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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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17
Userland/Libraries/LibJS/Bytecode/ASTCodegen.cpp
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@@ -525,7 +525,7 @@ Bytecode::CodeGenerationErrorOr<void> AssignmentExpression::generate_bytecode(By
VERIFY(m_lhs.has<NonnullRefPtr<Expression const>>());
auto& lhs = m_lhs.get<NonnullRefPtr<Expression const>>();
TRY(generator.emit_load_from_reference(lhs));
auto reference_registers = TRY(generator.emit_load_from_reference(lhs));
Bytecode::BasicBlock* rhs_block_ptr { nullptr };
Bytecode::BasicBlock* end_block_ptr { nullptr };
@@ -615,7 +615,10 @@ Bytecode::CodeGenerationErrorOr<void> AssignmentExpression::generate_bytecode(By
};
}
TRY(generator.emit_store_to_reference(lhs));
if (reference_registers.has_value())
TRY(generator.emit_store_to_reference(*reference_registers));
else
TRY(generator.emit_store_to_reference(lhs));
if (end_block_ptr) {
generator.emit<Bytecode::Op::Jump>(Bytecode::Label { *end_block_ptr });
@@ -1049,7 +1052,8 @@ Bytecode::CodeGenerationErrorOr<void> ArrayExpression::generate_bytecode(Bytecod
Bytecode::CodeGenerationErrorOr<void> MemberExpression::generate_bytecode(Bytecode::Generator& generator) const
{
Bytecode::Generator::SourceLocationScope scope(generator, *this);
return generator.emit_load_from_reference(*this);
(void)TRY(generator.emit_load_from_reference(*this));
return {};
}
Bytecode::CodeGenerationErrorOr<void> FunctionDeclaration::generate_bytecode(Bytecode::Generator& generator) const
@@ -2249,7 +2253,7 @@ Bytecode::CodeGenerationErrorOr<void> TaggedTemplateLiteral::generate_bytecode(B
Bytecode::CodeGenerationErrorOr<void> UpdateExpression::generate_bytecode(Bytecode::Generator& generator) const
{
Bytecode::Generator::SourceLocationScope scope(generator, *this);
TRY(generator.emit_load_from_reference(*m_argument));
auto reference_registers = TRY(generator.emit_load_from_reference(*m_argument));
Optional<Bytecode::Register> previous_value_for_postfix_reg;
if (!m_prefixed) {
@@ -2263,7 +2267,10 @@ Bytecode::CodeGenerationErrorOr<void> UpdateExpression::generate_bytecode(Byteco
else
generator.emit<Bytecode::Op::Decrement>();
TRY(generator.emit_store_to_reference(*m_argument));
if (reference_registers.has_value())
TRY(generator.emit_store_to_reference(*reference_registers));
else
TRY(generator.emit_store_to_reference(*m_argument));
if (!m_prefixed)
generator.emit<Bytecode::Op::Load>(*previous_value_for_postfix_reg);