feat(split-chunks): discover shared chunk intersections by JSerFeng · Pull Request #21991 · webpack/webpack

JSerFeng · 2026-09-06T17:13:37Z

Summary

entry: a b c d

a -> m1 m2
b -> m1 m2
c -> m1
d -> m2

m1 belongs to [a b c]
m2 belongs to [a b d]

What I want is to extra m1 + m2 into a separate chunk, but this can not happen in current algorithm.

As the get_combinations has trade-off for performance, it only calculate the intersections from existing combinations. which is as follows

combinations:
m1: [a b c] [a] [b] [c]
m2: [a b d] [a] [b] [d]

so the shared is
a: [m1 m2]
b: [m1 m2]
c: null
d: null
a b c: null
a b d: null

But can we extract m1 + m2 from a and b ?

Nope because they miss the minChunks: 2 in default cache group, so every module is not optimized by splitChunks.

But if there is a m3 that belongs to both a and b, and only a and b, not c or d, then there is another combination

m1: [a b] [a b c] [a] [b] [c]
m2: [a b] [a b d] [a] [b] [d]
m3: [a b]

so the shared is
a: [m1 m2]
b: [m1 m2]
ab: [m1 m2 m3]
c: null
d: null
abc: null
abd: null

Then we can successfully extract the m1+m2 from a and b, amazing right.

But in real app, that is not easy to meet, so there is huge optimization here, if we can get all right combinations not only depend on existing chunk combinations, we can maybe sacrifice a little performance I think, I'm trying my best to only regression about 1-5% performance

WIP

changeset-bot · 2026-09-06T17:13:43Z

🦋 Changeset detected

Latest commit: 9433e96

The changes in this PR will be included in the next version bump.

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lib/util/SetHelpers.js
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📝 Walkthrough

Walkthrough

The change adds bounded intersection discovery and propagation for split-chunk combinations. It updates SplitChunksPlugin to use propagated intersections and adds unit and configuration coverage for size thresholds, export modes, naming, reduction, and minChunks.

Changes

Split-chunk intersection discovery

Layer / File(s)	Summary
Intersection utility and unit coverage `lib/util/SetHelpers.js`, `test/SetHelpers.unittest.js`	`findIntersections` returns keyed intersections. `associateIntersections` maps them to containing original sets under bounded checks, comparisons, and weighted expansions. Unit tests cover deterministic limits and incomplete propagation.
SplitChunksPlugin combination integration `lib/optimize/SplitChunksPlugin.js`, `.changeset/012-split-chunk-intersections.md`	`SplitChunksPlugin` tracks module counts, associates intersections with original chunk sets, records propagation statistics, and appends intersections to intersection-aware combinations.
Split-chunks intersection configuration coverage `test/configCases/split-chunks/intersections-min-size/*`	The configuration case adds shared modules, pair and entry fixtures, and eight configurations. Assertions validate module chunk counts and chunks common to all three modules across the supported options.

Suggested reviewers: alexander-akait

Merge Risk: 🔵 Low · up to cde4e

Shared split-chunk intersections are now discovered before size validation, improving extraction of common modules. A minor build-time allocation concern remains in the per-module cache-group path, but no correctness or runtime behavior issue is established.

🚥 Pre-merge checks | ✅ 3 | ❌ 1

❌ Failed checks (1 inconclusive)

Check name	Status	Explanation	Resolution
Title check	❓ Inconclusive	The title uses the required Conventional Commit form, uses the allowed type `feat`, and accurately describes shared chunk intersection discovery. The branch prefix is not provided, so the type-to-bran…	Provide the branch name or confirm that its prefix is `feat`. If it is not, update the title type to match the branch prefix.

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The title uses the required Conventional Commit form, uses the allowed type feat, and accurately describes shared chunk intersection discovery. The branch prefix is not provided, so the type-to-branch-prefix match cannot be verified.

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🧹 Nitpick comments (1)

lib/optimize/SplitChunksPlugin.js (1)
1575-1578: 🚀 Performance & Scalability | 🔵 Trivial | ⚡ Quick win

Precompute the minSizeReduction predicate on the cache group.

includeIntersections runs for each module/cache-group pair in the modules phase. hasNonZeroSizes allocates an Object.keys array on every call, although the normalized minSizeReduction is cached with the cache group. This adds avoidable allocation and iteration work proportional to the number of pairs.

