diff --git a/moo/lib/exec.c b/moo/lib/exec.c
index 910f903..0e00a3a 100644
--- a/moo/lib/exec.c
+++ b/moo/lib/exec.c
@@ -1629,7 +1629,7 @@ static moo_oop_process_t start_initial_process (moo_t* moo, moo_oop_context_t c)
 	MOO_ASSERT (moo, moo->processor->active == moo->nil_process);
 
 	proc = make_process(moo, c, MOO_OBJ_FLAGS_PROC_INNATE);
-	if (!proc) return MOO_NULL;
+	if (MOO_UNLIKELY(!proc)) return MOO_NULL;
 
 	chain_into_processor (moo, proc, PROC_STATE_RUNNING);
 	moo->processor->active = proc;
@@ -1666,7 +1666,7 @@ static MOO_INLINE int activate_new_method (moo_t* moo, moo_oop_method_t mth, moo
 	moo_pushvolat (moo, (moo_oop_t*)&mth);
 	ctx = (moo_oop_context_t)moo_instantiate(moo, moo->_method_context, MOO_NULL, actual_ntmprs);
 	moo_popvolat (moo);
-	if (!ctx) return -1;
+	if (MOO_UNLIKELY(!ctx)) return -1;
 
 	MOO_STORE_OOP (moo, (moo_oop_t*)&ctx->sender, (moo_oop_t)moo->active_context); 
 	ctx->ip = MOO_SMOOI_TO_OOP(0);
@@ -1968,7 +1968,7 @@ static int start_initial_process_and_context (moo_t* moo, const moo_oocs_t* objn
 	moo_oop_process_t proc;
 	moo_oow_t tmp_count = 0;
 	moo_oop_t sym_startup;
-	
+
 #if defined(INVOKE_DIRECTLY)
 	moo_oop_association_t ass;
 #else
@@ -1984,7 +1984,7 @@ static int start_initial_process_and_context (moo_t* moo, const moo_oocs_t* objn
 		MOO_LOG2 (moo, MOO_LOG_DEBUG, "Cannot find a class '%.*js'", objname->len, objname->ptr);
 		return -1;
 	}
-	
+
 	sym_statup = moo_findsymbol(moo, mthname->ptr, mthname->len;
 	if (!sym_startup)
 	{
@@ -2476,7 +2476,7 @@ static moo_pfrc_t pf_method_get_ip_source_line (moo_t* moo, moo_mod_t* mod, moo_
 		if (ipv < di->code_loc_len && code_loc_ptr[ipv] <= MOO_SMOOI_MAX) 
 		{
 			retv = moo_oowtoint(moo, code_loc_ptr[ipv]);
-			if (!retv) return MOO_PF_FAILURE;
+			if (MOO_UNLIKELY(!retv)) return MOO_PF_FAILURE;
 		}
 	}
 
@@ -5258,16 +5258,17 @@ static MOO_INLINE int do_return (moo_t* moo, moo_oob_t bcode, moo_oop_t return_v
 	{
 		unwind_protect = 0;
 
-		/* set the instruction pointer to an invalid value.
-		 * this is stored into the current method context
-		 * before context switching and marks a dead context */
+
 		if (moo->active_context->origin == moo->active_context)
 		{
 			/* returning from a method */
 			MOO_ASSERT (moo, MOO_CLASSOF(moo, moo->active_context) == moo->_method_context);
 
-			/* mark that the context is dead. it will be 
-			 * save to the context object by SWITCH_ACTIVE_CONTEXT() */
+
+			/* set the instruction pointer to an invalid value.
+			 * this is stored into the current method context
+			 * before context switching  by SWITCH_ACTIVE_CONTEXT()
+			 * and marks the context dead */
 			moo->ip = -1;
 		}
 		else
diff --git a/moo/lib/gc.c b/moo/lib/gc.c
index 6f051c3..525f741 100644
--- a/moo/lib/gc.c
+++ b/moo/lib/gc.c
@@ -514,7 +514,7 @@ static int ignite_2 (moo_t* moo)
 	/* Create 'true' and 'false objects */
 	moo->_true = moo_instantiate(moo, moo->_true_class, MOO_NULL, 0);
 	moo->_false = moo_instantiate(moo, moo->_false_class, MOO_NULL, 0);
-	if (!moo->_true || !moo->_false) return -1;
+	if (MOO_UNLIKELY(!moo->_true) || MOO_UNLIKELY(!moo->_false)) return -1;
 
 	/* Prevent the object instations in the permspace. 
 	 *
@@ -537,7 +537,7 @@ static int ignite_2 (moo_t* moo)
 
 	/* Create the symbol table */
 	tmp = moo_instantiate(moo, moo->_symbol_table, MOO_NULL, 0);
-	if (!tmp) return -1;
+	if (MOO_UNLIKELY(!tmp)) return -1;
 	moo->symtab = (moo_oop_dic_t)tmp;
 
 	moo->symtab->tally = MOO_SMOOI_TO_OOP(0);
@@ -937,6 +937,11 @@ static moo_rbt_walk_t call_module_gc (moo_rbt_t* rbt, moo_rbt_pair_t* pair, void
 
 void moo_gc (moo_t* moo)
 {
+#if defined(MOO_ENABLE_GC_MARK_SWEEP)
+	/* TODO: */
+
+
+#else
 	/* 
 	 * move a referenced object to the new heap.
 	 * inspect the fields of the moved object in the new heap.
@@ -1103,6 +1108,7 @@ void moo_gc (moo_t* moo)
 	MOO_LOG4 (moo, MOO_LOG_GC | MOO_LOG_INFO, 
 		"Finished GC curheap base %p ptr %p newheap base %p ptr %p\n",
 		moo->heap->curspace.base, moo->heap->curspace.ptr, moo->heap->newspace.base, moo->heap->newspace.ptr); 
+#endif
 }
 
 void moo_pushvolat (moo_t* moo, moo_oop_t* oop_ptr)
diff --git a/moo/lib/moo-prv.h b/moo/lib/moo-prv.h
index 4ed34c3..afb263e 100644
--- a/moo/lib/moo-prv.h
+++ b/moo/lib/moo-prv.h
@@ -720,7 +720,6 @@ struct moo_compiler_t
 #	error Unsupported MOO_BCODE_LONG_PARAM_SIZE
 #endif
 
