LibJS/Bytecode: Always resolve this binding into dedicated register

We already have a dedicated register slot for `this`, so instead of
having ResolveThisBinding take a `dst` operand, just write the value
directly into the `this` register every time.

Userland/Libraries/LibJS/Bytecode/BasicBlock.h

@@ -58,8 +58,8 @@ public:
     auto const& source_map() const { return m_source_map; }
     void add_source_map_entry(size_t bytecode_offset, SourceRecord const& source_record) { m_source_map.set(bytecode_offset, source_record); }
 
-    auto const& this_() const { return m_this; }
-    void set_this(ScopedOperand operand) { m_this = operand; }
+    [[nodiscard]] bool has_resolved_this() const { return m_has_resolved_this; }
+    void set_has_resolved_this() { m_has_resolved_this = true; }
 
     [[nodiscard]] size_t last_instruction_start_offset() const { return m_last_instruction_start_offset; }
     void set_last_instruction_start_offset(size_t offset) { m_last_instruction_start_offset = offset; }
@@ -73,11 +73,10 @@ private:
     BasicBlock const* m_finalizer { nullptr };
     String m_name;
     bool m_terminated { false };
+    bool m_has_resolved_this { false };
 
     HashMap<size_t, SourceRecord> m_source_map;
 
-    Optional<ScopedOperand> m_this;
-
     size_t m_last_instruction_start_offset { 0 };
 };
 

Userland/Libraries/LibJS/Bytecode/Generator.cpp

@@ -24,6 +24,7 @@ Generator::Generator(VM& vm, GCPtr<ECMAScriptFunctionObject const> function, Mus
     , m_regex_table(make<RegexTable>())
     , m_constants(vm.heap())
     , m_accumulator(*this, Operand(Register::accumulator()))
+    , m_this_value(*this, Operand(Register::this_value()))
     , m_must_propagate_completion(must_propagate_completion == MustPropagateCompletion::Yes)
     , m_function(function)
 {
@@ -571,8 +572,7 @@ CodeGenerationErrorOr<Generator::ReferenceOperands> Generator::emit_super_refere
     // https://tc39.es/ecma262/#sec-super-keyword-runtime-semantics-evaluation
     // 1. Let env be GetThisEnvironment().
     // 2. Let actualThis be ? env.GetThisBinding().
-    auto actual_this = allocate_register();
-    emit<Bytecode::Op::ResolveThisBinding>(actual_this);
+    auto actual_this = get_this();
 
     Optional<ScopedOperand> computed_property_value;
 
@@ -1081,16 +1081,16 @@ void Generator::set_local_initialized(u32 local_index)
 
 ScopedOperand Generator::get_this(Optional<ScopedOperand> preferred_dst)
 {
-    if (m_current_basic_block->this_().has_value())
-        return m_current_basic_block->this_().value();
-    if (m_root_basic_blocks[0]->this_().has_value()) {
-        m_current_basic_block->set_this(m_root_basic_blocks[0]->this_().value());
-        return m_root_basic_blocks[0]->this_().value();
+    if (m_current_basic_block->has_resolved_this())
+        return this_value();
+    if (m_root_basic_blocks[0]->has_resolved_this()) {
+        m_current_basic_block->set_has_resolved_this();
+        return this_value();
     }
     auto dst = preferred_dst.has_value() ? preferred_dst.value() : allocate_register();
-    emit<Bytecode::Op::ResolveThisBinding>(dst);
-    m_current_basic_block->set_this(dst);
-    return dst;
+    emit<Bytecode::Op::ResolveThisBinding>();
+    m_current_basic_block->set_has_resolved_this();
+    return this_value();
 }
 
 ScopedOperand Generator::accumulator()
@@ -1098,6 +1098,11 @@ ScopedOperand Generator::accumulator()
     return m_accumulator;
 }
 
+ScopedOperand Generator::this_value()
+{
+    return m_this_value;
+}
+
 bool Generator::fuse_compare_and_jump(ScopedOperand const& condition, Label true_target, Label false_target)
 {
     auto& last_instruction = *reinterpret_cast<Instruction const*>(m_current_basic_block->data() + m_current_basic_block->last_instruction_start_offset());

