add a note

llvm-svn: 30515

llvm/lib/Target/X86/README.txt

@@ -734,3 +734,19 @@ This currently compiles to:
 	addl $12, %esp
 
 The add\sub pair is really unneeded here.
+
+//===---------------------------------------------------------------------===//
+
+We generate really bad code in some cases due to lowering SETCC/SELECT at 
+legalize time, which prevents the post-legalize dag combine pass from
+understanding the code.  As a silly example, this prevents us from folding 
+stuff like this:
+
+bool %test(ulong %x) {
+  %tmp = setlt ulong %x, 4294967296
+  ret bool %tmp
+}
+
+into x.h == 0
+
+