Treeshaking multiple entry modules with cross-referencing creates extra chunks

[tmsns]

Rollup Version

3.21.0

Operating System (or Browser)

Chrome 112

Node Version (if applicable)

N/A

Link To Reproduction

https://rollupjs.org/repl/?version=3.21.0&shareable=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

Expected Behaviour

We expect an output of 2 chunks where the internal used module integer-math.js is bundled into math.js

Actual Behaviour

When treeshaking is active, Rollup creates an extra chunk for the internal module (integer-math.js) and references this chunk instead of the originally referenced module (math.js)

[tmsns]

Some more background info:

Possible Cause

We think that Rollup tries to optimize the output bundle and as he notices a module (math.js) with only re-exports, he drills down directly to the referenced inner module (integer-math.js). But as two entries need this file, he will create a new chunk for both entries.

We tried to change the behavior using some of the options of Rollup, but none of them (output.hoistTransitiveImports, preserveentrysignatures, treeshake.*) created the expected bahavior.

Workarounds

As we were surprised by this result, we tried to workaround this behavior. We came up with a number of ways, none of which are ideal.

• mark math.js as side effect
  When adding a side effect to math.js (eg. void Console, a side effect that doesn't do anything), Rollup seems to create the expected output. Of course we don't like to have extra side effects in a file to just do nothing. This could also be achieved dynamically with a plugin.
• disable treeshaking
  Of course, we still want to be able to tree shake, so this solution was dismissed.
• a manualChunk function
  For now we settled with a manualChunk function that will make sure that modules that are reference from an entry module are bundled together. But we considered this more as a workaround than an actual long-term solution.

import { basename } from 'path';

/**
 * Bundles all first level imports of an entry module together with its entry in the same chunk.
 *
 * @type {import('rollup').GetManualChunk}
 */
export function bundleImportsOfEntries(id, { getModuleInfo }) {
  const moduleInfo = getModuleInfo(id);
  if (moduleInfo.isEntry) {
    return basename(id);
  }

  const entryPoint = moduleInfo.importers.find(importer => getModuleInfo(importer).isEntry);

  if (entryPoint) {
    return basename(entryPoint);
  }

  return undefined;
} 

/cc @jorenbroekema @bashmish

[rollup-bot]

This issue has been resolved via #4989 as part of rollup@3.22.0-0. Note that this is a pre-release, so to test it, you need to install Rollup via npm install rollup@3.22.0-0 or npm install rollup@beta. It will likely become part of a regular release later.

[rollup-bot]

This issue has been resolved via #4989 as part of rollup@3.22.0. You can test it via npm install rollup.