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Implement try-statement-deoptimization for feature detection, tree-shake unused arguments #2892

merged 8 commits into from Jun 5, 2019


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@lukastaegert lukastaegert commented Jun 4, 2019

This PR contains:

  • bugfix
  • feature
  • refactor
  • documentation
  • other

Are tests included?

  • yes (bugfixes and features will not be merged without tests)
  • no

Breaking Changes?

  • yes (breaking changes will not be merged unless absolutely necessary)
  • no

List any relevant issue numbers:
resolves #2891
resolves #2869
resolves #2540
resolves #2273
resolves #1771


This PR adds a heuristic around try-catch blocks that allows for feature detection workflows, or workflows that depend on uncommonly thrown errors such as checking if class test extends someVar {} throws an error to see if someVar is a valid constructor.

I checked that with this modification, all feature detections of core-js should work as intended. Besides the try-catch deoptimization, this PR will also tree-shake unused function arguments.

Try-statement deoptimization

This PR adds a heuristic based on the idea that in order to detect an uncommonly thrown error, you will probably put it into the try-statement of a try-catch construct (alternatively promises would work as well but the heuristic will not cover those).


  • Tree-shaking will be disabled inside try-statements, i.e. code directly nested inside a try-statement will be retained.

  • If a variable is

    • called from inside try-statement
    • called directly, i.e. myVar() and not e.g. (myVar || otherVar)().
    • can be uniquely resolved to a function declaration/expression (i.e. variable is never reassigned)

    then tree-shaking will also be disabled in that function; however, other variables called from such a function will NOT be deoptimized to contain the infectious nature of this feature.

  • If a parameter of a function is called inside a try-statement, this parameter is marked in a special way. In all places this function is called, the argument provided for this parameter will be treated the same way as if it was called from within the try-statement. This is to allow core-js style feature detection:

    function fails(exec) {
      try {
        return !!exec();
      } catch (e) {
        return true;
    var MAX_UINT32 = 0xffffffff;
    var SUPPORTS_Y = !fails(function() {
    	RegExp(MAX_UINT32, 'y'); // this will now be retained by Rollup

Unused argument tree-shaking

To use some of the logic built for this feature for other cool stuff, this PR will not also tree-shake unused arguments from function calls. To that end, each time the call target of a call can be uniquely resolved, unused arguments will be removed from the call. More precisely:

  • A parameter that is either

    • not defined or
    • an identifier (i.e. the parameter is not a pattern or the paramter has a default value) and defined but not used by a function

    is "unused" unless arguments is accessed inside that function.

  • Call arguments corresponding to "unused" parameters will be removed if

    • they have no side effects (e.g. they are not the result of a side-effect-ful function call)
    • there are no other arguments after that call that are included (to maintain the correspondence between arguments and parameters).
  • Parameters themselves will not be removed even if they are unused to avoid difficult edge-cases when some logic relies on functionName.length to determine the number of parameters for e.g. currying.

This enables tree-shaking in scenarios such as

// input
const options = {
  prefix: 'cool '

const useOptions = false;

function doIt(value, options) {
  if (useOptions) console.log(options.prefix + value);
  else console.log(value);

doIt('feature', options);

// result
function doIt(value, options) {


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@guybedford guybedford left a comment

Very pleased to see work in these directions, and agreed its about matching the common feature detection forms here.

It would be good to see some tests on try-catch dynamic require / non-analyzed dynamic import cases that are passed through to the built environment, although in theory that would be no different right?

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lukastaegert commented Jun 4, 2019

At the moment it would not make a difference for dynamic imports. Except to make sure that dynamic imports inside try-blocks are always included if the try-block is included.

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ljharb commented Jun 4, 2019

It’d be great to test this on es5-shim, es6-shim, and all of airbnb-browser-shims, since that would catch the bug referenced in #2540.

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lukastaegert commented Jun 4, 2019

You can try it out by installing rollup via npm install rollup/rollup#try-block-deoptimization.

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lukastaegert commented Jun 4, 2019

at least es6-shim should be fine, throwsError is exactly something that should be caught by the heuristic.

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lukastaegert commented Jun 4, 2019

Any help here is welcome, a simple way to test it could be to write an entry point that contains the shim, e.g. main.js, and then run

npx rollup/rollup#try-block-deoptimization main.js --format esm > output.js
npx rollup/rollup#try-block-deoptimization main.js --format esm --no-treeshake > full-output.js

and diff the two files output.js and full-output.js with your favourite diffing tool to see if something vital is missing in output.js.

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