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Inline hoisted, post-return declarations properly #391

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merged 7 commits into from Jan 28, 2017
Merged

Inline hoisted, post-return declarations properly #391

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81cd100
Inline hoisted, post-return declarations properly. Fixes #192
kangax Jan 26, 2017
3404d3d
Add another test
kangax Jan 26, 2017
5b735d1
Change algorithm to check references
kangax Jan 26, 2017
9d49be4
undefined --> void 0
kangax Jan 26, 2017
eddd59f
Fix comment typo
kangax Jan 26, 2017
80dd806
Deopt if reference is in different scope
kangax Jan 27, 2017
ffefb6d
Fix travis
kangax Jan 27, 2017
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43 changes: 43 additions & 0 deletions packages/babel-plugin-minify-dead-code-elimination/__tests__/dead-code-elimination-test.js
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Expand Up @@ -2460,4 +2460,47 @@ describe("dce-plugin", () => {
`);
expect(transform(source)).toBe(expected);
});

it("should not remove vars after return statement", () => {

const source = unpad(`
function f() {
return x;
var x = 1;
}
`);

const expected = unpad(`
function f() {
return undefined;
}
`);

expect(transform(source)).toBe(expected);
});

it("should not remove vars after return statement #2", () => {

const source = unpad(`
var x = 0;
function f1(){
function f2(){
return x;
};
return f2();
var x = 1;
}
`);

const expected = unpad(`
var x = 0;
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This is not ideal, of course. But we could revisit later.

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It's actually correct. This is not removed because it's a global.

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Created #392

function f1() {
return function () {
return undefined;
}();
}
`);

expect(transform(source)).toBe(expected);
});
});
30 changes: 27 additions & 3 deletions packages/babel-plugin-minify-dead-code-elimination/src/index.js
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Expand Up @@ -3,6 +3,15 @@
const some = require("lodash.some");
const { markEvalScopes, isMarked: isEvalScopesMarked, hasEval } = require("babel-helper-mark-eval-scopes");

function forEachPrevSibling(path, callback) {
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Considering that DCE has a lot of cases of path.parentPath.getSibling(path.parentPath.key - 1) and path.parentPath.getSibling(path.parentPath.key + 1), I think it makes sense to add helper traversal methods to Babel.

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Filed babel/babel#5223

const parentPath = path.parentPath;
let key = parentPath.key;

while ((path = parentPath.getSibling(--key)).type) {
callback(path);
}
}

module.exports = ({ types: t, traverse }) => {
const removeOrVoid = require("babel-helper-remove-or-void")(t);
const shouldRevisit = Symbol("shouldRevisit");
Expand Down Expand Up @@ -238,10 +247,25 @@ module.exports = ({ types: t, traverse }) => {
}

if (binding.references === 1 && binding.kind !== "param" && binding.kind !== "module" && binding.constant) {

let replacement = binding.path.node;
let replacementPath = binding.path;
let isAfterReturn = false;

if (t.isVariableDeclarator(replacement)) {
replacement = replacement.init;

forEachPrevSibling(replacementPath, (path) => {
if (t.isReturnStatement(path)) {
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this will fail for cases like -

function foo() {
  bar = x;
  var x = 1;
}

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You're right. But then we have a much "deeper" problem:

function foo() {
  (function() {
    alert(function(){ return x }());
  })();
  var x = 1;
}

This brings us again to CFG and knowing when an identifier is referenced before declaration (having to traverse entire tree before declaration).

I think in this case it would be simpler to just hoist vars before doing DCE. So this:

function foo() {
  bar = x;
  var x = 1;
}

would first become:

function foo() {
  var x;
  bar = x;
  x = 1;
}

which could then be transformed to:

function foo() {
  bar = undefined;
}

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Case 1 can be solved without a CFG ... siblingPath.isAncestor(referencedPath) will tell you this.

function foo() {
  (() => {
    return x // x - ReferencedPath
  })() // CallExpression SiblingPath
  var x = 1;
}

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@kangax kangax Jan 26, 2017 •
edited

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Right, for each binding reference we can traverse a subtree upwards and check if parent is a previous sibling. We also need to check that identifier is not already defined:

function foo() {
  ((x) => {
    return x // x - ReferencedPath but is a local var
  })(2) // CallExpression SiblingPath
  var x = 1;
}

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Yes! But, this case we are dealing with is "one-time-used-constant" - binding.references === 1 and binding.constant, and we already have the path to the reference (binding.referencePaths[0]). So the local variable case wouldn't be this referencePath.

isAfterReturn = true;
}
});

// simulate hoisting by replacing value
// with undefined if declaration is after return
replacement = isAfterReturn
? t.identifier("undefined")
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void 0 is safe. We can't assume that there is no local variable called undefined

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Good point, thanks

: replacement.init;

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a few lines below this, replacementPath = replacementPath.get("init") - we might have to fix this here too. Else replacement and replacementPath might point to different things.

// Bail out for ArrayPattern and ObjectPattern
// TODO: maybe a more intelligent approach instead of simply bailing out
if (!replacementPath.get("id").isIdentifier()) {
Expand All @@ -253,7 +277,7 @@ module.exports = ({ types: t, traverse }) => {
continue;
}

if (!scope.isPure(replacement, true)) {
if (!scope.isPure(replacement, true) && !isAfterReturn) {
continue;
}

Expand Down Expand Up @@ -358,7 +382,7 @@ module.exports = ({ types: t, traverse }) => {
return;
}

// Not last in it's block? (See BlockStatement visitor)
// Not last in its block? (See BlockStatement visitor)
if (path.container.length - 1 !== path.key &&
!canExistAfterCompletion(path.getSibling(path.key + 1)) &&
path.parentPath.isBlockStatement()
Expand Down