When assigning a pointer to an object that is allocated on the stack to the heap, send a -copy message. This lets you cast blocks to id and then assign them to ivars without anything breaking. Apple's 'solution' to this is to segfault, which is not particularly elegant. This code is also quite an ugly hack (it just checks whether the object is allocated within a couple of pages of the current stack frame), so I might remove it before release.

gc_boehm.c

@@ -23,6 +23,7 @@ static dispatch_queue_t finalizer_queue;
  */
 static BOOL finalizeThreaded;
 
+struct objc_slot* objc_get_slot(Class cls, SEL selector);
 
 /*
  * Citing boehm-gc's README.linux:
@@ -151,23 +152,45 @@ BOOL objc_atomicCompareAndSwapInstanceVariableBarrier(id predicate, id replaceme
 	return objc_atomicCompareAndSwapPtr(predicate, replacement, objectLocation);
 }
 
+SEL copy;
+
+static inline id copy_to_heap(id val)
+{
+	if ((0 == val) || ((1 & (intptr_t)val) == 1) || (0 == val->isa)) { return val; }
+	struct objc_object a;
+	if (((&a - 2048) < val) && ((&a + 2048) > val))
+	{
+		struct objc_slot *slot = objc_msg_lookup_sender(&val, copy, nil);
+		if (NULL != slot)
+		{
+			val = slot->method(val, copy);
+		}
+	}
+	return val;
+}
 
 id objc_assign_strongCast(id val, id *ptr)
 {
+	val = copy_to_heap(val);
 	*ptr = val;
 	return val;
 }
 
 id objc_assign_global(id val, id *ptr)
 {
+	val = copy_to_heap(val);
 	//fprintf(stderr, "Storign %p in global %p\n", val, ptr);
+	if (isGCEnabled)
+	{
 		GC_add_roots(ptr, ptr+1);
+	}
 	*ptr = val;
 	return val;
 }
 
 id objc_assign_ivar(id val, id dest, ptrdiff_t offset)
 {
+	val = copy_to_heap(val);
 	*(id*)((char*)dest+offset) = val;
 	return val;
 }
@@ -195,11 +218,20 @@ static void *readWeakLocked(void *ptr)
 
 id objc_read_weak(id *location)
 {
+	if (!isGCEnabled)
+	{
+		return *location;
+	}
 	return GC_call_with_alloc_lock(readWeakLocked, location);
 }
 
 id objc_assign_weak(id value, id *location)
 {
+	if (!isGCEnabled)
+	{
+		*location = value;
+		return value;
+	}
 	// Temporarily zero this pointer and get the old value
 	id old = __sync_swap(location, 0);
 	if (0 != old)
@@ -229,7 +261,6 @@ static SEL cxx_destruct;
 
 Class zombie_class;
 
-struct objc_slot* objc_get_slot(Class cls, SEL selector);
 
 static void runFinalize(void *addr, void *context)
 {
@@ -526,6 +557,7 @@ static void init(void)
 	GC_clear_roots();
 	finalize = sel_registerName("finalize");
 	cxx_destruct = sel_registerName(".cxx_destruct");
+	copy = sel_registerName("copy");
 	GC_finalizer_notifier = runFinalizers;
 }