From 2bdc69c42c98467d8f114cb6b6b267b5d07851c3 Mon Sep 17 00:00:00 2001
From: Aliaksandr Kalenik <kalenik.aliaksandr@gmail.com>
Date: Fri, 28 Jul 2023 23:59:43 +0200
Subject: [PATCH] LibJS: Add caching of this value in ResolveThisBinding
 instruction

Because "this" value cannot be changed during function execution it is
safe to compute it once and then use for future access.

This optimization makes ai-astar.js run 8% faster.
---
 Userland/Libraries/LibJS/Bytecode/Interpreter.cpp | 2 ++
 Userland/Libraries/LibJS/Bytecode/Interpreter.h   | 3 +++
 Userland/Libraries/LibJS/Bytecode/Op.cpp          | 9 +++++++--
 3 files changed, 12 insertions(+), 2 deletions(-)

diff --git a/Userland/Libraries/LibJS/Bytecode/Interpreter.cpp b/Userland/Libraries/LibJS/Bytecode/Interpreter.cpp
index c818d9c527..b747ce9b2e 100644
--- a/Userland/Libraries/LibJS/Bytecode/Interpreter.cpp
+++ b/Userland/Libraries/LibJS/Bytecode/Interpreter.cpp
@@ -208,6 +208,8 @@ Interpreter::ValueAndFrame Interpreter::run_and_return_frame(Realm& realm, Execu
     else
         push_call_frame(make<CallFrame>(), executable.number_of_registers);
 
+    TemporaryChange restore_this_value { m_this_value, {} };
+
     for (;;) {
         Bytecode::InstructionStreamIterator pc(m_current_block->instruction_stream());
         TemporaryChange temp_change { m_pc, &pc };
diff --git a/Userland/Libraries/LibJS/Bytecode/Interpreter.h b/Userland/Libraries/LibJS/Bytecode/Interpreter.h
index dfd57fe936..1c4e05883a 100644
--- a/Userland/Libraries/LibJS/Bytecode/Interpreter.h
+++ b/Userland/Libraries/LibJS/Bytecode/Interpreter.h
@@ -95,6 +95,8 @@ public:
 
     VM::InterpreterExecutionScope ast_interpreter_scope(Realm&);
 
+    Optional<Value>& this_value() { return m_this_value; }
+
     void visit_edges(Cell::Visitor&);
 
 private:
@@ -119,6 +121,7 @@ private:
     Span<Value> m_current_call_frame;
     Optional<BasicBlock const*> m_pending_jump;
     BasicBlock const* m_scheduled_jump { nullptr };
+    Optional<Value> m_this_value;
     Optional<Value> m_return_value;
     Optional<Value> m_saved_exception;
     Executable* m_current_executable { nullptr };
diff --git a/Userland/Libraries/LibJS/Bytecode/Op.cpp b/Userland/Libraries/LibJS/Bytecode/Op.cpp
index 1e37fd6aee..164ed5b829 100644
--- a/Userland/Libraries/LibJS/Bytecode/Op.cpp
+++ b/Userland/Libraries/LibJS/Bytecode/Op.cpp
@@ -732,8 +732,13 @@ ThrowCompletionOr<void> Jump::execute_impl(Bytecode::Interpreter& interpreter) c
 
 ThrowCompletionOr<void> ResolveThisBinding::execute_impl(Bytecode::Interpreter& interpreter) const
 {
-    auto& vm = interpreter.vm();
-    interpreter.accumulator() = TRY(vm.resolve_this_binding());
+    if (!interpreter.this_value().has_value()) {
+        // OPTIMIZATION: Because the value of 'this' cannot be reassigned during a function execution, it's
+        //               resolved once and then saved for subsequent use.
+        auto& vm = interpreter.vm();
+        interpreter.this_value() = TRY(vm.resolve_this_binding());
+    }
+    interpreter.accumulator() = interpreter.this_value().value();
     return {};
 }