bundler: order a chunk's cross-chunk imports by source evaluation order - #40547
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With code splitting, a chunk's `import` statements were sorted by chunk index, which follows the entry-bits byte order. An entry that imports two shared chunks ran them in whichever order their bitsets sorted, not the order its source imports them. The walk that orders a chunk's parts now also records the other chunks it reaches, in the order it finishes their first file with top-level side effects. `compute_cross_chunk_dependencies` sorts the chunk's cross-chunk imports by that order, so the shared chunks run in the order the unbundled modules would.
ESM hoists every `import` above a module's own code, so whatever a chunk imports from other chunks runs before any of its own files. With `e1.js` importing `s1.js` (used only by `e1`) and then `s2.js` (shared with `e2`), the `e1` chunk inlined `s1` and imported the `s2` chunk, and `s2` ran first. esbuild has the same limitation (evanw/esbuild#399). `split_chunks_for_evaluation_order` runs after `compute_chunks` groups files by entry bits. For each entry point it walks the files the entry loads in evaluation order and cuts a chunk into a new chunk wherever a file with top-level side effects from another chunk interrupts its run. A file that runs nothing when loaded joins a run as long as everything it imports has run its side effects by the time the run is evaluated, so such files rarely cost a chunk. Files that an entry guaranteed to have finished already loads (`merge_small_chunks`'s `guaranteed`) take no part in an `import()` target's order. The new chunks keep their parent's entry bits under a longer key, and chunk membership is `files_with_parts_in_chunk` from then on. `find_imported_parts_in_js_order` walks from the first entry point that loads a chunk before the chunk's own files, so a shared chunk comes out in that entry's order; where two entries run two files of one chunk in opposite orders, the files are cut apart too. It also follows a live part's `require()` whichever chunk holds it, so every walk reaches each file at the same place.
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Reproduced on bun 1.4.1 and main with three entries: With this branch The earlier revision also cut chunks so that code inlined into an entry runs before the shared chunks it precedes in the source (the |
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Included review availability: Your plan provides up to 5 included reviews per hour; 2 remain after this review. WalkthroughThe bundler now tracks module evaluation order across JavaScript chunks, splits chunks when entry-point orders conflict, and sorts cross-chunk imports by evaluation rank. Tests and documentation cover the new behavior. ChangesBundler evaluation order
Suggested reviewers: 🚥 Pre-merge checks | ✅ 4✅ Passed checks (4 passed)
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In `@src/bundler/linker_context/computeCrossChunkDependencies.rs`:
- Around line 635-642: Update the CrossChunkImport::sorted_cross_chunk_imports
call in computeCrossChunkDependencies to propagate its bun_alloc::AllocError
with ?, replacing the expect("unreachable") panic while preserving the
function’s existing Result<(), bun_alloc::AllocError> return path.
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docs/bundler/esbuild.mdxdocs/bundler/index.mdxsrc/bundler/Chunk.rssrc/bundler/lib.rssrc/bundler/linker_context/README.mdsrc/bundler/linker_context/computeChunks.rssrc/bundler/linker_context/computeCrossChunkDependencies.rssrc/bundler/linker_context/findAllImportedPartsInJSOrder.rssrc/bundler/linker_context/mergeSmallChunks.rssrc/bundler/linker_context/splitChunksForEvaluationOrder.rstest/bundler/bundler_splitting.test.ts
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The function has no fallible step. Drop the `Result` and the
`expect("unreachable")` at its one call site.
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Updated 1:53 PM PT - Aug 26th, 2026
@Jarred-Sumner, your commit 277d016 is building: |
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…first `find_imported_parts_in_js_order` started a shared chunk's walk at the entry point with the lowest bit. `split_chunks_for_evaluation_order` drops a chunk from an entry's order when an entry that is guaranteed to have finished already loads it, so its order check could take a later entry as the reference while the chunk was laid out in the first one's order. The pass now records the entry it used on the chunk, and the walk starts there.
A chunk that is not an entry point is a browser chunk of a server build when any of its files targets the browser. After the evaluation-order pass moves files out of such a chunk, the flag is computed again from the files that remain, as it is for the new chunk.
