diff --git a/docs/writeup.txt b/docs/writeup.txt
index dd2fb07..a6ca7d0 100644
--- a/docs/writeup.txt
+++ b/docs/writeup.txt
@@ -28,3 +28,9 @@ Constant foldable expressions, e.g. "size_t myOffset = ((size_t)&((struct task_s
 
 CONSTRUCTORs are only turned into COMPONENT_REF at gimplification
 	-> early gimple pass (before cfg) to replace offsets with calls
+
+Custom UNSPECs can not get through vregs or a few later passes
+	-> use asm
+	-> architecture specific, dynamic operands though
+	-> can not get optimized (O1+ does optimizations in RTL, mostly after vregs)
+	-> could replace asm with normal "set" RTL close to final pass to remove architecture dependence