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feat(minifier): compress x = x || 1 to x ||= 1 (#8368)
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The simplified version of the evaluation of `a = a || b` is:

> AssignmentExpression : LeftHandSideExpression = LogicalORExpression || LogicalANDExpression
> 1. Let lRef be ? Evaluation of LeftHandSideExpression.
> 2. Let llRef be ? Evaluation of LogicalORExpression.
> 3. Let llVal be ? GetValue(llRef).
> 4. If ToBoolean(llVal) is true
>   a. Perform ? PutValue(lRef, llVal).
>   b. return llVal.
> 5. Let lrRef be ? Evaluation of LogicalANDExpression.
> 6. Let rRef be ? GetValue(lrRef).
> 7. Let rVal be ? GetValue(rRef). [Note GetValue(rRef) returns rRef itself]
> 8. Perform ? PutValue(lRef, rVal).
> 9. Return rVal.

The simplified version of the evaluation of `a ||= b` is:

> AssignmentExpression : LeftHandSideExpression ||= AssignmentExpression
> 1. Let lRef be ? Evaluation of LeftHandSideExpression.
> 2. Let lVal be ? GetValue(lRef).
> 3. If ToBoolean(lVal) is true, return lVal.
> 4. Let rRef be ? Evaluation of AssignmentExpression.
> 5. Let rVal be ? GetValue(rRef).
> 6. Perform ? PutValue(lRef, rVal).
> 7. Return rVal.

The difference of these is that

- the evaluation of `a` is done twice for `a = a || b`, one with `1. Let lRef be ? Evaluation of LeftHandSideExpression` and one with `2. Let llRef be ? Evaluation of LogicalORExpression.`. This is same with #8366, #8367.
- `PutValue(lRef, llVal)` is performed when `ToBoolean(lVal)` is `true`.

So `x = x || 1` can be compressed to `x ||= 1` when the conditions written in #8366 are met and `PutValue(lRef, llVal)` does not have a side effect. When `a` is a non-global identifier (and not a reference created by a `with` statement), these conditions are met.

**References**
- [Spec of `||`](https://tc39.es/ecma262/multipage/ecmascript-language-expressions.html#sec-binary-logical-operators-runtime-semantics-evaluation)
- [Spec of `=` / `||=`](https://tc39.es/ecma262/multipage/ecmascript-language-expressions.html#sec-binary-logical-operators-runtime-semantics-evaluation)
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@sapphi-red
sapphi-red committed Jan 9, 2025
1 parent a596821 commit 814da55
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58 changes: 58 additions & 0 deletions crates/oxc_minifier/src/ast_passes/peephole_substitute_alternate_syntax.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -138,6 +138,7 @@ impl<'a> Traverse<'a> for PeepholeSubstituteAlternateSyntax {
}
Expression::AssignmentExpression(e) => {
self.try_compress_normal_assignment_to_combined_assignment(e, ctx);
self.try_compress_normal_assignment_to_combined_logical_assignment(e, ctx);
}
_ => {}
}
Expand Down Expand Up @@ -631,6 +632,42 @@ impl<'a, 'b> PeepholeSubstituteAlternateSyntax {
self.changed = true;
}

/// Compress `a = a || b` to `a ||= b`
///
/// This can only be done for resolved identifiers as this would avoid setting `a` when `a` is truthy.
fn try_compress_normal_assignment_to_combined_logical_assignment(
&mut self,
expr: &mut AssignmentExpression<'a>,
ctx: Ctx<'a, 'b>,
) {
if self.target < ESTarget::ES2020 {
return;
}
if !matches!(expr.operator, AssignmentOperator::Assign) {
return;
}

let Expression::LogicalExpression(logical_expr) = &mut expr.right else { return };
let new_op = logical_expr.operator.to_assignment_operator();

let (
AssignmentTarget::AssignmentTargetIdentifier(write_id_ref),
Expression::Identifier(read_id_ref),
) = (&expr.left, &logical_expr.left)
else {
return;
};
// It should also early return when the reference might refer to a reference value created by a with statement
// when the minifier supports with statements
if write_id_ref.name != read_id_ref.name || ctx.is_global_reference(write_id_ref) {
return;
}

expr.operator = new_op;
expr.right = ctx.ast.move_expression(&mut logical_expr.right);
self.changed = true;
}

fn try_compress_assignment_to_update_expression(
expr: &mut AssignmentExpression<'a>,
ctx: Ctx<'a, 'b>,
Expand Down Expand Up @@ -1210,6 +1247,27 @@ mod test {
// test_same("var x; with (z) { x.y || (x.y = 3) }");
}

