CFP focuses on finding when a field always contains a constant, and then replaces a struct.get with that constant. If we find there are two constant values, then in some cases we can still optimize, if we have a way to pick between them. All we have is the struct.get and its reference, so we must use a ref.test: (struct.get $T x (..ref..)) => (select (..constant1..) (..constant2..) (ref.test $U (..ref..)) ) This is valid if, of all the subtypes of $T, those that pass the test have constant1 in that field, and those that fail the test have constant2. For example, a simple case is where $T has two subtypes, $T is never created itself, and each of the two subtypes has a different constant value. This is a somewhat risky operation, as ref.test is not necessarily cheap. To mitigate that, this is a new pass, --cfp-reftest that is not run by default, and also we only optimize when we can use a ref.test on what we think will be a final type (because ref.test on a final type can be faster in VMs).