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Both issues I raised earlier are now addressed — 09f2610 records layout_entry_source_index on the chunk so the part-ordering walk starts from the same entry the inversion check used as its reference, and 554f4a1 recomputes IS_BROWSER_CHUNK_FROM_SERVER_BUILD on the parent from its remaining files_with_parts_in_chunk after split_off moves files out (guarded to non-entry-point chunks, matching how compute_chunks derives the flag). I found nothing further this pass.
Given the scope — a new ~470-line fixed-point linker pass that changes chunk membership and cross-chunk import ordering for every --splitting build — a human look at the run-cutting algorithm and the chunk-count trade-off is still worthwhile.
Checked: has_browser_file receives &[u32] from ArrayHashMap::keys(); the parent-flag recompute runs after all remove() calls so it sees the post-split file set; the entry-point guard matches computeChunks.rs where entry chunks derive the flag from config target rather than file contents.
Extended reasoning...
Overview
This PR adds a new bundler linker pass, split_chunks_for_evaluation_order, that splits code-splitting chunks so bundled output preserves source evaluation order of top-level side effects (fixing the equivalent of esbuild#399). It touches Chunk.rs (new reached_chunks_in_order and layout_entry_source_index fields, cross-chunk import sort now keyed by evaluation rank), computeChunks.rs (invokes the new pass), findAllImportedPartsInJSOrder.rs (walk starts from layout_entry_source_index, membership test switched to files_with_parts_in_chunk.contains, records reached chunks in post-order), mergeSmallChunks.rs (returns guaranteed_loaded bitsets for reuse), and computeCrossChunkDependencies.rs (sort by evaluation rank, no longer returns Result). Docs and ~215 lines of itBundled tests are added. Since my last review, two follow-up commits landed: 09f2610 (align layout entry with inversion-check reference entry) and 554f4a1 (recompute parent's browser-chunk flag after split).
Security risks
None. This is pure bundler-internal chunk layout logic operating on the module graph already parsed and validated upstream. No user-controlled paths, network, credentials, or crypto are involved. The worst-case failure mode is an incorrect bundle (wrong evaluation order or misplaced output file), not a security exposure.
Level of scrutiny
High. The new pass is a ~470-line fixed-point algorithm with run-cutting, per-entry evaluation-order walks, inversion detection across entries, and chunk splitting that changes files_with_parts_in_chunk membership — the field several downstream passes (findAllImportedPartsInJSOrder, computeCrossChunkDependencies, generateChunksInParallel) depend on. It changes output for every --splitting build and increases chunk count (PR notes +9-13% on synthetic apps, up to ~2× on random-graph worst cases). REVIEW.md's "Architecture & layering" and "Correctness: the bug class" sections apply directly: this is a structural redesign of how chunk boundaries are drawn, and the reference implementation (esbuild) explicitly does not do this, so there is no upstream to diff against. A maintainer should weigh the chunk-count/correctness trade-off and sanity-check the fixed-point termination and the pure-file-joins-run heuristic.
Other factors
Two prior findings from this system were both addressed by targeted follow-up commits, and I verified the latest fix (554f4a1): ArrayHashMap::keys() returns &[K] so has_browser_file(parent.files_with_parts_in_chunk.keys()) iterates the post-removal file set, and the !is_entry_point() guard correctly excludes entry chunks whose flag compute_chunks derives from config target. The PR includes six new itBundled tests plus the mechgate evidence showing 5 tests fail on main and pass on the branch. The many github-actions bot inline comments were all author-self-resolved, which is not independent evidence they were addressed, but no human CHANGES_REQUESTED review is outstanding. Exit reason was dry_streak. Given the size and that this introduces a new algorithm rather than a mechanical fix, deferring rather than approving is appropriate per the guidelines.
…-chunk import ordering Reverts aa3461d, 848376d (the pass's part), 09f2610 and 554f4a1. The s1/s2 hoisting is esbuild#399 behaviour that 1.4.0 already had; cutting extra chunks for it (+9-13% files on ordinary apps, 2x on interleaved multi-entry graphs) is a product decision, not a fix, and is dropped here. What remains: a chunk's cross-chunk import statements follow the order the entry reaches those chunks instead of entry-bit order (no new chunks).