Add _validateSizeReduction to the CacheGroup typedef and initialize it in _getCacheGroup:
 			_validateSize: hasNonZeroSizes(minSize),
+			_validateSizeReduction: hasNonZeroSizes(minSizeReduction),
 			_validateRemainingSize: hasNonZeroSizes(minRemainingSize),
Then use the cached predicate:
 						const includeIntersections =
 							cacheGroup.getName === defaultGetName &&
-							(cacheGroup._validateSize ||
-								hasNonZeroSizes(cacheGroup.minSizeReduction));
+							(cacheGroup._validateSize ||
+								cacheGroup._validateSizeReduction);
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In `@lib/optimize/SplitChunksPlugin.js` around lines 1575 - 1578, Precompute the
minSizeReduction non-zero predicate on each cache group in _getCacheGroup, store
it as the new _validateSizeReduction field in the CacheGroup typedef, and
replace the per-pair hasNonZeroSizes(cacheGroup.minSizeReduction) call in
includeIntersections with that cached field while preserving the existing
_validateSize condition.

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Nitpick comments:
In `@lib/optimize/SplitChunksPlugin.js`:
- Around line 1575-1578: Precompute the minSizeReduction non-zero predicate on
each cache group in _getCacheGroup, store it as the new _validateSizeReduction
field in the CacheGroup typedef, and replace the per-pair
hasNonZeroSizes(cacheGroup.minSizeReduction) call in includeIntersections with
that cached field while preserving the existing _validateSize condition.

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📒 Files selected for processing (17)

.changeset/012-split-chunk-intersections.md
lib/optimize/SplitChunksPlugin.js
lib/util/SetHelpers.js
test/SetHelpers.unittest.js
test/configCases/split-chunks/intersections-min-size/a.js
test/configCases/split-chunks/intersections-min-size/b.js
test/configCases/split-chunks/intersections-min-size/c0.js
test/configCases/split-chunks/intersections-min-size/c1.js
test/configCases/split-chunks/intersections-min-size/c2.js
test/configCases/split-chunks/intersections-min-size/m0.js
test/configCases/split-chunks/intersections-min-size/m1.js
test/configCases/split-chunks/intersections-min-size/m2.js
test/configCases/split-chunks/intersections-min-size/pair01.js
test/configCases/split-chunks/intersections-min-size/pair02.js
test/configCases/split-chunks/intersections-min-size/pair12.js
test/configCases/split-chunks/intersections-min-size/test.config.js
test/configCases/split-chunks/intersections-min-size/webpack.config.js

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codspeed · 2026-09-06T18:07:11Z

Merging this PR will regress 1 benchmark

⚡ 2 improved benchmarks
❌ 1 regressed benchmark
✅ 391 untouched benchmarks

Warning

Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Mode	Benchmark	`BASE`	`HEAD`	Efficiency
❌	Memory	`benchmark "future-defaults", scenario '{"name":"mode-development","mode":"development"}'`	881.7 KB	1,388.2 KB	-36.48%
⚡	Memory	`benchmark "side-effects-reexport", scenario '{"name":"mode-development-rebuild","mode":"development","watch":true}'`	468.4 KB	164.2 KB	×2.9
⚡	Memory	`benchmark "asset-modules-source", scenario '{"name":"mode-production","mode":"production"}'`	8.4 MB	6.3 MB	+32.84%

Tip

Investigate this regression by commenting @codspeedbot fix this regression on this PR, or directly use the CodSpeed MCP with your agent.

_{Comparing JSerFeng:feat/split-chunks-intersections (c22632f) with main (ee66ae8)}

codecov · 2026-09-06T18:08:58Z

Codecov Report

❌ Patch coverage is 89.07104% with 20 lines in your changes missing coverage. Please review.
✅ Project coverage is 60.42%. Comparing base (4448f8b) to head (c22632f).
⚠️ Report is 127 commits behind head on main.

