ConstantFieldPropagation: Add a variation that picks between 2 values using RefTest (#6692)

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).

1 files changed, 1 insertions, 0 deletions

diff --git a/src/passes/passes.h b/src/passes/passes.h
index 02b164279..bd3aabf9f 100644
--- a/src/passes/passes.h
+++ b/src/passes/passes.h
@@ -32,6 +32,7 @@ Pass* createCodeFoldingPass();
 Pass* createCodePushingPass();
 Pass* createConstHoistingPass();
 Pass* createConstantFieldPropagationPass();
+Pass* createConstantFieldPropagationRefTestPass();
 Pass* createDAEPass();
 Pass* createDAEOptimizingPass();
 Pass* createDataFlowOptsPass();