- remove all symbols that get linked into the symbol table from the garbage collector.

Symbols are very easy to manage once they are in a symbol table and there's lots of them so this reduces the amount of work the GC needs to do quite considerably.
After cleaning out compile-time-only symbols there will still be more than 2000 left, one for each function and one for each member variable of a class or struct.
This means more than 2000 object that won't need to tracked constantly by the garbage collector.

Note that loose fields which do occur during code generation will be GC'd just as before.

src/dobject.cpp

@@ -301,42 +301,17 @@ DObject::~DObject ()
 		PClass *type = GetClass();
 		if (!(ObjectFlags & OF_Cleanup) && !PClass::bShutdown)
 		{
-			DObject **probe;
-
-			if (!(ObjectFlags & OF_YesReallyDelete))
+			if (!(ObjectFlags & (OF_YesReallyDelete|OF_Released)))
 			{
 				Printf("Warning: '%s' is freed outside the GC process.\n",
 					type != NULL ? type->TypeName.GetChars() : "==some object==");
 			}
 
-			// Find all pointers that reference this object and NULL them.
-			StaticPointerSubstitution(this, NULL);
-
-			// Now unlink this object from the GC list.
-			for (probe = &GC::Root; *probe != NULL; probe = &((*probe)->ObjNext))
+			if (!(ObjectFlags & OF_Released))
 			{
-				if (*probe == this)
-				{
-					*probe = ObjNext;
-					if (&ObjNext == GC::SweepPos)
-					{
-						GC::SweepPos = probe;
-					}
-					break;
-				}
-			}
-
-			// If it's gray, also unlink it from the gray list.
-			if (this->IsGray())
-			{
-				for (probe = &GC::Gray; *probe != NULL; probe = &((*probe)->GCNext))
-				{
-					if (*probe == this)
-					{
-						*probe = GCNext;
-						break;
-					}
-				}
+				// Find all pointers that reference this object and NULL them.
+				StaticPointerSubstitution(this, NULL);
+				Release();
 			}
 		}
 		
@@ -347,6 +322,41 @@ DObject::~DObject ()
 	}
 }
 
+void DObject::Release()
+{
+	DObject **probe;
+
+	// Unlink this object from the GC list.
+	for (probe = &GC::Root; *probe != NULL; probe = &((*probe)->ObjNext))
+	{
+		if (*probe == this)
+		{
+			*probe = ObjNext;
+			if (&ObjNext == GC::SweepPos)
+			{
+				GC::SweepPos = probe;
+			}
+			break;
+		}
+	}
+
+	// If it's gray, also unlink it from the gray list.
+	if (this->IsGray())
+	{
+		for (probe = &GC::Gray; *probe != NULL; probe = &((*probe)->GCNext))
+		{
+			if (*probe == this)
+			{
+				*probe = GCNext;
+				break;
+			}
+		}
+	}
+	ObjNext = nullptr;
+	GCNext = nullptr;
+	ObjectFlags |= OF_Released;
+}
+
 //==========================================================================
 //
 //

src/dobject.h

@@ -468,6 +468,9 @@ public:
 		Class = NULL;
 	}
 
+	// Releases the object from the GC, letting the caller care of any maintenance.
+	void Release();
+
 	// For catching Serialize functions in derived classes
 	// that don't call their base class.
 	void CheckIfSerialized () const;

src/dobjgc.cpp

@@ -277,7 +277,9 @@ static DObject **SweepList(DObject **p, size_t count, size_t *finalize_count)
 void Mark(DObject **obj)
 {
 	DObject *lobj = *obj;
-	if (lobj != NULL)
+
+	assert(lobj == nullptr || !(lobj->ObjectFlags & OF_Released));
+	if (lobj != nullptr && !(lobj->ObjectFlags & OF_Released))
 	{
 		if (lobj->ObjectFlags & OF_EuthanizeMe)
 		{
@@ -551,6 +553,8 @@ void Barrier(DObject *pointing, DObject *pointed)
 	assert(pointing == NULL || (pointing->IsBlack() && !pointing->IsDead()));
 	assert(pointed->IsWhite() && !pointed->IsDead());
 	assert(State != GCS_Finalize && State != GCS_Pause);
+	assert(!(pointed->ObjectFlags & OF_Released));	// if a released object gets here, something must be wrong.
+	if (pointed->ObjectFlags & OF_Released) return;	// don't do anything with non-GC'd objects.
 	// The invariant only needs to be maintained in the propagate state.
 	if (State == GCS_Propagate)
 	{