Files with missing lines	Patch %	Lines
lib/optimize/SplitChunksPlugin.js	83.33%	12 Missing ⚠️
lib/util/SetHelpers.js	92.79%	8 Missing ⚠️

❌ Your patch check has failed because the patch coverage (89.07%) is below the target coverage (90.00%). You can increase the patch coverage or adjust the target coverage.

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syntax-equivalence 1 0

integration 20 0

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##             main   #21991       +/-   ##
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  Branches    28944    25548     -3396     
===========================================
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- Misses       4490    33016    +28526

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JSerFeng · 2026-09-07T09:20:26Z

This is bad for performance, wait for me to improve it.

Use frozen intersection rounds and complete original-set support. Rebuild candidates from remaining original module-chunk edges between cache-group priorities.

JSerFeng · 2026-09-18T07:25:03Z

@alexander-akait @hai-x I've updated the summary to simply show what this PR does and what problem we met.

In severa internal projects in my company, it turns out that there is a huge size just for duplicated modules, they use the default splitChunks config with chunks:'all', which is also my recommendation for most projects before.

eg. 3k modules total, there is about 6k modules duplicated, as one module might be placed in multiple chunks, (even 20+ chunks in my internal project), so the duplicated small modules are even more than total modules.

The current solution is to find intersections of all chunk combinations, but just find intersection in 1 level, that is for {abc} intersect {abcd}, we find {abc} but not {ab}, because find {abc} is enough for most cases, find {ab} will cost much more compulation. But even so, the performance regression for large project with lots of chunks is obivious, but is acceptable by me, like about 1-5% total build time regression.

So we can discuss it more, and you can test this in some of your large projects to see the gain as well, and consider if we should make this default in production mode.

Also checked Turbopack chunking, they uses similar algorithm with limited computation cost

(Oh by the way, after used this optimization, the output still contains much more duplications than total modules, so there is still much room for us to improve)

hai-x · 2026-09-21T09:51:59Z

I think 1-5% overhead is acceptable, what do you think? @alexander-akait

alexander-akait · 2026-09-22T05:42:15Z

Fixed in the main. With some peer improvements in the plugin at all, thank for the PR and raising this, feel free to feedback

github-actions Bot added the area: optimization Tree-shaking, splitChunks, concatenation, ids (lib/optimize, lib/ids) label Sep 6, 2026

coderabbitai Bot added the needs: tests Fix or PR exists but lacks the required tests label Sep 6, 2026

coderabbitai Bot reviewed Sep 6, 2026

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coderabbitai Bot removed the needs: tests Fix or PR exists but lacks the required tests label Sep 6, 2026

coderabbitai Bot added needs: tests Fix or PR exists but lacks the required tests and removed needs: tests Fix or PR exists but lacks the required tests labels Sep 6, 2026

JSerFeng marked this pull request as draft September 7, 2026 09:20

JSerFeng mentioned this pull request Sep 10, 2026

feat(split-chunks): add configurable dedupDepth web-infra-dev/rspack#15497

Merged

github-actions Bot added area: config Options, defaults, validation (lib/config, schemas/) area: types types.d.ts, JSDoc annotations, hand-maintained declarations labels Sep 18, 2026

JSerFeng added 7 commits September 18, 2026 14:52

feat(split-chunks): discover shared chunk intersections

358a0c4

perf(split-chunks): cache size reduction validation

f1a3de8

perf(split-chunks): bound intersection propagation

3fe9115

perf(split-chunks): bound intersection expansion

9dd8d1f

perf(split-chunks): bound propagated chunk visits

17f2235

fix(split-chunks): discover singleton intersections

4098381

feat(split-chunks): sync configurable dedupDepth from rspack

9433e96

Use frozen intersection rounds and complete original-set support. Rebuild candidates from remaining original module-chunk edges between cache-group priorities.

JSerFeng force-pushed the feat/split-chunks-intersections branch from 702703c to 9433e96 Compare September 18, 2026 06:58

alexander-akait mentioned this pull request Sep 21, 2026

feat(split-chunks): discover the chunk sets that modules share #22240

Merged

alexander-akait closed this Sep 22, 2026

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feat(split-chunks): discover shared chunk intersections - #21991

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