bundler: don't create a code-splitting chunk for a file with no live parts - #40581
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Chunk membership was assigned per reachable file, so a file whose every part was tree-shaken still created (or joined) a code-splitting chunk with nothing in it. Two such chunks have identical content and therefore the same [hash]: "Multiple files share the same output path". esbuild builds chunks from live parts; do the same by skipping part-less files.
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WalkthroughChangesChunk generation
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Beyond the inline finding, I checked the other consumers of files_with_parts_in_chunk for fallout from dropping these files: computeCrossChunkDependencies.rs and findAllImportedPartsInJSOrder.rs both iterate only over live parts within each listed file (or gate on chunk_of_file != u32::MAX), so a 0-live-part file already contributed nothing there — the metafile builder is the only downstream that changes behavior.
Extended reasoning...
The change skips fully-tree-shaken files when assigning code-splitting chunk membership. files_with_parts_in_chunk is read in several downstream passes; I traced each. Cross-chunk dependency computation (computeCrossChunkDependencies.rs:142, 339, 366) and part ordering (findAllImportedPartsInJSOrder.rs:31, 98) both immediately filter to live parts or guard on chunk_of_file[i] != u32::MAX, so removing a file with zero live parts is a no-op for them. The one consumer that does not filter by liveness — MetafileBuilder::generate, which unions chunk membership to build the metafile's top-level inputs — is covered by the inline finding.
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Updated 4:44 PM PT - Aug 26th, 2026
@Jarred-Sumner, your commit 3485c8c is building: |
…build hash snapshots
| exports[`Bun.build BuildArtifact properties: hash 1`] = `"g9c33042"`; | ||
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| exports[`Bun.build BuildArtifact properties + entry.naming: hash 1`] = `"7gfnt0h6"`; | ||
| exports[`Bun.build BuildArtifact properties + entry.naming: hash 1`] = `"2ksyc1td"`; | ||
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| exports[`Bun.build BuildArtifact properties sourcemap: hash index.js 1`] = `"est79qzq"`; | ||
| exports[`Bun.build BuildArtifact properties sourcemap: hash index.js 1`] = `"g9c33042"`; |
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🔴 Unjustified snapshot updates: the three BuildArtifact properties*: hash tests use neither splitting nor metafile, so this PR's code changes cannot alter their output hashes
Extended reasoning...
The computeChunks.rs change is gated on this.graph.code_splitting and the MetafileBuilder.rs change only runs when options.metafile is true (bundle_v2.rs:4065/5204). The three tests at bun-build-api.test.ts:530/553/578 build fixtures/trivial/index.js with no splitting and no metafile, so their blob.hash is byte-identical before and after this PR. On base the hash is est79qzq/7gfnt0h6; after merging, bun bd test bun-build-api.test.ts will still produce est79qzq/7gfnt0h6 and the three toMatchSnapshot("hash …") assertions will fail against the newly-committed g9c33042/2ksyc1td. REVIEW.md: "snapshot updates must be justified by the behavior change, not blindly regenerated" — these three snapshot edits should be reverted.
Verification: normal — The gating is unambiguous and the snapshot updates cannot be caused by this PR's source changes. computeChunks.rs — the added guard is inside the code_splitting branch: ``` 316 if this.graph.code_splitting { 317 // A file none of whose parts survived tree shaking... 320 if this.graph.parts_live[source_index.get() as usize].count() == 0 { 321 continue;…
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The hashes did change on main, independently of this PR: #40518 replaced the per-chunk closure walk that computes every output file's final [hash] (final_chunk_hashes), and that runs for all builds, not only --splitting. Same fixture, release binaries: 1.4.0 → prmf23xc, main before #40518 → prmf23xc, main after #40518 → wgrch6v3; this PR's first CI run (before the snapshot update) failed with exactly Expected "est79qzq" / Received "g9c33042", and a build of plain origin/main produces the same g9c33042. The snapshot was simply stale on main (that test file didn't run in #40518's CI); updating it here is what makes it pass again.
…iles (they print nothing either)
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Actionable comments posted: 1
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Inline comments:
In `@src/bundler/linker_context/computeChunks.rs`:
- Around line 301-319: Move the contributes_code AutoBitSet calculation into the
js_chunks.count() > 0 && this.graph.code_splitting branch where it is consumed,
or guard it so builds without JavaScript code splitting do not allocate the
bitset or scan reachable parts. Preserve the existing should_include_part logic
when chunk assignment requires the result.
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src/bundler/linker_context/MetafileBuilder.rssrc/bundler/linker_context/computeChunks.rstest/bundler/bundler_splitting.test.ts
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| itBundled("splitting/NoChunkForFilesWithNoLiveParts", { | ||
| files: { | ||
| "/entry.js": `await import('./a.js'); await import('./b.js'); await import('./e.js')`, | ||
| "/a.js": `import './c.js'; import './d.js'; console.log('a')`, | ||
| "/b.js": `import './c.js'; console.log('b')`, | ||
| "/e.js": `import './d.js'; console.log('e')`, | ||
| "/c.js": `export function dead() {}`, | ||
| // only live part is a bare import of an unwrapped file: prints nothing either | ||
| "/d.js": `import './shared.js'; export function dead() {}`, | ||
| "/shared.js": `console.log('shared')`, | ||
| }, | ||
| entryPoints: ["/entry.js"], | ||
| splitting: true, | ||
| minifySyntax: true, | ||
| outdir: "/out", | ||
| chunkNaming: "chunk-[hash].[ext]", | ||
| onAfterBundle(api) { | ||
| // entry + the three import() targets + shared.js's chunk; nothing for c.js / d.js | ||
| expect(readdirSync(api.outdir).filter(f => f.endsWith(".js"))).toHaveLength(5); | ||
| }, | ||
| run: { file: "/out/entry.js", stdout: "shared\na\nb\ne" }, | ||
| }); |
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🟡 Test fixture never produces two empty chunks and never exercises the should_include_part half of contributes_code: it fails on base for a different reason (file count 6≠5), and passes if the guard is weakened back to parts_live.count()==0
Extended reasoning...
