Optimise field access

Author: penelopeysm

src/gc.cpp

@@ -119,7 +119,8 @@ void GC::gc() {
     case ObjType::INSTANCE: {
       auto p = static_cast<ObjInstance*>(objptr);
       mark_as_grey(p->klass);
-      for (const auto& [_, value] : p->fields.map) {
+      for (const auto& [fieldname, value] : p->fields) {
+        mark_as_grey(fieldname);
         mark_as_grey(value);
       }
     }

src/value.hpp

@@ -146,9 +146,7 @@ class ObjClass : public Obj {
 class ObjInstance : public Obj {
 public:
   ObjClass* klass;
-  // TODO: we could use std::unordered_map<ObjString*, Value> here to avoid
-  // creating the same string multiple times across different instances?
-  StringMap<Value> fields;
+  std::unordered_map<ObjString*, Value> fields;
 
   ObjInstance(ObjClass* klass) : Obj(ObjType::INSTANCE), klass(klass) {}
 

src/vm.cpp

@@ -461,8 +461,8 @@ InterpretResult VM::run() {
         }
         auto instanceptr = static_cast<ObjInstance*>(objptr);
         ObjString* property_name = read_constant_string();
-        auto it = instanceptr->fields.map.find(property_name);
-        if (it == instanceptr->fields.map.end()) {
+        auto it = instanceptr->fields.find(property_name);
+        if (it == instanceptr->fields.end()) {
           throw std::runtime_error("undefined property '" +
                                    property_name->value + "'");
         } else {
@@ -487,7 +487,7 @@ InterpretResult VM::run() {
         ObjString* property_name = read_constant_string();
         // NOTE: operator[] does not allow for heterogeneous lookup, so we need
         // to actually access the underlying std::string
-        instanceptr->fields.map[property_name->value] = value_to_set;
+        instanceptr->fields[property_name] = value_to_set;
         // Pop the instance and value, but leave the value on the stack since
         // (a.x = b) evaluates to b
         stack_pop();