbuild/css`, `css-modules`, `metafile`, `bundler_regressions` on the debug build.
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What does this PR do?
Builds on #40430 (cherry-picked here, credit @sosukesuzuki) and makes the parts of it that never cost an entry point anything always on, plus removes cross-chunk syntax that
--splittingemitted for nothing.--min-chunk-sizestays opt-in and keeps meaning "also hoist small side-effect-free chunks into a chunk more entries load".Always on with
--splittingnow:import()targets that can only load after it ({main, lazyA},{main, lazyB},{main, lazyA, lazyB}, …) no longer gets one chunk per importer set. If the entry has no exports of its own it lands in the entry's chunk (no moremain.jsthat is justimport {a,b} from "./main-x.js"; export {a,b}); if the entry has exports, its module namespace is left exactly as written and those chunks fold into one shared chunk instead (same policy as Rollup'spreserveEntrySignatures: "exports-only"). An entry whose static graph uses top-level await guarantees nothing (the lazy chunk would import a module that is still mid-evaluation), and--compileentry chunks are left alone as in bundler: add minChunkSize / --min-chunk-size to fold small code-splitting chunks #40430.import "./chunk-x.js"for chunks that run nothing. An entry imported every chunk carrying its bit "for side effects" even when tree-shaking left it using nothing from it (e.g. 3 of 5 entries bare-importing the runtime-helpers chunk). Dropped when loading the chunk — and anything its files statically import — runs nothing.import()of another chunk no longer drags__requireinto the bundle (esbuild only counts it whenimport()is lowered; we counted it always). This is what created the extra ~200-byte runtime chunk in split builds.export {}per ESM entry chunk — cross-chunk exports join the entry's own clause.Purity got stricter than #40430 while doing this: a static import of a wrapped (
__esm/__commonJS) or external module, a top-levelrequire(), top-level await, and being an entry point all count as side effects even under"sideEffects": false(the annotation vouches for the file's statements, not for what its imports run).Numbers
Same-revision release builds (base = this branch with the last commit reverted), Linux x64.
Synthetic app — 1 entry (with exports), 40
import()routes, 300 shared modules, 1-in-7 with a top-level side effect:importstatements (all files)main.jsstatic closure)instructions:u(median of 9)Route return values identical across unbundled / before / after (checked all 40 + nested).
No dynamic imports (5 entries over hono/zod/rxjs/lodash-es/dayjs; 8 entries over lodash-es): output byte-identical except the dropped bare imports of the runtime chunk;
instructions:uwithin noise (+0.1% / −0.8%). Non-splitting build: unchanged (−0.1%).Facade case (
main.tsw/o exports +import("./lazy")sharing code): 3 files → 2,main.jscontains its code,lazyimports from./main.js.Things to look at
import {…} from "./<entry>.js"when the entry has no exports (Rollup/Vite do the same). If a page loads the entry under a different URL than its siblings resolve (main.js?v=123, inlined<script type=module>), the entry would be instantiated twice. Entries with exports are never touched.export { … }of the bindings its lazy chunks need."sideEffects": falsemodule that an entry imports but uses nothing from is no longer fetched by that entry — same as the non-splitting build and what the annotation permits, but different from before.How did you verify your code works?
bun bd test test/bundler/bundler_splitting.test.ts— new:FoldsSharedIntoEntry,EntryWithExportsKeepsSignature,FoldsSharedCommonJSIntoEntry,FoldsDynamicallyImportedCommonJSIntoEntry,TopLevelAwaitKeepsSharedChunk,NoBareImportOfSideEffectFreeChunk,DynamicImportDoesNotNeedRequireShim,MinChunkSizeRequiresSplittingAPI; bundler: add minChunkSize / --min-chunk-size to fold small code-splitting chunks #40430's tests updated for the always-on fold and the missing runtime chunk. New tests fail onUSE_SYSTEM_BUN=1.esbuild/splitting,bundler_compile_splitting,esbuild/default,bundler_edgecase,bundler_naming,esbuild/css,bundler_html,bundler_html_server,bun-serve-html*,bundler_cjs2esm,esbuild/importstar,esbuild/dce,bake/dev-and-prod,bake/dev/{bundle,esm}green locally.