shared.js is imported only by d.js, so it inherits d.js's entry_bits {a,e} and on the base branch lands in the same chunk as d.js — that chunk therefore contains console.log('shared') and is not empty. Only c.js (entry_bits {a,b}) is alone in its group, so base emits exactly one boilerplate-only chunk: no hash collision, the build succeeds with 6 .js files, and the test fails only via toHaveLength(5) — not via the "Multiple files share the same output path" error the comment claims and the PR fixes. Worse, revert contributes_code to the weaker parts_live[i].count()==0 check (the shape the [PRIOR pre_existing] review showed is insufficient): c.js has 0 live parts → skipped; d.js's bare-import part is live (shared.js has side effects) so count()==1 → not skipped, d.js joins shared.js in chunk {a,e}; total is still entry+a+b+e+{a,e}=5, stdout still shared\na\nb\ne, and the test passes. So neither the collision nor the should_include_part refinement is guarded. Fix the fixture so shared.js's entry_bits differ from both dead files' — e.g. add `import…
Verification: nit — the entry-bits trace bears the candidate out. shared.js and d.js land in the same chunk on base. Chunk membership is keyed on file_entry_bits (computeChunks.rs:330,340: let entry_bits = &file_entry_bits[source_index]; … js_chunk_key = entry_bits.bytes(...)). Reachability from the four entry points (entry, a, b, e — the three import() targets become entries): - c.js: reached from…
| let contributes_code = { | ||
| let mut bits = AutoBitSet::init_empty(this.graph.files.len())?; | ||
| let parts = this.graph.ast.items_parts(); | ||
| for source_index in this.graph.reachable_files.slice() { | ||
| let i = source_index.get() as usize; | ||
| let parts_live = &this.graph.parts_live[i]; | ||
| if parts[i].as_slice().iter().enumerate().any(|(p, part)| { | ||
| parts_live.is_set(p) && this.should_include_part(source_index.get(), part) | ||
| }) { | ||
| bits.set(i); | ||
| } | ||
| } | ||
| bits | ||
| }; |
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🟡 contributes_code is computed unconditionally (allocates a bitset, iterates every part of every reachable file, calls should_include_part on each) but is only ever read inside if this.graph.code_splitting { … } at line 336 — pure wasted work on every non-splitting build (the default).
Extended reasoning...
Relative to base, every Bun.build/bun build invocation without splitting: true now pays an extra O(total-parts) pass over the whole graph plus a bitset allocation whose result is discarded; on large single-entry builds this adds measurable link-time overhead for no behavioral benefit. The block should be gated on the existing code_splitting local (line 294) so non-splitting builds skip it entirely.
Verification: nit — The only read of contributes_code is at computeChunks.rs:336, inside if this.graph.code_splitting { … } (grep confirms exactly two hits: the definition at :305 and the read at :336). The computation at :305-318 runs unconditionally: it allocates an AutoBitSet sized to this.graph.files.len(), iterates every reachable file, and for each iterates every part calling… | nit — The…
Instead of scanning a file's parts when it would start a chunk, set a per-file bit in mark_part_live_step as each part becomes live: one tag check on the part's single statement, skipped once the file's bit is set, and only with code splitting. compute_chunks reads the bit and otherwise goes back to the #40581 flow: a file without it joins no chunk.
Instead of scanning a file's parts when it would start a chunk, set a per-file bit in mark_part_live_step as each part becomes live: one tag check on the part's single statement, skipped once the file's bit is set, and only with code splitting. compute_chunks reads the bit and otherwise goes back to the #40581 flow: a file without it joins no chunk.
2 participants
What does this PR do?
With
--splitting, chunk membership was assigned per reachable file, so a file whose every part was removed by tree shaking (e.g.export function dead(){}imported only for side effects it doesn't have) still created — or joined — a chunk containing nothing but the banner andexport{};. Two such chunks have byte-identical content and therefore the same[hash], and the build fails with "Multiple files share the same output path". esbuild builds chunks from live parts; this does the same by skipping part-less files when assigning chunks, so the empty chunk and the bareimport "./chunk-….js"its importers carried are simply not emitted.Pre-existing (the repro below fails on 1.4.0 as well); recent splitting changes (#40519, #40518) just make real apps more likely to produce two of them.
How did you verify your code works?
New
splitting/NoChunkForFilesWithNoLiveParts(fails on system bun, passes here);bundler_splitting,esbuild/splitting,bundler_compile_splitting,bundler_html,esbuild/dce,bundler_edgecase,bake/dev-and-prodgreen (352 tests).