Make sure we keep a root for the delegates when calling them.

	* method-to-ir.c (mono_emit_method_call_full): Make sure
	we keep a root for delegates when calling them using the
	fast delegate dispatch.

	Fixes #667921

mono/mini/method-to-ir.c

@@ -2401,6 +2401,8 @@ mono_emit_method_call_full (MonoCompile *cfg, MonoMethod *method, MonoMethodSign
 
 #ifdef MONO_ARCH_HAVE_CREATE_DELEGATE_TRAMPOLINE
 		if ((method->klass->parent == mono_defaults.multicastdelegate_class) && (!strcmp (method->name, "Invoke"))) {
+			MonoInst *dummy_use;
+
 			MONO_EMIT_NULL_CHECK (cfg, this_reg);
 
 			/* Make a call to delegate->invoke_impl */
@@ -2409,6 +2411,20 @@ mono_emit_method_call_full (MonoCompile *cfg, MonoMethod *method, MonoMethodSign
 			call->inst.inst_offset = G_STRUCT_OFFSET (MonoDelegate, invoke_impl);
 			MONO_ADD_INS (cfg->cbb, (MonoInst*)call);
 
+			/* We must emit a dummy use here because the delegate trampoline will
+			replace the 'this' argument with the delegate target making this activation
+			no longer a root for the delegate.
+			This is an issue for delegates that target collectible code such as dynamic
+			methods of GC'able assemblies.
+
+			For a test case look into #667921.
+
+			FIXME: a dummy use is not the best way to do it as the local register allocator
+			will put it on a caller save register and spil it around the call. 
+			Ideally, we would either put it on a callee save register or only do the store part.  
+			 */
+			EMIT_NEW_DUMMY_USE (cfg, dummy_use, args [0]);
+
 			return (MonoInst*)call;
 		}
 #endif