Jarred-Sumner
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Reviewed the reduced scope after the split pass was reverted — no bugs found. Both of my earlier inline comments were about splitChunksForEvaluationOrder.rs and its layout-entry interaction; that file is gone, so they no longer apply.
What was reviewed:
evaluation_rankfill/reset incomputeCrossChunkDependencies.rs— reset walks the samereachedlist it filled from, so state is clean between chunks; unreached chunks keepu32::MAXand fall back to the oldchunk_indextiebreak, so output stays deterministic.reached_chunksrecording in the visitor's!is_file_in_chunkbranch — post-order, deduped viareached_chunk_set, andchunk_of_fileis all-u32::MAXwithout splitting so it's a no-op there.- The
MinChunkSizeFoldsPureChunkIntoSupersetsnapshot flip:entry1.jsimports./common.jsbefore./impure.jsin source, socommon evaluatedfirst is the correct unbundled order — this is the fix landing, not a regression baked in. inert_chunksVec<bool>→AutoBitSet:get_or_insert_withreplaced with explicitis_none()+?because the closure form can't propagateAllocError; the following.unwrap()is guarded.
Extended reasoning...
Overview
The PR now only (1) records reached_chunks_in_order during the existing per-chunk post-order walk in findAllImportedPartsInJSOrder.rs, (2) uses it as the primary sort key for cross-chunk import statements in Chunk.rs / computeCrossChunkDependencies.rs, and (3) refactors inert_chunks from Vec<bool> to AutoBitSet. The earlier splitChunksForEvaluationOrder.rs pass — which both of my previous findings targeted — was reverted in 7946300, and 5451030/277d016e swapped two Vec<bool> uses for AutoBitSet.
Security risks
None. This is bundler output-ordering logic with no untrusted-input parsing, no auth/crypto, and no FFI surface change. Allocation fallibility from AutoBitSet::init_empty is threaded through ? to the existing Result<_, AllocError> return or bun_core::handle_oom at the worker boundary, matching src/CLAUDE.md's OOM rule.
Level of scrutiny
Moderate. The remaining change is small and mechanical relative to the earlier revisions, but it is still a behavioral change to code-splitting output order in a parallel linker path, and a maintainer is already actively reshaping it. I'm deferring rather than approving so the human already on the PR signs off on the final shape; the note is mainly to close the loop on my two now-moot inline threads.
Other factors
The updated MinChunkSizeFoldsPureChunkIntoSuperset expectation matches source import order (./common.js before ./impure.js), so it's the intended fix rather than a snapshot capturing a regression. The new splitting/EvaluationOrderOfSharedChunkImports test asserts both the emitted import order and the runtime ["b","a"] log, and the entry2 line of the existing test is unchanged — a reasonable check that only the previously-wrong ordering moved.
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What does this PR do?
With
--splitting, a chunk's cross-chunkimportstatements were emitted in chunk-index order (entry-bit byte order), so two shared chunks evaluated in whichever order their bitsets sorted. They now follow the order in which the entry's evaluation walk reaches those chunks (reached_chunks_in_order, recorded byfindAllImportedPartsInJSOrder), which is the order the unbundled modules would run them in. No change in chunk count or size.Not addressed (unchanged, same as esbuild — evanw/esbuild#399, and identical in 1.4.0): code inlined into a chunk still runs after every chunk it imports, since an ES module's
imports evaluate before its body. An earlier revision of this PR also cut extra chunks to reproduce that order exactly (+9–13% output files on typical apps, up to 2× on interleaved multi-entry graphs); that pass was reverted here as a product decision rather than a fix.How did you verify your code works?
test/bundler/bundler_splitting.test.ts(splitting/EvaluationOrderOfSharedChunkImportsfails on 1.4.1), plusesbuild/splitting,bundler_compile_splitting,bundler_html,bake/dev-and-prod; 100-route split fixture unchanged (359 files, same bytes, same output).