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Add unsafe optimization to inline undefined for missing objlit props #589

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Add unsafe optimization to inline undefined for missing objlit props #589

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e829ee3
Add "unsafe" optimization to inline undefined for missing objlit prop…
shicks Mar 7, 2020
0edb6ac
add some test files, not sure if they're supposed to be included at t…
shicks May 26, 2022
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27 changes: 27 additions & 0 deletions lib/compress/index.js
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Expand Up @@ -6878,6 +6878,7 @@ def_optimize(AST_Sub, function(self, compressor) {
if (key !== prop) {
var sub = self.flatten_object(property, compressor);
if (sub) {
if (sub instanceof AST_Undefined) return sub;
expr = self.expression = sub.expression;
prop = self.property = sub.property;
}
Expand Down Expand Up @@ -6941,14 +6942,37 @@ AST_Lambda.DEFMETHOD("contains_this", function() {
});
});

const objectPrototypeProperties = new Set([
"__count__",
"__defineGetter__",
"__defineSetter__",
"__lookupGetter__",
"__lookupSetter__",
"__noSuchMethod__",
"__parent__",
"__proto__",
"constructor",
"eval",
"hasOwnProperty",
"isPrototypeOf",
"propertyIsEnumerable",
"toLocaleString",
"toSource",
"toString",
"unwatch",
"valueOf",
"watch",
]);
AST_PropAccess.DEFMETHOD("flatten_object", function(key, compressor) {
if (!compressor.option("properties")) return;
var arrows = compressor.option("unsafe_arrows") && compressor.option("ecma") >= 2015;
var expr = this.expression;
if (expr instanceof AST_Object) {
var props = expr.properties;
var collapse_missing_props = compressor.option("unsafe");
for (var i = props.length; --i >= 0;) {
var prop = props[i];
if (prop.value instanceof AST_Accessor) collapse_missing_props = false;
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I don't think there's a need for this condition here. If any of the properties is an accessor, it can still be doomed to oblivion :)

if ("" + (prop instanceof AST_ConciseMethod ? prop.key.name : prop.key) == key) {
if (!props.every((prop) => {
return prop instanceof AST_ObjectKeyVal
Expand All @@ -6973,6 +6997,9 @@ AST_PropAccess.DEFMETHOD("flatten_object", function(key, compressor) {
});
}
}
if (i < 0 && collapse_missing_props && !objectPrototypeProperties.has(key)) {
return make_node(AST_Undefined, this);
}
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This will get rid of side effects in other properties of the object. Picture this (too contrived) use case:

const x = {
  iHaveSideEffects: sideEffects(),
  foo
}.bar

If we turned this object into undefined outright here, the call with side effects disappears. There's a method which can fix this, AST_Node#drop_side_effect_free which when called on the object X is being initialised to, would return simply sideEffects(). Be careful because when nothing has side effects this method just returns null.

But! We want the result of the expression to really be undefined.

So we can call make_sequence with an array of two which results in (sideEffects(), undefined). The sequence expression is terrible in terms of readability, but it's used extensively in Terser for being an expression instead of a statement.

There is, however, some calling code of flatten_object which expects the return to be either null or a AST_Sub with a number. I'll leave that one up to you but maybe returning a pair would work ;)

}
});

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17 changes: 17 additions & 0 deletions test/compress/properties.js
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Expand Up @@ -1224,6 +1224,23 @@ prop_side_effects_2: {
]
}

missing_prop: {
options = {
inline: true,
properties: true,
unsafe: true,
}
input: {
console.log({x: 42}.y);
}
expect: {
console.log(void 0);
}
expect_stdout: [
"undefined",
]
}

accessor_1: {
options = {
properties: true,
Expand Down