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LibJS: Skip allocating locals for arguments that allowed to be local

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This allows us to get rid of instructions that move arguments to locals
and allocate smaller JS::Value vector in ExecutionContext by reusing
slots that were already allocated for arguments.

With this change for following function:
```js
function f(x, y) {
    return x + y;
}
```

we now produce following bytecode:
```
[   0]    0: Add dst:reg6, lhs:arg0, rhs:arg1
[  10]       Return value:reg6
```

instead of:
```
[   0]    0: GetArgument 0, dst:x~1
[  10]       GetArgument 1, dst:y~0
[  20]       Add dst:reg6, lhs:x~1, rhs:y~0
[  30]       Return value:reg6
```
This commit is contained in:
Aliaksandr Kalenik 2025-04-25 17:10:47 +02:00 • committed by Andreas Kling
parent 3f04d18ef7
commit 2d732b2251
Notes: github-actions[bot] 2025-04-26 09:03:23 +00:00
6 changed files with 102 additions and 39 deletions
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18
Libraries/LibJS/Bytecode/ASTCodegen.cpp
View file

@ -461,8 +461,14 @@ Bytecode::CodeGenerationErrorOr<Optional<ScopedOperand>> Identifier::generate_by
Bytecode::Generator::SourceLocationScope scope(generator, *this);
if (is_local()) {
auto local = generator.local(local_variable_index());
if (!generator.is_local_initialized(local_variable_index())) {
auto local_index = this->local_index();
auto local = generator.local(local_index);
if (!generator.is_local_initialized(local_index)) {
if (local_index.is_argument()) {
// Arguments are initialized to undefined by default, so here we need to replace it with the empty value to
// trigger the TDZ check.
generator.emit<Bytecode::Op::Mov>(local, generator.add_constant(js_special_empty_value()));
}
generator.emit<Bytecode::Op::ThrowIfTDZ>(local);
}
return local;
@ -1605,7 +1611,7 @@ Bytecode::CodeGenerationErrorOr<Optional<ScopedOperand>> VariableDeclaration::ge
if (declaration_kind() != DeclarationKind::Var) {
if (auto const* identifier = declarator->target().get_pointer<NonnullRefPtr<Identifier const>>()) {
if ((*identifier)->is_local()) {
init_dst = generator.local((*identifier)->local_variable_index());
init_dst = generator.local((*identifier)->local_index());
}
}
}
@ -1625,7 +1631,7 @@ Bytecode::CodeGenerationErrorOr<Optional<ScopedOperand>> VariableDeclaration::ge
if (auto const* identifier = declarator->target().get_pointer<NonnullRefPtr<Identifier const>>()) {
if ((*identifier)->is_local()) {
generator.set_local_initialized((*identifier)->local_variable_index());
generator.set_local_initialized((*identifier)->local_index());
}
}
}
@ -1736,7 +1742,7 @@ Bytecode::CodeGenerationErrorOr<Optional<ScopedOperand>> CallExpression::generat
call_type = Bytecode::Op::CallType::DirectEval;
}
if (identifier.is_local()) {
auto local = generator.local(identifier.local_variable_index());
auto local = generator.local(identifier.local_index());
if (!generator.is_local_initialized(local.operand().index())) {
generator.emit<Bytecode::Op::ThrowIfTDZ>(local);
}
@ -2648,7 +2654,7 @@ Bytecode::CodeGenerationErrorOr<Optional<ScopedOperand>> TryStatement::generate_
TRY(m_handler->parameter().visit(
[&](NonnullRefPtr<Identifier const> const& parameter) -> Bytecode::CodeGenerationErrorOr<void> {
if (parameter->is_local()) {
auto local = generator.local(parameter->local_variable_index());
auto local = generator.local(parameter->local_index());
generator.emit_mov(local, caught_value);
} else {
generator.begin_variable_scope();