LibJS: Store bytecode VM program counter in ExecutionContext

This way it's always automatically correct, and we don't have to
manually flush it in push_execution_context().

~7% speedup on the MicroBench/call* tests :^)

Libraries/LibJS/Bytecode/Interpreter.cpp

@@ -381,9 +381,8 @@ FLATTEN_ON_CLANG void Interpreter::run_bytecode(size_t entry_point)
     auto& executable = current_executable();
     auto const* bytecode = executable.bytecode.data();
 
-    size_t program_counter = entry_point;
-
-    TemporaryChange change(m_program_counter, Optional<size_t&>(program_counter));
+    size_t& program_counter = running_execution_context.program_counter;
+    program_counter = entry_point;
 
     // Declare a lookup table for computed goto with each of the `handle_*` labels
     // to avoid the overhead of a switch statement.

Libraries/LibJS/Bytecode/Interpreter.h

@@ -76,7 +76,6 @@ public:
 
     Executable& current_executable() { return *m_current_executable; }
     Executable const& current_executable() const { return *m_current_executable; }
-    Optional<size_t> program_counter() const { return m_program_counter; }
     Span<Value> allocate_argument_values(size_t argument_count)
     {
         m_argument_values_buffer.resize_and_keep_capacity(argument_count);
@@ -100,7 +99,6 @@ private:
     GC::Ptr<Realm> m_realm { nullptr };
     GC::Ptr<Object> m_global_object { nullptr };
     GC::Ptr<DeclarativeEnvironment> m_global_declarative_environment { nullptr };
-    Optional<size_t&> m_program_counter;
     Span<Value> m_registers_and_constants_and_locals_arguments;
     Vector<Value> m_argument_values_buffer;
     ExecutionContext* m_running_execution_context { nullptr };

Libraries/LibJS/Runtime/ExecutionContext.h

@@ -58,7 +58,7 @@ public:
     // Non-standard: This points at something that owns this ExecutionContext, in case it needs to be protected from GC.
     GC::Ptr<Cell> context_owner;
 
-    Optional<size_t> program_counter;
+    size_t program_counter { 0 };
 
     mutable RefPtr<CachedSourceRange> cached_source_range;
 

Libraries/LibJS/Runtime/VM.cpp

@@ -480,8 +480,8 @@ void VM::dump_backtrace() const
 {
     for (ssize_t i = m_execution_context_stack.size() - 1; i >= 0; --i) {
         auto& frame = m_execution_context_stack[i];
-        if (frame->executable && frame->program_counter.has_value()) {
-            auto source_range = frame->executable->source_range_at(frame->program_counter.value()).realize();
+        if (frame->executable) {
+            auto source_range = frame->executable->source_range_at(frame->program_counter).realize();
             dbgln("-> {} @ {}:{},{}", frame->function_name ? frame->function_name->utf8_string() : ""_string, source_range.filename(), source_range.start.line, source_range.start.column);
         } else {
             dbgln("-> {}", frame->function_name ? frame->function_name->utf8_string() : ""_string);
@@ -746,34 +746,17 @@ void VM::load_imported_module(ImportedModuleReferrer referrer, ModuleRequest con
     finish_loading_imported_module(referrer, module_request, payload, module);
 }
 
-void VM::push_execution_context(ExecutionContext& context)
-{
-    if (!m_execution_context_stack.is_empty())
-        m_execution_context_stack.last()->program_counter = bytecode_interpreter().program_counter();
-    m_execution_context_stack.append(&context);
-}
-
-void VM::pop_execution_context()
-{
-    m_execution_context_stack.take_last();
-    if (m_execution_context_stack.is_empty() && on_call_stack_emptied)
-        on_call_stack_emptied();
-}
-
 static RefPtr<CachedSourceRange> get_source_range(ExecutionContext const* context)
 {
     // native function
     if (!context->executable)
         return {};
 
-    if (!context->program_counter.has_value())
-        return {};
-
     if (!context->cached_source_range
-        || context->cached_source_range->program_counter != context->program_counter.value()) {
-        auto unrealized_source_range = context->executable->source_range_at(context->program_counter.value());
+        || context->cached_source_range->program_counter != context->program_counter) {
+        auto unrealized_source_range = context->executable->source_range_at(context->program_counter);
         context->cached_source_range = adopt_ref(*new CachedSourceRange(
-            context->program_counter.value(),
+            context->program_counter,
             move(unrealized_source_range)));
     }
     return context->cached_source_range;

Libraries/LibJS/Runtime/VM.h

@@ -115,12 +115,21 @@ public:
         if (did_reach_stack_space_limit()) [[unlikely]] {
             return throw_completion<InternalError>(ErrorType::CallStackSizeExceeded);
         }
-        push_execution_context(context);
+        m_execution_context_stack.append(&context);
         return {};
     }
 
-    void push_execution_context(ExecutionContext&);
-    void pop_execution_context();
+    void push_execution_context(ExecutionContext& context)
+    {
+        m_execution_context_stack.append(&context);
+    }
+
+    void pop_execution_context()
+    {
+        m_execution_context_stack.take_last();
+        if (m_execution_context_stack.is_empty() && on_call_stack_emptied)
+            on_call_stack_emptied();
+    }
 
     // https://tc39.es/ecma262/#running-execution-context
     // At any point in time, there is at most one execution context per agent that is actually executing code.

Libraries/LibWeb/HTML/WindowOrWorkerGlobalScope.cpp

@@ -913,8 +913,8 @@ static ErrorInformation extract_error_information(JS::VM& vm, JS::Value exceptio
     else {
         for (ssize_t i = vm.execution_context_stack().size() - 1; i >= 0; --i) {
             auto& frame = vm.execution_context_stack()[i];
-            if (frame->executable && frame->program_counter.has_value()) {
-                auto source_range = frame->executable->source_range_at(frame->program_counter.value()).realize();
+            if (frame->executable) {
+                auto source_range = frame->executable->source_range_at(frame->program_counter).realize();
                 attributes.filename = MUST(String::from_byte_string(source_range.filename()));
                 attributes.lineno = source_range.start.line;
                 attributes.colno = source_range.start.column;