src/dobjtype.cpp

@@ -185,18 +185,6 @@ PType::~PType()
 {
 }
 
-//==========================================================================
-//
-// PType :: PropagateMark
-//
-//==========================================================================
-
-size_t PType::PropagateMark()
-{
-	size_t marked = Symbols.MarkSymbols();
-	return marked + Super::PropagateMark();
-}
-
 //==========================================================================
 //
 // PType :: WriteValue
@@ -2413,7 +2401,7 @@ PField *PStruct::AddField(FName name, PType *type, DWORD flags)
 
 	if (Symbols.AddSymbol(field) == nullptr)
 	{ // name is already in use
-		delete field;
+		field->Destroy();
 		return nullptr;
 	}
 	Fields.Push(field);
@@ -2444,18 +2432,6 @@ PField *PStruct::AddNativeField(FName name, PType *type, size_t address, DWORD f
 	return field;
 }
 
-//==========================================================================
-//
-// PStruct :: PropagateMark
-//
-//==========================================================================
-
-size_t PStruct::PropagateMark()
-{
-	GC::MarkArray(Fields);
-	return Fields.Size() * sizeof(void*) + Super::PropagateMark();
-}
-
 //==========================================================================
 //
 // NewStruct

src/dobjtype.h

@@ -171,8 +171,6 @@ public:
 
 	const char *DescriptiveName() const;
 
-	size_t PropagateMark();
-
 	static void StaticInit();
 };
 
@@ -520,8 +518,6 @@ public:
 	virtual PField *AddField(FName name, PType *type, DWORD flags=0);
 	virtual PField *AddNativeField(FName name, PType *type, size_t address, DWORD flags = 0, int bitvalue = 0);
 
-	size_t PropagateMark();
-
 	void WriteValue(FSerializer &ar, const char *key,const void *addr) const override;
 	bool ReadValue(FSerializer &ar, const char *key,void *addr) const override;
 	void SetDefaultValue(void *base, unsigned offset, TArray<FTypeAndOffset> *specials) const override;

src/scripting/backend/codegen.cpp

@@ -6558,23 +6558,10 @@ FxStackVariable::FxStackVariable(PType *type, int offset, const FScriptPosition
 
 FxStackVariable::~FxStackVariable()
 {
-	// Q: Is this good or bad? Needs testing if this is fine or better left to the GC anyway. DObject's destructor is anything but cheap.
 	membervar->ObjectFlags |= OF_YesReallyDelete;
 	delete membervar;
 }
 
-//==========================================================================
-//
-//
-//==========================================================================
-
-void FxStackVariable::ReplaceField(PField *newfield)
-{
-	membervar->ObjectFlags |= OF_YesReallyDelete;
-	delete membervar;
-	membervar = newfield;
-}
-
 //==========================================================================
 //
 //

src/scripting/backend/codegen.h

@@ -1417,7 +1417,6 @@ class FxStackVariable : public FxMemberBase
 public:
 	FxStackVariable(PType *type, int offset, const FScriptPosition&);
 	~FxStackVariable();
-	void ReplaceField(PField *newfield);
 	FxExpression *Resolve(FCompileContext&);
 	bool RequestAddress(FCompileContext &ctx, bool *writable);
 	ExpEmit Emit(VMFunctionBuilder *build);

src/scripting/symbols.cpp

@@ -149,34 +149,19 @@ PSymbolTable::~PSymbolTable ()
 