#[test]
fn test_compress_normal_assignment_to_combined_logical_assignment() {
test("var x; x = x || 1", "var x; x ||= 1");
test("var x; x = x && 1", "var x; x &&= 1");
test("var x; x = x ?? 1", "var x; x ??= 1");

// `x` is a global reference and might have a setter
// Example case: `Object.defineProperty(globalThis, 'x', { get: () => true, set: () => console.log('x') }); x = x || 1`
test_same("x = x || 1");
// setting x.y might have a side effect
test_same("var x; x.y = x.y || 1");
// This case is not supported, since the minifier does not support with statements
// test_same("var x; with (z) { x = x || 1 }");

let allocator = Allocator::default();
let target = ESTarget::ES2019;
let mut pass = super::PeepholeSubstituteAlternateSyntax::new(target, false);
let code = "var x; x = x || 1";
tester::test(&allocator, code, code, &mut pass);
}

#[test]
fn test_fold_subtraction_assignment() {
test("x -= 1", "--x");
Expand Down
18 changes: 9 additions & 9 deletions tasks/minsize/minsize.snap
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Original file line number Diff line number Diff line change
Expand Up @@ -3,25 +3,25 @@ Original | minified | minified | gzip | gzip | Fixture
-------------------------------------------------------------------------------------
72.14 kB | 23.70 kB | 23.70 kB | 8.61 kB | 8.54 kB | react.development.js

173.90 kB | 59.80 kB | 59.82 kB | 19.41 kB | 19.33 kB | moment.js
173.90 kB | 59.79 kB | 59.82 kB | 19.40 kB | 19.33 kB | moment.js

287.63 kB | 90.10 kB | 90.07 kB | 32.04 kB | 31.95 kB | jquery.js
287.63 kB | 90.08 kB | 90.07 kB | 32.02 kB | 31.95 kB | jquery.js

342.15 kB | 118.11 kB | 118.14 kB | 44.45 kB | 44.37 kB | vue.js
342.15 kB | 118.09 kB | 118.14 kB | 44.43 kB | 44.37 kB | vue.js

544.10 kB | 71.74 kB | 72.48 kB | 26.14 kB | 26.20 kB | lodash.js

555.77 kB | 273.19 kB | 270.13 kB | 90.91 kB | 90.80 kB | d3.js

1.01 MB | 460.18 kB | 458.89 kB | 126.76 kB | 126.71 kB | bundle.min.js
1.01 MB | 460.17 kB | 458.89 kB | 126.75 kB | 126.71 kB | bundle.min.js

1.25 MB | 652.82 kB | 646.76 kB | 163.51 kB | 163.73 kB | three.js
1.25 MB | 652.81 kB | 646.76 kB | 163.50 kB | 163.73 kB | three.js

2.14 MB | 726.27 kB | 724.14 kB | 180.14 kB | 181.07 kB | victory.js
2.14 MB | 726.23 kB | 724.14 kB | 180.12 kB | 181.07 kB | victory.js

3.20 MB | 1.01 MB | 1.01 MB | 331.80 kB | 331.56 kB | echarts.js
3.20 MB | 1.01 MB | 1.01 MB | 331.78 kB | 331.56 kB | echarts.js

6.69 MB | 2.32 MB | 2.31 MB | 492.65 kB | 488.28 kB | antd.js
6.69 MB | 2.32 MB | 2.31 MB | 492.64 kB | 488.28 kB | antd.js

10.95 MB | 3.49 MB | 3.49 MB | 907.49 kB | 915.50 kB | typescript.js
10.95 MB | 3.49 MB | 3.49 MB | 907.46 kB | 915.50 kB | typescript.js

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