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feat(minifier): drop var r = [...arguments] if r is not used #13115

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Merged
merged 1 commit into from
Aug 15, 2025
Merged

feat(minifier): drop var r = [...arguments] if r is not used #13115

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157 changes: 86 additions & 71 deletions crates/oxc_minifier/src/peephole/substitute_alternate_syntax.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -549,7 +549,7 @@ impl<'a> PeepholeOptimizations {
assign_expr
};

let (r_id_name, a_id_name, lhs_offset) = {
let (r_id_name, a_id_name, offset) = {
let AssignmentTarget::ComputedMemberExpression(lhs_member_expr) =
&body_assign_expr.left
else {
Expand All @@ -576,7 +576,7 @@ impl<'a> PeepholeOptimizations {
(lhs_member_expr_obj.name, base_name, offset)
};

let rhs_offset = {
{
let Expression::ComputedMemberExpression(rhs_member_expr) = &body_assign_expr.right
else {
return;
Expand All @@ -589,56 +589,13 @@ impl<'a> PeepholeOptimizations {
{
return;
}
match &rhs_member_expr.expression {
Expression::Identifier(id) => {
if id.name != a_id_name {
return;
}
0.0
}
Expression::BinaryExpression(b) => {
if b.operator != BinaryOperator::Addition {
return;
}
let Some(((), offset)) = Self::commutative_pair(
(&b.left, &b.right),
|a| {
if let Expression::Identifier(id) = a {
if id.name != a_id_name {
return None;
}
Some(())
} else {
None
}
},
|b| {
if let Expression::NumericLiteral(n) = b {
if n.value.fract() == 0.0 && n.value >= 0.0 {
Some(n.value)
} else {
None
}
} else {
None
}
},
) else {
return;
};
offset
}
_ => return,
}
};

let offset = if lhs_offset == 0.0 {
rhs_offset
} else {
if rhs_offset != 0.0 {
let Expression::Identifier(rhs_member_expr_expr_id) = &rhs_member_expr.expression
else {
return;
};
if rhs_member_expr_expr_id.name != a_id_name {
return;
}
lhs_offset
};

// Parse update: `a++`
Expand Down Expand Up @@ -765,7 +722,11 @@ impl<'a> PeepholeOptimizations {
if de_id.name != r_id_name {
return;
}
de_r.id.take_in(ctx.ast)
// `var r = [...arguments]` / `var r = [...arguments].slice(n)` is not needed
// if r is not used by other places because `[...arguments]` does not have a sideeffect
// `r` is used once in the for-loop (assignment for each index)
(ctx.scoping().get_resolved_references(de_id.symbol_id()).count() > 1)
.then(|| de_r.id.take_in(ctx.ast))
};

// Build `var r = [...arguments]` (with optional `.slice(offset)`) as the only declarator and drop test/update/body.
Expand Down Expand Up @@ -803,8 +764,13 @@ impl<'a> PeepholeOptimizations {
base_arr
};