 //==========================================================================
 //
-//
-//
-//==========================================================================
-
-size_t PSymbolTable::MarkSymbols()
-{
-	size_t count = 0;
-	MapType::Iterator it(Symbols);
-	MapType::Pair *pair;
-
-	while (it.NextPair(pair))
-	{
-		GC::Mark(pair->Value);
-		count++;
-	}
-	return count * sizeof(*pair);
-}
-
-//==========================================================================
-//
-//
+// this must explicitly delete all content because the symbols have
+// been released from the GC.
 //
 //==========================================================================
 
 void PSymbolTable::ReleaseSymbols()
 {
-	// The GC will take care of deleting the symbols. We just need to
-	// clear our references to them.
+	auto it = GetIterator();
+	MapType::Pair *pair;
+	while (it.NextPair(pair))
+	{
+		delete pair->Value;
+	}
 	Symbols.Clear();
 }
 
@@ -243,6 +228,7 @@ PSymbol *PSymbolTable::AddSymbol (PSymbol *sym)
 		return nullptr;
 	}
 	Symbols.Insert(sym->SymbolName, sym);
+	sym->Release();	// no more GC, please!
 	return sym;
 }
 
@@ -257,6 +243,7 @@ void PSymbolTable::RemoveSymbol(PSymbol *sym)
 	auto mysym = Symbols.CheckKey(sym->SymbolName);
 	if (mysym == nullptr || *mysym != sym) return;
 	Symbols.Remove(sym->SymbolName);
+	delete sym;
 }
 
 //==========================================================================
@@ -265,20 +252,20 @@ void PSymbolTable::RemoveSymbol(PSymbol *sym)
 //
 //==========================================================================
 
-PSymbol *PSymbolTable::ReplaceSymbol(PSymbol *newsym)
+void PSymbolTable::ReplaceSymbol(PSymbol *newsym)
 {
 	// If a symbol with a matching name exists, take its place and return it.
 	PSymbol **symslot = Symbols.CheckKey(newsym->SymbolName);
 	if (symslot != nullptr)
 	{
 		PSymbol *oldsym = *symslot;
+		delete oldsym;
 		*symslot = newsym;
-		return oldsym;
 	}
 	// Else, just insert normally and return nullptr since there was no
 	// symbol to replace.
+	newsym->Release();	// no more GC, please!
 	Symbols.Insert(newsym->SymbolName, newsym);
-	return nullptr;
 }
 
 //==========================================================================
@@ -306,18 +293,6 @@ PNamespace::PNamespace(int filenum, PNamespace *parent)
 //
 //==========================================================================
 
-size_t PNamespace::PropagateMark()
-{
-	GC::Mark(Parent);
-	return Symbols.MarkSymbols() + 1;
-}
-
-//==========================================================================
-//
-//
-//
-//==========================================================================
-
 FNamespaceManager::FNamespaceManager()
 {
 	GlobalNamespace = nullptr;
@@ -370,6 +345,7 @@ size_t FNamespaceManager::MarkSymbols()
 
 void FNamespaceManager::ReleaseSymbols()
 {
+	RemoveSymbols();
 	GlobalNamespace = nullptr;
 	AllNamespaces.Clear();
 }

src/scripting/symbols.h

@@ -192,8 +192,6 @@ struct PSymbolTable
 	PSymbolTable(PSymbolTable *parent);
 	~PSymbolTable();
 
-	size_t MarkSymbols();
-
 	// Sets the table to use for searches if this one doesn't contain the
 	// requested symbol.
 	void SetParentTable (PSymbolTable *parent);
@@ -218,7 +216,7 @@ struct PSymbolTable
 
 	// Similar to AddSymbol but always succeeds. Returns the symbol that used
 	// to be in the table with this name, if any.
-	PSymbol *ReplaceSymbol(PSymbol *sym);
+	void ReplaceSymbol(PSymbol *sym);
 
 	void RemoveSymbol(PSymbol *sym);
 
@@ -255,7 +253,6 @@ public:
 
 	PNamespace() {}
 	PNamespace(int filenum, PNamespace *parent);
-	size_t PropagateMark();
 };
 
 struct FNamespaceManager