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LibJS: Elide empty lexical environment in for..in/of blocks

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When all the variables in a for..in/of block's lexical scope have been
turned into locals, we don't need to create and immediately abandon an
empty environment for them.

This avoid environment allocation in cases like this:

    function foo(a) {
        for (const x of a) {
        }
    }
This commit is contained in:
Andreas Kling 2023-12-06 15:07:42 +01:00
parent facdbe13c7
commit c9f0f0fc70
1 changed files with 32 additions and 20 deletions
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52
Userland/Libraries/LibJS/Bytecode/ASTCodegen.cpp
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@ -2872,8 +2872,6 @@ static Bytecode::CodeGenerationErrorOr<void> for_in_of_body_evaluation(Bytecode:
// iii. Let iterationEnv be NewDeclarativeEnvironment(oldEnv).
// iv. Perform ForDeclarationBindingInstantiation of lhs with argument iterationEnv.
// v. Set the running execution context's LexicalEnvironment to iterationEnv.
generator.begin_variable_scope();
has_lexical_binding = true;
// 14.7.5.4 Runtime Semantics: ForDeclarationBindingInstantiation, https://tc39.es/ecma262/#sec-runtime-semantics-fordeclarationbindinginstantiation
// 1. Assert: environment is a declarative Environment Record.
@ -2881,24 +2879,35 @@ static Bytecode::CodeGenerationErrorOr<void> for_in_of_body_evaluation(Bytecode:
auto& variable_declaration = static_cast<VariableDeclaration const&>(*lhs.get<NonnullRefPtr<ASTNode const>>());
// 2. For each element name of the BoundNames of ForBinding, do
// NOTE: Nothing in the callback throws an exception.
MUST(variable_declaration.for_each_bound_identifier([&](auto const& identifier) {
if (identifier.is_local())
return;
auto interned_identifier = generator.intern_identifier(identifier.string());
// a. If IsConstantDeclaration of LetOrConst is true, then
if (variable_declaration.is_constant_declaration()) {
// i. Perform ! environment.CreateImmutableBinding(name, true).
generator.emit<Bytecode::Op::CreateVariable>(interned_identifier, Bytecode::Op::EnvironmentMode::Lexical, true, false, true);
}
// b. Else,
else {
// i. Perform ! environment.CreateMutableBinding(name, false).
generator.emit<Bytecode::Op::CreateVariable>(interned_identifier, Bytecode::Op::EnvironmentMode::Lexical, false);
}
}));
// 3. Return unused.
// NOTE: No need to do that as we've inlined this.
auto has_non_local_variables = false;
MUST(variable_declaration.for_each_bound_identifier([&](auto const& identifier) {
if (!identifier.is_local())
has_non_local_variables = true;
}));
if (has_non_local_variables) {
generator.begin_variable_scope();
has_lexical_binding = true;
MUST(variable_declaration.for_each_bound_identifier([&](auto const& identifier) {
if (identifier.is_local())
return;
auto interned_identifier = generator.intern_identifier(identifier.string());
// a. If IsConstantDeclaration of LetOrConst is true, then
if (variable_declaration.is_constant_declaration()) {
// i. Perform ! environment.CreateImmutableBinding(name, true).
generator.emit<Bytecode::Op::CreateVariable>(interned_identifier, Bytecode::Op::EnvironmentMode::Lexical, true, false, true);
}
// b. Else,
else {
// i. Perform ! environment.CreateMutableBinding(name, false).
generator.emit<Bytecode::Op::CreateVariable>(interned_identifier, Bytecode::Op::EnvironmentMode::Lexical, false);
}
}));
// 3. Return unused.
// NOTE: No need to do that as we've inlined this.
}
// vi. If destructuring is false, then
if (!destructuring) {
// 1. Assert: lhs binds a single name.
@ -2906,7 +2915,10 @@ static Bytecode::CodeGenerationErrorOr<void> for_in_of_body_evaluation(Bytecode:
auto lhs_name = variable_declaration.declarations().first()->target().get<NonnullRefPtr<Identifier const>>();
// 3. Let lhsRef be ! ResolveBinding(lhsName).
// NOTE: We're skipping all the completion stuff that the spec does, as the unwinding mechanism will take case of doing that.
generator.emit_set_variable(*lhs_name, Bytecode::Op::SetVariable::InitializationMode::Initialize, Bytecode::Op::EnvironmentMode::Lexical);
if (lhs_name->is_local())
generator.emit<Bytecode::Op::SetLocal>(lhs_name->local_variable_index());
else
generator.emit_set_variable(*lhs_name, Bytecode::Op::SetVariable::InitializationMode::Initialize, Bytecode::Op::EnvironmentMode::Lexical);
}
}
// i. If destructuring is false, then