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feat(minifier): compress a.b = a.b + c to a.b += c (#8367)

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

> AssignmentExpression : LeftHandSideExpression AssignmentOperator AssignmentExpression
> 1. Let lRef be ? Evaluation of LeftHandSideExpression.
> 2. Let lVal be ? GetValue(lRef).
> 3. Let rRef be ? Evaluation of AssignmentExpression.
> 4. Let rVal be ? GetValue(rRef).
> 5. Let r be ? ApplyStringOrNumericBinaryOperator(lVal, opText, rVal).
> 6. Perform ? PutValue(lRef, r).
> 7. Return r.

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

> AssignmentExpression : LeftHandSideExpression = AssignmentExpressionLeft + AssignmentExpressionRight
> 1. Let lRef be ? Evaluation of LeftHandSideExpression.
> 2. Let alRef be ? Evaluation of AssignmentExpressionLeft.
> 3. Let alVal be ? GetValue(alRef).
> 4. Let arRef be ? Evaluation of AssignmentExpressionRight.
> 5. Let arVal be ? GetValue(arRef).
> 6. Let rRef be ? ApplyStringOrNumericBinaryOperator(alVal, opText, arVal).
> 7. Let rVal be ? GetValue(rRef). [Note GetValue(rRef) returns rRef itself]
> 8. Perform ? PutValue(lRef, rVal).
> 9. 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 alRef be ? Evaluation of AssignmentExpressionLeft.`

So this is same with #8366 and can be compressed similarly when the conditions are met (`a.b = a.b + c` -> `a.b += c`).

**References**
- [Spec of `=`, `+=`](https://tc39.es/ecma262/multipage/ecmascript-language-expressions.html#sec-assignment-operators-runtime-semantics-evaluation)
- [Spec of `+`](https://tc39.es/ecma262/multipage/ecmascript-language-expressions.html#sec-addition-operator-plus-runtime-semantics-evaluation)
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sapphi-red 2025-01-09 03:47:11 +00:00
parent 579eb603d5
commit a596821d85
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2 changed files with 53 additions and 29 deletions
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78
crates/oxc_minifier/src/ast_passes/peephole_substitute_alternate_syntax.rs
View file

@ -481,18 +481,39 @@ impl<'a, 'b> PeepholeSubstituteAlternateSyntax {
let new_op = expr.operator.to_assignment_operator();
match (&assignment_expr.left, &expr.left) {
// `a || (a = b)` -> `a ||= b`
if !Self::has_no_side_effect_for_evaluation_same_target(
&assignment_expr.left,
&expr.left,
ctx,
) {
return None;
}
assignment_expr.span = expr.span;
assignment_expr.operator = new_op;
Some(ctx.ast.move_expression(&mut expr.right))
}
/// Returns `true` if the assignment target and expression have no side effect for *evaluation* and points to the same reference.
///
/// Evaluation here means `Evaluation` in the spec.
/// <https://tc39.es/ecma262/multipage/syntax-directed-operations.html#sec-evaluation>
///
/// Matches the following cases:
///
/// - `a`, `a`
/// - `a.b`, `a.b`
/// - `a.#b`, `a.#b`
fn has_no_side_effect_for_evaluation_same_target(
assignment_target: &AssignmentTarget,
expr: &Expression,
ctx: Ctx<'a, 'b>,
) -> bool {
match (&assignment_target, &expr) {
(
AssignmentTarget::AssignmentTargetIdentifier(write_id_ref),
Expression::Identifier(read_id_ref),
) => {
if write_id_ref.name != read_id_ref.name {
return None;
}
}
// `a.b || (a.b = c)` -> `a.b ||= c`
// `a.#b || (a.#b = c)` -> `a.#b ||= c`
) => write_id_ref.name == read_id_ref.name,
(
AssignmentTarget::StaticMemberExpression(_),
Expression::StaticMemberExpression(_),
@ -501,24 +522,17 @@ impl<'a, 'b> PeepholeSubstituteAlternateSyntax {
AssignmentTarget::PrivateFieldExpression(_),
Expression::PrivateFieldExpression(_),
) => {
let write_expr = assignment_expr.left.to_member_expression();
let read_expr = expr.left.to_member_expression();
let write_expr = assignment_target.to_member_expression();
let read_expr = expr.to_member_expression();
let Expression::Identifier(write_expr_object_id) = &write_expr.object() else {
return None;
return false;
};
// It should also return None when the reference might refer to a reference value created by a with statement
// It should also return false when the reference might refer to a reference value created by a with statement
// when the minifier supports with statements
if ctx.is_global_reference(write_expr_object_id) || write_expr.content_ne(read_expr)
{
return None;
}
!ctx.is_global_reference(write_expr_object_id) && write_expr.content_eq(read_expr)
}
_ => return None,
_ => false,
}
assignment_expr.span = expr.span;
assignment_expr.operator = new_op;
Some(ctx.ast.move_expression(&mut expr.right))
}
fn commutative_pair<A, F, G, RetF: 'a, RetG: 'a>(
@ -603,14 +617,12 @@ impl<'a, 'b> PeepholeSubstituteAlternateSyntax {
if !matches!(expr.operator, AssignmentOperator::Assign) {
return;
}
let AssignmentTarget::AssignmentTargetIdentifier(write_id_ref) = &mut expr.left else {
return;
};
let Expression::BinaryExpression(binary_expr) = &mut expr.right else { return };
let Some(new_op) = binary_expr.operator.to_assignment_operator() else { return };
let Expression::Identifier(read_id_ref) = &mut binary_expr.left else { return };
if write_id_ref.name != read_id_ref.name {
if !Self::has_no_side_effect_for_evaluation_same_target(&expr.left, &binary_expr.left, ctx)
{
return;
}
@ -1184,6 +1196,18 @@ mod test {
test_same("x = g() & x");
test_same("x = (x -= 2) ^ x");
// GetValue(x) has no sideeffect when x is a resolved identifier
test("var x; x.y = x.y + 3", "var x; x.y += 3");
test("var x; x.#y = x.#y + 3", "var x; x.#y += 3");
test_same("x.y = x.y + 3");
// this can be compressed if `y` does not have side effect
test_same("var x; x[y] = x[y] + 3");
// GetValue(x) has a side effect in this case
// Example case: `var a = { get b() { console.log('b'); return { get c() { console.log('c') } } } }; a.b.c = a.b.c + 1`
test_same("var x; x.y.z = x.y.z + 3");
// This case is not supported, since the minifier does not support with statements
// test_same("var x; with (z) { x.y || (x.y = 3) }");
}
#[test]

4
tasks/minsize/minsize.snap
View file

@ -15,7 +15,7 @@ Original | minified | minified | gzip | gzip | Fixture
1.01 MB | 460.18 kB | 458.89 kB | 126.76 kB | 126.71 kB | bundle.min.js
1.25 MB | 652.84 kB | 646.76 kB | 163.52 kB | 163.73 kB | three.js
1.25 MB | 652.82 kB | 646.76 kB | 163.51 kB | 163.73 kB | three.js
2.14 MB | 726.27 kB | 724.14 kB | 180.14 kB | 181.07 kB | victory.js
@ -23,5 +23,5 @@ Original | minified | minified | gzip | gzip | Fixture
6.69 MB | 2.32 MB | 2.31 MB | 492.65 kB | 488.28 kB | antd.js
10.95 MB | 3.49 MB | 3.49 MB | 907.50 kB | 915.50 kB | typescript.js
10.95 MB | 3.49 MB | 3.49 MB | 907.49 kB | 915.50 kB | typescript.js