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Scope hoisting less effective since rewrite #7481

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101arrowz opened this issue Dec 24, 2021 · 6 comments · Fixed by #7833
Closed

Scope hoisting less effective since rewrite #7481

101arrowz opened this issue Dec 24, 2021 · 6 comments · Fixed by #7833
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🐛 Bug ✨ Parcel 2 swc

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@101arrowz
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🐛 bug report

Scope hoisting is dramatically less effective since #6230. This seems to be because #__PURE__ annotations don't work properly anymore.

🎛 Configuration (.babelrc, package.json, cli command)

I'm testing with zero configuration. Here's package.json:

{
  "name": "fflt",
  "version": "1.0.0",
  "license": "MIT",
  "devDependencies": {
    "parcel": "2.0.0-nightly.655"
  },
  "dependencies": {
    "fflate": "^0.7.2"
  }
}

🤔 Expected Behavior

The scope hoisting rewrite should not adversely affect scope hoisting for any case.

😯 Current Behavior

Scope hoisting is dramatically less effective in every version since ba57b65 (#6230).

💁 Possible Solution

The behavior is similar to what happens when you remove all #__PURE__ annotations from fflate. Maybe all comments are now being stripped before terser processes the code?

🔦 Context

Without the ability to utilize /*#__PURE__*/ comments, every function call is considered a side effect, which makes it impossible to tree-shake out IIFEs and variables used in exported closures that are initialized to the result of a function call.

💻 Code Sample

Here's my index.html:

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    <script type="module" src="./index.js"></script>
</body>
</html>

Here's index.js:

import { gzipSync, strToU8 } from 'fflate';

console.log(gzipSync(strToU8('hello world')));

To build, I use parcel build index.html. When using parcel@2.0.0-nightly.654 (gitHead = fdbcbd6):
6.78kB JS file

Running the same command using parcel@2.0.0-nightly.655 (gitHead = ba57b65):
21.79kB JS file

🌍 Your Environment

Software Version(s)
Parcel Any version since 2.0.0-nightly.655
Node v16.6.2
npm/Yarn Yarn 1.22.15
Operating System Ubuntu 20.04 on WSL
@mischnic mischnic mentioned this issue Dec 24, 2021
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@mischnic
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mischnic commented Dec 24, 2021 •
edited
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I have identified one problem where pure comments of IIFE are stripped during the build:

// index.js
import { strToU8 } from './other.js';
console.log((strToU8('hello world')));


// other.js
var u32 = Uint32Array;
var flm = /*#__PURE__*/ hMap(flt, 9, 0);
var deo = /*#__PURE__*/ new u32([
    65540, 131080, 131088
]);
export function strToU8(a) {
    return "converted";
}
var Unzip = /*#__PURE__*/ (function () {
    console.log("UNUSED")
}());
export { Unzip };

results in

(() => {
var $da3800f92a93f12a$var$u32 = Uint32Array;
var $da3800f92a93f12a$var$flm = /*#__PURE__*/ hMap(flt, 9, 0);
var $da3800f92a93f12a$var$deo = /*#__PURE__*/ new $da3800f92a93f12a$var$u32([
    65540,
    131080,
    131088
]);
function $da3800f92a93f12a$export$366b39a6daa8ed7a(a) {
    return "converted";
}
var $da3800f92a93f12a$export$9b7485f84cbaaa56 = function() { // <-------  !!!
    console.log("UNUSED");
}();


console.log($da3800f92a93f12a$export$366b39a6daa8ed7a('hello world'));

})();
//# sourceMappingURL=index.js.map

See the linked swc issue, we'll see if there are more problems once that particular one is fixed.

@101arrowz
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Thanks for researching the problem! For future reference, how did you get the bundled but pre-terser code?

@mischnic
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parcel build ... --no-optimize

This was referenced Dec 24, 2021
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@mischnic
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If I comment that expr_simplifier pass out, the output is 6.78 KB which seems to be what you were expecting.

@101arrowz
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Since swc-project/swc#3115 has been resolved if you add paren_remover, this should be fixable now in Parcel? Or is paren_remover on by default?

@mischnic
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swc needs to be bumped and that pass has be added

@mischnic mischnic mentioned this issue Mar 16, 2022
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Bump swc and prevent pure comment removal parcel-bundler/parcel
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@mischnic @101arrowz