Userland/Libraries/LibJS/Bytecode/Generator.h

@@ -46,6 +46,7 @@ public:
     [[nodiscard]] ScopedOperand allocate_register();
     [[nodiscard]] ScopedOperand local(u32 local_index);
     [[nodiscard]] ScopedOperand accumulator();
+    [[nodiscard]] ScopedOperand this_value();
 
     void free_register(Register);
 
@@ -377,6 +378,7 @@ private:
     MarkedVector<Value> m_constants;
 
     ScopedOperand m_accumulator;
+    ScopedOperand m_this_value;
     Vector<Register> m_free_registers;
 
     u32 m_next_register { Register::reserved_register_count };

Userland/Libraries/LibJS/Bytecode/Interpreter.cpp

@@ -1563,18 +1563,17 @@ ThrowCompletionOr<void> DeleteByIdWithThis::execute_impl(Bytecode::Interpreter&
 ThrowCompletionOr<void> ResolveThisBinding::execute_impl(Bytecode::Interpreter& interpreter) const
 {
     auto& cached_this_value = interpreter.reg(Register::this_value());
-    if (cached_this_value.is_empty()) {
-        // OPTIMIZATION: Because the value of 'this' cannot be reassigned during a function execution, it's
-        //               resolved once and then saved for subsequent use.
-        auto& running_execution_context = interpreter.running_execution_context();
-        if (auto function = running_execution_context.function; function && is<ECMAScriptFunctionObject>(*function) && !static_cast<ECMAScriptFunctionObject&>(*function).allocates_function_environment()) {
-            cached_this_value = running_execution_context.this_value;
-        } else {
-            auto& vm = interpreter.vm();
-            cached_this_value = TRY(vm.resolve_this_binding());
-        }
+    if (!cached_this_value.is_empty())
+        return {};
+    // OPTIMIZATION: Because the value of 'this' cannot be reassigned during a function execution, it's
+    //               resolved once and then saved for subsequent use.
+    auto& running_execution_context = interpreter.running_execution_context();
+    if (auto function = running_execution_context.function; function && is<ECMAScriptFunctionObject>(*function) && !static_cast<ECMAScriptFunctionObject&>(*function).allocates_function_environment()) {
+        cached_this_value = running_execution_context.this_value;
+    } else {
+        auto& vm = interpreter.vm();
+        cached_this_value = TRY(vm.resolve_this_binding());
     }
-    interpreter.set(dst(), cached_this_value);
     return {};
 }
 
@@ -2655,9 +2654,9 @@ ByteString IteratorNext::to_byte_string_impl(Executable const& executable) const
         format_operand("iterator_record"sv, m_iterator_record, executable));
 }
 
-ByteString ResolveThisBinding::to_byte_string_impl(Bytecode::Executable const& executable) const
+ByteString ResolveThisBinding::to_byte_string_impl(Bytecode::Executable const&) const
 {
-    return ByteString::formatted("ResolveThisBinding {}", format_operand("dst"sv, m_dst, executable));
+    return "ResolveThisBinding"sv;
 }
 
 ByteString ResolveSuperBase::to_byte_string_impl(Bytecode::Executable const& executable) const

Userland/Libraries/LibJS/Bytecode/Op.h

@@ -2550,23 +2550,14 @@ private:
 
 class ResolveThisBinding final : public Instruction {
 public:
-    explicit ResolveThisBinding(Operand dst)
+    ResolveThisBinding()
         : Instruction(Type::ResolveThisBinding)
-        , m_dst(dst)
     {
     }
 
     ThrowCompletionOr<void> execute_impl(Bytecode::Interpreter&) const;
     ByteString to_byte_string_impl(Bytecode::Executable const&) const;
-    void visit_operands_impl(Function<void(Operand&)> visitor)
-    {
-        visitor(m_dst);
-    }
-
-    Operand dst() const { return m_dst; }
-
-private:
-    Operand m_dst;
+    void visit_operands_impl(Function<void(Operand&)>) { }
 };
 
 class ResolveSuperBase final : public Instruction {

Userland/Libraries/LibJS/Bytecode/ScopedOperand.cpp

@@ -11,7 +11,7 @@ namespace JS::Bytecode {
 
 ScopedOperandImpl::~ScopedOperandImpl()
 {
-    if (!m_generator.is_finished() && m_operand.is_register() && m_operand.as_register().index() != 0)
+    if (!m_generator.is_finished() && m_operand.is_register() && m_operand.as_register().index() >= Register::reserved_register_count)
         m_generator.free_register(m_operand.as_register());
 }