let new_decl = ctx.ast.variable_declarator(SPAN, var_init.kind, r_id_pat, Some(arr), false);
var_init.declarations = ctx.ast.vec1(new_decl);
var_init.declarations = if let Some(r_id_pat) = r_id_pat {
let new_decl =
ctx.ast.variable_declarator(SPAN, var_init.kind, r_id_pat, Some(arr), false);
ctx.ast.vec1(new_decl)
} else {
ctx.ast.vec()
};
for_stmt.test =
Some(ctx.ast.expression_numeric_literal(for_stmt.span, 0.0, None, NumberBase::Decimal));
for_stmt.update = None;
Expand Down Expand Up @@ -2236,40 +2202,89 @@ mod test {
#[test]
fn test_rewrite_arguments_copy_loop() {
test(
"for (var e = arguments.length, r = Array(e), a = 0; a < e; a++) r[a] = arguments[a];",
"var r = [...arguments]",
"for (var e = arguments.length, r = Array(e), a = 0; a < e; a++) r[a] = arguments[a]; console.log(r)",
"var r = [...arguments]; console.log(r)",
);
test(
"for (var e = arguments.length, r = Array(e), a = 0; a < e; a++) { r[a] = arguments[a]; } console.log(r)",
"var r = [...arguments]; console.log(r)",
);
test(
"for (var e = arguments.length, r = Array(e), a = 0; a < e; a++) { r[a] = arguments[a] } console.log(r)",
"var r = [...arguments]; console.log(r)",
);
test(
"for (var e = arguments.length, r = new Array(e), a = 0; a < e; a++) r[a] = arguments[a]; console.log(r)",
"var r = [...arguments]; console.log(r)",
);
test(
"for (var e = arguments.length, r = Array(e > 1 ? e - 1 : 0), a = 1; a < e; a++) r[a - 1] = arguments[a]; console.log(r)",
"var r = [...arguments].slice(1); console.log(r)",
);
test(
"for (var e = arguments.length, r = Array(e > 2 ? e - 2 : 0), a = 2; a < e; a++) r[a - 2] = arguments[a]; console.log(r)",
"var r = [...arguments].slice(2); console.log(r)",
);
test(
"for (var e = arguments.length, r = Array(e), a = 0; a < e; a++) { r[a] = arguments[a] }",
"var r = [...arguments]",
"for (var e = arguments.length, r = [], a = 0; a < e; a++) r[a] = arguments[a]; console.log(r)",
"var r = [...arguments]; console.log(r)",
);
test(
"for (var e = arguments.length, r = new Array(e), a = 0; a < e; a++) r[a] = arguments[a];",
"var r = [...arguments]",
"for (var r = [], a = 0; a < arguments.length; a++) r[a] = arguments[a]; console.log(r)",
"var r = [...arguments]; console.log(r)",
);
test(
"for (var e = arguments.length, r = Array(e > 1 ? e - 1 : 0), a = 1; a < e; a++) r[a - 1] = arguments[a];",
"var r = [...arguments].slice(1)",
"for (var r = [], a = 1; a < arguments.length; a++) r[a - 1] = arguments[a]; console.log(r)",
"var r = [...arguments].slice(1); console.log(r)",
);
test(
"for (var e = arguments.length, r = Array(e > 2 ? e - 2 : 0), a = 2; a < e; a++) r[a - 2] = arguments[a];",
"var r = [...arguments].slice(2)",
"for (var r = [], a = 2; a < arguments.length; a++) r[a - 2] = arguments[a]; console.log(r)",
"var r = [...arguments].slice(2); console.log(r)",
);
test(
"for (var e = arguments.length, r = [], a = 0; a < e; a++) r[a] = arguments[a];",
"var r = [...arguments]",
"for (var e = arguments.length, r = Array(e), a = 0; a < e; a++) r[a] = arguments[a];",
"",
);
test(
"for (var r = [], a = 0; a < arguments.length; a++) r[a] = arguments[a];",
"var r = [...arguments]",
"for (var e = arguments.length, r = Array(e > 1 ? e - 1 : 0), a = 1; a < e; a++) r[a - 1] = arguments[a]",
"",
);
test_same(
"for (var e = arguments.length, r = Array(e), a = 0; a < e; a++) console.log(r[a]);",
);
test(
"for (var r = [], a = 1; a < arguments.length; a++) r[a - 1] = arguments[a];",
"var r = [...arguments].slice(1)",
"for (var e = arguments.length, r = Array(e), a = 0; a < e; a++) { r[a] = arguments[a]; console.log(r); }",
"for (var e = arguments.length, r = Array(e), a = 0; a < e; a++) (r[a] = arguments[a], console.log(r))",
);
test_same(
"for (var e = arguments.length, r = Array(e), a = 0; a < e; a++) r[a] += arguments[a];",
);
test_same(
"for (var e = arguments.length, r = Array(e), a = 0; a < e; a++) r[a + 1] = arguments[a];",
);
test_same(
"for (var e = arguments.length, r = Array(e), a = 0; a < e; a++) r[a - 0.5] = arguments[a];",
);
test(
"for (var r = [], a = 2; a < arguments.length; a++) r[a - 2] = arguments[a];",
"var r = [...arguments].slice(2)",
"var arguments; for (var e = arguments.length, r = Array(e), a = 0; a < e; a++) r[a] = arguments[a];",
"for (var arguments, e = arguments.length, r = Array(e), a = 0; a < e; a++) r[a] = arguments[a];",
);
test_same("for (var e = arguments.length, r = Array(e), a = 0; a < e; a++) r[a] = foo[a];");
test_same(
"for (var e = arguments.length, r = Array(e), a = 0; a < e; e--) r[a] = arguments[a];",
);
test_same(
"for (var e = arguments.length, r = Array(e), a = 0; a < e; r++) r[a] = arguments[a];",
);
test_same(
"for (var e = arguments.length, r = Array(e), a = 0; a < r; r++) r[a] = arguments[a];",
);
test(
"var arguments; for (var r = [], a = 0; a < arguments.length; a++) r[a] = arguments[a];",
"for (var arguments, r = [], a = 0; a < arguments.length; a++) r[a] = arguments[a];",
);
test_same(
"for (var e = arguments.length, r = Array(e > 1 ? e - 2 : 0), a = 2; a < e; a++) r[a - 2] = arguments[a];",
);
}
}
4 changes: 2 additions & 2 deletions tasks/minsize/minsize.snap
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Original file line number Diff line number Diff line change
Expand Up @@ -19,9 +19,9 @@ Original | minified | minified | gzip | gzip | Iterations | Fi

2.14 MB | 715.37 kB | 724.14 kB | 161.66 kB | 181.07 kB | 2 | victory.js

3.20 MB | 1.01 MB | 1.01 MB | 323.99 kB | 331.56 kB | 3 | echarts.js
3.20 MB | 1.01 MB | 1.01 MB | 323.98 kB | 331.56 kB | 2 | echarts.js

6.69 MB | 2.23 MB | 2.31 MB | 461.69 kB | 488.28 kB | 3 | antd.js

10.95 MB | 3.35 MB | 3.49 MB | 860.60 kB | 915.50 kB | 3 | typescript.js
10.95 MB | 3.35 MB | 3.49 MB | 860.59 kB | 915.50 kB | 2 | typescript.js

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