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LibJS: Don't evaluate computed MemberExpression LHS twice in assignments

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The following snippet would cause "i" to be incremented twice(!):

    let a = []
    let i = 0
    a[++i] += 0

This patch solves the issue by remembering the base object and property
name for computed MemberExpression LHS in codegen. We the store the
result of the assignment to the same object and property (instead of
computing the LHS again).

3 new passes on test262. :^)
This commit is contained in:
Andreas Kling
2023-10-04 15:22:07 +02:00
parent 2083376618
commit 732b39d120
3 changed files with 42 additions and 15 deletions
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26
Userland/Libraries/LibJS/Bytecode/Generator.cpp
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@@ -189,12 +189,12 @@ CodeGenerationErrorOr<Generator::ReferenceRegisters> Generator::emit_super_refer
};
}
CodeGenerationErrorOr<void> Generator::emit_load_from_reference(JS::ASTNode const& node)
CodeGenerationErrorOr<Optional<Generator::ReferenceRegisters>> Generator::emit_load_from_reference(JS::ASTNode const& node)
{
if (is<Identifier>(node)) {
auto& identifier = static_cast<Identifier const&>(node);
TRY(identifier.generate_bytecode(*this));
return {};
return Optional<ReferenceRegisters> {};
}
if (is<MemberExpression>(node)) {
auto& expression = static_cast<MemberExpression const&>(node);
@@ -213,15 +213,24 @@ CodeGenerationErrorOr<void> Generator::emit_load_from_reference(JS::ASTNode cons
auto identifier_table_ref = intern_identifier(verify_cast<Identifier>(expression.property()).string());
emit_get_by_id_with_this(identifier_table_ref, super_reference.this_value);
}
return super_reference;
} else {
TRY(expression.object().generate_bytecode(*this));
if (expression.is_computed()) {
auto object_reg = allocate_register();
emit<Bytecode::Op::Store>(object_reg);
TRY(expression.property().generate_bytecode(*this));
auto property_reg = allocate_register();
emit<Bytecode::Op::Store>(property_reg);
emit<Bytecode::Op::GetByValue>(object_reg);
return ReferenceRegisters {
.base = object_reg,
.referenced_name = property_reg,
.this_value = object_reg,
};
} else if (expression.property().is_identifier()) {
auto identifier_table_ref = intern_identifier(verify_cast<Identifier>(expression.property()).string());
emit_get_by_id(identifier_table_ref);
@@ -235,7 +244,7 @@ CodeGenerationErrorOr<void> Generator::emit_load_from_reference(JS::ASTNode cons
};
}
}
return {};
return Optional<ReferenceRegisters> {};
}
VERIFY_NOT_REACHED();
}
@@ -306,6 +315,15 @@ CodeGenerationErrorOr<void> Generator::emit_store_to_reference(JS::ASTNode const
};
}
CodeGenerationErrorOr<void> Generator::emit_store_to_reference(ReferenceRegisters const& reference_registers)
{
if (reference_registers.base == reference_registers.this_value)
emit<Bytecode::Op::PutByValue>(reference_registers.base, reference_registers.referenced_name.value());
else
emit<Bytecode::Op::PutByValueWithThis>(reference_registers.base, reference_registers.referenced_name.value(), reference_registers.this_value);
return {};
}
CodeGenerationErrorOr<void> Generator::emit_delete_reference(JS::ASTNode const& node)
{
if (is<Identifier>(node)) {