Add GlobalSubtyping pass (#4306)

This specializes the fields of structs based on the types written to them. That is,
if a field is of type A but in practice we always write some subtype B to it
then we can change the type of the field to that.

On j2wasm this manages to improve at least one field in 2% of types. Not a
large amount, but this does lead to further benefits in later opts (e.g. about a third
of the improvements are to turn a field non-nullable).

1 files changed, 8 insertions, 0 deletions

diff --git a/src/ir/lubs.h b/src/ir/lubs.h
index b75932091..a45b62c52 100644
--- a/src/ir/lubs.h
+++ b/src/ir/lubs.h
@@ -39,6 +39,14 @@ struct LUBFinder {
 
   // Returns the lub that we found.
   Type get() { return lub; }
+
+  // Combines the information in another LUBFinder into this one, and returns
+  // whether we changed anything.
+  bool combine(const LUBFinder& other) {
+    auto old = lub;
+    note(other.lub);
+    return lub != old;
+  }
 };
 
 } // namespace wasm