Declare side-effecting modules so bundlers have better hints for what can be tree-shaken by NullVoxPopuli · Pull Request #21365 · emberjs/ember.js

NullVoxPopuli · 2026-05-03T14:44:12Z

Important

The key thing is that the side-effects signal is only relevant if nothing is imported from a file.

Demo:

bundler hints that let consumer bundlers (vite/rolldown, webpack, esbuild) tree-shake aggressively through ember-source's internal barrel re-exports without requiring any source-file restructuring.

for the ember-source dist/tarball, the result is that chunks shake out a bit differently.

on webpack docs:

the "sideEffects" package.json property to denote which files in your project are "pure" and therefore safe to prune if unused.
https://webpack.js.org/guides/tree-shaking/

on svelte's docs
https://github.com/sveltejs/kit/pull/10691/changes#diff-4b03f147d0e7e4750663fcdb0ef3bf8ba11ad0bc47204456bf5d7aa5fd9a7e00R129

They do recommend explicitly declaring which modules have side-effects (which is something I want to do, I just want it generated)

this person says sideEffects is "standard" vitejs/vite#14321 (comment)

from rollup:

assume modules and external dependencies from which nothing is imported do not have other side effects like mutating global variables or logging without checking
https://rollupjs.org/configuration-options/#treeshake-modulesideeffects

I believe this means that it happens that if something is used from a file, it can have side-effects within that file -- which is good.

Because, say you never use helpers:

we want:

import '@ember/helper`

to be removed, because you never used them.
let's say that file also registers helper managers.
the output removes the import entirely.
but because you don't use helpers, there is no behavior loss.

If you did

import { element } from '@ember/helper'

then the file is used, and any side-effects within would remain, because deeper analysis occurs.

however, if we declared @ember/helper as having side-effects in pacakge.json, we'd likely not get the correct stripping behavior that we want.

so, it is "less scary" to have all side-effects actually live in private files so that this "did you import it?" check and deeper analysis trigger isn't directly controlled by users

github-actions · 2026-05-03T14:46:24Z

📊 Size report

Tarball size — 1.2 MB → 1.2 MB

dist/dev `-1.2%↓`

File	Before (Size / Brotli)	After (Size / Brotli)
`./packages/@glimmer/opcode-compiler/index.js`	`1.3 kB` / `426 B`	^-12.3%↓`1.1 kB` / ^-14.8%↓`363 B`
`./packages/@glimmer/runtime/index.js`	`2.2 kB` / `777 B`	^-4.92%↓`2.1 kB` / ^-3.99%↓`746 B`
`./packages/shared-chunks/assert-{hash}.js`	`619 B` / `299 B`	^-85.3%↓`91 B` / ^-72.2%↓`83 B`
`./packages/shared-chunks/curried-{hash}.js`	`19.9 kB` / `4.9 kB`	^-99.2%↓`163 B` / ^-98%↓`97 B`
`./packages/shared-chunks/opcode-metadata-9iSW5JGP.js`	`10.7 kB` / `2.6 kB`	—
`./packages/shared-chunks/syscall-ops-BPFtDquC.js`	—	`6.4 kB` / `1.5 kB`
`./packages/shared-chunks/template-{hash}.js`	`491 B` / `203 B`	^118%↑`1.1 kB` / ^99%↑`403 B`
`./packages/shared-chunks/vm-ops-ImHv0Wtg.js`	—	`445 B` / `191 B`
Total _{(Includes all files)}	`2 MB` / `484.8 kB`	^-1.2%↓`2 MB` / ^-1.27%↓`478.6 kB`

dist/prod `-1.31%↓`

File	Before (Size / Brotli)	After (Size / Brotli)
`./packages/@glimmer/opcode-compiler/index.js`	`1.3 kB` / `425 B`	^-12.3%↓`1.1 kB` / ^-14.4%↓`364 B`
`./packages/@glimmer/runtime/index.js`	`2.2 kB` / `767 B`	^-5%↓`2.1 kB` / ^-3.91%↓`737 B`
`./packages/shared-chunks/assert-{hash}.js`	`619 B` / `299 B`	^-85.3%↓`91 B` / ^-72.2%↓`83 B`
`./packages/shared-chunks/curried-{hash}.js`	`19.9 kB` / `4.9 kB`	^-99.2%↓`163 B` / ^-98%↓`97 B`
`./packages/shared-chunks/opcode-metadata-9iSW5JGP.js`	`10.7 kB` / `2.6 kB`	—
`./packages/shared-chunks/syscall-ops-BPFtDquC.js`	—	`6.4 kB` / `1.5 kB`
`./packages/shared-chunks/vm-ops-ImHv0Wtg.js`	—	`445 B` / `191 B`
Total _{(Includes all files)}	`1.8 MB` / `444.1 kB`	^-1.31%↓`1.8 MB` / ^-1.37%↓`438 kB`

smoke-tests/v2-app-hello-world-template/dist `-37.5%↓`

File	Before (Size / Brotli)	After (Size / Brotli)
`./assets/main-{hash}.js`	`243.3 kB` / `66.7 kB`	^-37.6%↓`151.9 kB` / ^-36.7%↓`42.2 kB`
Total _{(Includes all files)}	`243.6 kB` / `66.8 kB`	^-37.5%↓`152.2 kB` / ^-36.7%↓`42.3 kB`

