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Support marking a call as pure
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A function call or IIFE with an immediately preceding comment
containing `@__PURE__` or `#__PURE__` is deemed to be a
side-effect-free pure function call and can potentially be
dropped.

Depends on `side_effects` option.

`[#@]__PURE__` hint will be removed from comment when pure
call is dropped.

fixes #1261
closes #1448
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@kzc @alexlamsl
kzc authored and alexlamsl committed Feb 21, 2017
1 parent d48a308 commit 1e51586
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Showing 3 changed files with 151 additions and 2 deletions.
20 changes: 18 additions & 2 deletions lib/compress.js
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Original file line number Diff line number Diff line change
Expand Up @@ -1360,6 +1360,22 @@ merge(Compressor.prototype, {
});
});

AST_Call.DEFMETHOD("has_pure_annotation", function(compressor) {
if (!compressor.option("side_effects")) return false;
if (this.pure !== undefined) return this.pure;
var pure = false;
var comments, last_comment;
if (this.start
&& (comments = this.start.comments_before)
&& comments.length
&& /[@#]__PURE__/.test((last_comment = comments[comments.length - 1]).value)) {
compressor.warn("Dropping __PURE__ call [{file}:{line},{col}]", this.start);
last_comment.value = last_comment.value.replace(/[@#]__PURE__/g, ' ');
pure = true;
}
return this.pure = pure;
});

// determine if expression has side effects
(function(def){
def(AST_Node, return_true);
Expand All @@ -1369,7 +1385,7 @@ merge(Compressor.prototype, {
def(AST_This, return_false);

def(AST_Call, function(compressor){
if (compressor.pure_funcs(this)) return true;
if (!this.has_pure_annotation(compressor) && compressor.pure_funcs(this)) return true;
for (var i = this.args.length; --i >= 0;) {
if (this.args[i].has_side_effects(compressor))
return true;
Expand Down Expand Up @@ -1904,7 +1920,7 @@ merge(Compressor.prototype, {
def(AST_Constant, return_null);
def(AST_This, return_null);
def(AST_Call, function(compressor, first_in_statement){
if (compressor.pure_funcs(this)) return this;
if (!this.has_pure_annotation(compressor) && compressor.pure_funcs(this)) return this;
var args = trim(this.args, compressor, first_in_statement);
return args && AST_Seq.from_array(args);
});
Expand Down
118 changes: 118 additions & 0 deletions test/compress/issue-1261.js
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Original file line number Diff line number Diff line change
@@ -0,0 +1,118 @@
pure_function_calls: {
options = {
evaluate : true,
conditionals : true,
comparisons : true,
side_effects : true,
booleans : true,
unused : true,
if_return : true,
join_vars : true,
cascade : true,
negate_iife : true,
}
input: {
// pure top-level IIFE will be dropped
// @__PURE__ - comment
(function() {
console.log("iife0");
})();

// pure top-level IIFE assigned to unreferenced var will not be dropped
var iife1 = /*@__PURE__*/(function() {
console.log("iife1");
function iife1() {}
return iife1;
})();

(function(){
// pure IIFE in function scope assigned to unreferenced var will be dropped
var iife2 = /*#__PURE__*/(function() {
console.log("iife2");
function iife2() {}
return iife2;
})();
})();

// comment #__PURE__ comment
bar(), baz(), quux();
a.b(), /* @__PURE__ */ c.d.e(), f.g();
}
expect: {
var iife1 = function() {
console.log("iife1");
function iife1() {}
return iife1;
}();

baz(), quux();
a.b(), f.g();
}
expect_warnings: [
"WARN: Dropping __PURE__ call [test/compress/issue-1261.js:17,8]",
"WARN: Dropping side-effect-free statement [test/compress/issue-1261.js:17,8]",
"WARN: Dropping __PURE__ call [test/compress/issue-1261.js:30,37]",
"WARN: Dropping unused variable iife2 [test/compress/issue-1261.js:30,16]",
"WARN: Dropping side-effect-free statement [test/compress/issue-1261.js:28,8]",
"WARN: Dropping __PURE__ call [test/compress/issue-1261.js:38,8]",
"WARN: Dropping __PURE__ call [test/compress/issue-1261.js:39,31]",
]
}

pure_function_calls_toplevel: {
options = {
evaluate : true,
conditionals : true,
comparisons : true,
side_effects : true,
booleans : true,
unused : true,
if_return : true,
join_vars : true,
cascade : true,
negate_iife : true,
toplevel : true,
}
input: {
// pure top-level IIFE will be dropped
// @__PURE__ - comment
(function() {
console.log("iife0");
})();

// pure top-level IIFE assigned to unreferenced var will be dropped
var iife1 = /*@__PURE__*/(function() {
console.log("iife1");
function iife1() {}
return iife1;
})();

(function(){
// pure IIFE in function scope assigned to unreferenced var will be dropped
var iife2 = /*#__PURE__*/(function() {
console.log("iife2");
function iife2() {}
return iife2;
})();
})();

// comment #__PURE__ comment
bar(), baz(), quux();
a.b(), /* @__PURE__ */ c.d.e(), f.g();
}
expect: {
baz(), quux();
a.b(), f.g();
}
expect_warnings: [
"WARN: Dropping __PURE__ call [test/compress/issue-1261.js:79,8]",
"WARN: Dropping side-effect-free statement [test/compress/issue-1261.js:79,8]",
"WARN: Dropping __PURE__ call [test/compress/issue-1261.js:92,37]",
"WARN: Dropping unused variable iife2 [test/compress/issue-1261.js:92,16]",
"WARN: Dropping side-effect-free statement [test/compress/issue-1261.js:90,8]",
"WARN: Dropping __PURE__ call [test/compress/issue-1261.js:100,8]",
"WARN: Dropping __PURE__ call [test/compress/issue-1261.js:101,31]",
"WARN: Dropping __PURE__ call [test/compress/issue-1261.js:84,33]",
"WARN: Dropping unused variable iife1 [test/compress/issue-1261.js:84,12]",
]
}
15 changes: 15 additions & 0 deletions test/mocha/minify.js
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Original file line number Diff line number Diff line change
Expand Up @@ -95,4 +95,19 @@ describe("minify", function() {
assert.strictEqual(code, "var a=function(n){return n};");
});
});

describe("#__PURE__", function() {
it("should drop #__PURE__ hint after use", function() {
var result = Uglify.minify('//@__PURE__ comment1 #__PURE__ comment2\n foo(), bar();', {
fromString: true,
output: {
comments: "all",
beautify: false,
}
});
var code = result.code;
assert.strictEqual(code, "// comment1 comment2\nbar();");
});
});

});

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