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, 3 insertions, 0 deletions

diff --git a/test/lit/help/optimization-opts.test b/test/lit/help/optimization-opts.test
index 34559ab0a..380101a53 100644
--- a/test/lit/help/optimization-opts.test
+++ b/test/lit/help/optimization-opts.test
@@ -275,6 +275,9 @@
 ;; CHECK-NEXT:
 ;; CHECK-NEXT:   --generate-stack-ir                           generate Stack IR
 ;; CHECK-NEXT:
+;; CHECK-NEXT:   --global-subtyping                            apply more specific subtypes to
+;; CHECK-NEXT:                                                 type fields where possible
+;; CHECK-NEXT:
 ;; CHECK-NEXT:   --gto                                         globally optimize GC types
 ;; CHECK-NEXT:
 ;; CHECK-NEXT:   --heap2local                                  replace GC allocations with