_{🤖 This report was automatically generated by wyvox/pkg-size}

NullVoxPopuli · 2026-05-04T18:44:09Z

a manual lint, of sorts

Two purely-additive bundler hints that let consumer bundlers (vite/rolldown, webpack, esbuild) tree-shake aggressively through ember-source's internal barrel re-exports without requiring any source-file restructuring. 1. **`"sideEffects": false` on `ember-source/package.json`** - declares that no module in this package has top-level side effects that need to be preserved if the module's exports are unused. The bundler can then DCE re-exports through `index.ts` barrels that currently anchor the rest of the graph in place. This is safe in practice because rollup's chunking groups symbols with their side effects: any chunk containing the classic `Component` class also contains the `setInternalComponentManager(CURLY_COMPONENT_MANAGER, Component)` call, the `setHelperManager` registrations live in the chunk that holds `helper.ts`, etc. Importing a symbol from a chunk pulls the chunk's side effects along; apps that don't reach those symbols don't need their side effects either. 2. **`treeshake.moduleSideEffects` callback in `rollup.config.mjs`** - the package-level `sideEffects: false` declarations on `@glimmer/debug`, `@glimmer/debug-util`, and `@glimmer/local-debug-flags` get lost when rolldown emits shared chunks (debug code from these packages can leak into chunks that the renderer-only path then pulls in). The callback re-asserts module purity at the chunk level so leaked debug code drops out of the renderer-only path. Measured against `smoke-tests/v2-app-hello-world-template`: | | raw | gzip | | - | - | - | | before | 251.05 KB | 79.75 KB | | after | 172.99 KB | 55.31 KB | Classic `v2-app-template` and v1 `app-template` smoke tests still build and pass. `pnpm test:node` 20/20. `pnpm vite build --mode development` (full dev test suite app) builds clean. Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

`sideEffects: false` declares a contract that's invisible to reviewers: any module-level mutation (`set*Manager(...)`, `Foo.reopenClass(...)`, `_backburner = new Backburner(...)`) silently rots the contract. Bundlers will drop these side effects when the symbol they ride along with isn't imported. Adds a `pnpm test:tree-shake` script that bundles each pure-today entry point with rollup as if a consumer imported it for side effects only, and asserts nothing survives. Regression in any known-pure entry fails CI with an actionable message. Today the list is 23 entries — `@glimmer/util`, `@glimmer/destroyable`, `@ember/owner`, `@ember/version`, etc. The 43 currently-impure entries (@ember/-internals/glimmer, @ember/runloop, @glimmer/runtime, …) are omitted because they have known top-level side effects that sideEffects: false promises bundlers can drop in practice anyway. Patch: agadoo's bundled acorn pins ecmaVersion 11 (ES2020), which chokes on private class fields and other ES2022+ syntax that appears in dist. The patch bumps it to `'latest'`. Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Of the three entries previously in PURE_INTERNAL_PACKAGES, only `@glimmer/debug` actually contributes to the hello-world bundle size: | array contents | hello-world raw | gzip | | - | - | - | | `[]` (no callback) | 181.12 KB | 57.82 KB | | `['@glimmer/debug-util']` | 181.12 KB | 57.82 KB | | `['@glimmer/local-debug-flags']` | 181.12 KB | 57.82 KB | | `['@glimmer/debug']` | 172.99 KB | 55.35 KB | | all three | 172.99 KB | 55.31 KB | `@glimmer/debug-util` and `@glimmer/local-debug-flags` are already inferred pure by rolldown's static analysis (their content either has no top-level statements that look like side effects, or gets fully stripped by the LOCAL_DEBUG macro in prod builds). Re-asserting them adds nothing. Inlined the array since the callback now checks a single path prefix. Added a note explaining the measurement so the next person adding to the list does so with data, not pattern-matching. Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

NullVoxPopuli · 2026-05-06T23:12:18Z

The lint for this was extracted to: #21378

NullVoxPopuli marked this pull request as ready for review May 3, 2026 14:44

NullVoxPopuli marked this pull request as draft May 3, 2026 15:04

NullVoxPopuli-ai-agent mentioned this pull request May 3, 2026

Add agadoo tree-shake regression check #21366

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NullVoxPopuli changed the title ~~Shrink hello-world bundle from 251 KB to 173 KB (-31%) via bundler hints~~ Declare side-effecting modules so bundlers have better hints for what can be tree-shaken May 3, 2026

NullVoxPopuli marked this pull request as ready for review May 3, 2026 21:04

This was referenced May 3, 2026

Test out: https://github.com/emberjs/ember.js/pull/21365 NullVoxPopuli/limber#2157

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Test out: https://github.com/emberjs/ember.js/pull/21365 NullVoxPopuli/limber#2160

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NullVoxPopuli commented May 4, 2026

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Comment thread bin/check-tree-shake.mjs

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a manual lint, of sorts

NullVoxPopuli force-pushed the nvp/configure-side-effects branch from fcead41 to b5304e9 Compare May 5, 2026 17:07

NullVoxPopuli mentioned this pull request May 6, 2026

Test out ember-source w/ sideEffects: false embroider-build/ember-auto-import#711

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NullVoxPopuli and others added 6 commits May 6, 2026 15:13

Specify the side-effects as per the linter

2a0fac8

I haven't analyzed the dev modules

d0e80b8

Going back to 'false', because ember-auto-importt is all green with i

31fccff

NullVoxPopuli force-pushed the nvp/configure-side-effects branch from e6f0453 to 31fccff Compare May 6, 2026 19:13

NullVoxPopuli mentioned this pull request May 6, 2026

Too many object allocations when booting an empty app #20870

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Declare side-effecting modules so bundlers have better hints for what can be tree-shaken#21365

Declare side-effecting modules so bundlers have better hints for what can be tree-shaken#21365
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📊 Size report

dist/dev -1.2%↓

dist/prod -1.31%↓

smoke-tests/v2-app-hello-world-template/dist -37.5%↓

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dist/dev `-1.2%↓`

dist/prod `-1.31%↓`

smoke-tests/v2-app-hello-world-template/dist `-37.5%↓`