-
 /*
 ----------------------------------------------------------------------------------------------------------------
 SHORT INSTRUCTION CODE                                        LONG INSTRUCTION CODE
diff --git a/moo/lib/moo.h b/moo/lib/moo.h
index 5779ec0..8bbe6b0 100644
--- a/moo/lib/moo.h
+++ b/moo/lib/moo.h
@@ -34,6 +34,9 @@
 /* TODO: move this macro out to the build files.... */
 #define MOO_INCLUDE_COMPILER
 
+/*#define MOO_ENABLE_GC_MARK_SWEEP*/
+
+
 typedef struct moo_mod_t moo_mod_t;
 
 /* ========================================================================== */
@@ -198,6 +201,19 @@ enum moo_method_type_t
 };
 typedef enum moo_method_type_t moo_method_type_t;
 
+
+/* =========================================================================
+ * HEADER FOR SOME GC IMPLEMENTATIONS
+ * ========================================================================= */
+#if defined(MOO_ENABLE_GC_MARK_SWEEP)
+typedef struct moo_gchdr_t moo_gchdr_t;
+struct moo_gchdr_t
+{
+	moo_gchdr_t* next;
+};
+/* The size of moo_gchdr_t must be aligned to MOO_SIZEOF_OOP_T */
+#endif
+
 /* =========================================================================
  * OBJECT STRUCTURE
  * ========================================================================= */
@@ -1780,6 +1796,10 @@ struct moo_t
 		moo_uintmax_t inst_counter;
 	} stat;
 
+#if defined(MOO_ENABLE_GC_MARK_SWEEP)
+	moo_gchdr_t* gch;
+#endif
+
 #if defined(MOO_INCLUDE_COMPILER)
 	moo_compiler_t* c;
 #endif
diff --git a/moo/lib/obj.c b/moo/lib/obj.c
index 500ddde..693773b 100644
--- a/moo/lib/obj.c
+++ b/moo/lib/obj.c
@@ -29,11 +29,38 @@
 void* moo_allocbytes (moo_t* moo, moo_oow_t size)
 {
 	moo_uint8_t* ptr;
+#if defined(MOO_ENABLE_GC_MARK_SWEEP)
+	moo_gchdr_t* gch;
+#endif
 
 #if defined(MOO_BUILD_DEBUG)
 	if ((moo->option.trait & MOO_TRAIT_DEBUG_GC) && !(moo->option.trait & MOO_TRAIT_NOGC)) moo_gc (moo);
 #endif
 
+#if defined(MOO_ENABLE_GC_MARK_SWEEP)
+	if (MOO_UNLIKELY(moo->igniting))
+	{
+		gch = (moo_gchdr_t*)moo_allocmem(moo, MOO_SIZEOF(*gch) + size); 
+		if (MOO_UNLIKELY(!gch)) return MOO_NULL;
+	}
+	else
+	{
+// TODO: perform GC if allocation got above threshold...
+		gch = (moo_gchdr_t*)moo_allocmem(moo, MOO_SIZEOF(*gch) + size); 
+		if (!gch && moo->errnum == MOO_EOOMEM && !(moo->option.trait & MOO_TRAIT_NOGC))
+		{
+			moo_gc (moo);
+			MOO_LOG0 (moo, MOO_LOG_GC | MOO_LOG_INFO, "GC completed\n"); /* TODO: add more inforamtion */
+			gch = (moo_gchdr_t*)moo_allocmem(moo, MOO_SIZEOF(*gch) + size); 
+			if (MOO_UNLIKELY(!gch)) return MOO_NULL;
+		}
+	}
+
+	gch->next = moo->gch;
+	moo->gch = gch;
+
+	ptr = (moo_uint8_t*)(gch + 1);
+#else
 	if (MOO_UNLIKELY(moo->igniting))
 	{
 		/* you must increase the size of the permspace if this allocation fails */
@@ -55,6 +82,8 @@ void* moo_allocbytes (moo_t* moo, moo_oow_t size)
 	/* TODO: grow heap if ptr is still null. */
 		}
 	}
+#endif
+
 	return ptr;
 }
 
@@ -74,7 +103,7 @@ moo_oop_t moo_allocoopobj (moo_t* moo, moo_oow_t size)
 	 * of the allocated space to be an even number. 
 	 * see MOO_OOP_IS_NUMERIC() and MOO_OOP_IS_POINTER() */
 	hdr = (moo_oop_oop_t)moo_allocbytes(moo, MOO_SIZEOF(moo_obj_t) + nbytes_aligned);
-	if (!hdr) return MOO_NULL;
+	if (MOO_UNLIKELY(!hdr)) return MOO_NULL;
 
 	hdr->_flags = MOO_OBJ_MAKE_FLAGS(MOO_OBJ_TYPE_OOP, MOO_SIZEOF(moo_oop_t), 0, 0, moo->igniting, 0, 0, 0, 0, 0);
 	MOO_OBJ_SET_SIZE (hdr, size);