[Custom Descriptors] AbstractTypeRefining: Fix up ref.get_desc (WebAssembly#7922)

When removing a descriptor type, we may need to fix up uses of it.

Author: kripken

src/passes/AbstractTypeRefining.cpp

@@ -394,6 +394,22 @@ struct AbstractTypeRefining : public Pass {
         }
       }
 
+      // If we optimize away a descriptor type, then we must fix up any
+      // ref.get_desc of it, as ReFinalize would fix us up to return it. This
+      // can only happen when no such descriptor is created, which means the
+      // instruction will never be reached (no struct with such a descriptor was
+      // created).
+      void visitRefGetDesc(RefGetDesc* curr) {
+        auto optimized = getOptimized(curr->type);
+        if (!optimized) {
+          return;
+        }
+
+        Builder builder(*getModule());
+        replaceCurrent(builder.makeSequence(builder.makeDrop(curr->ref),
+                                            builder.makeUnreachable()));
+      }
+
       void visitBrOn(BrOn* curr) {
         if (curr->op == BrOnNull || curr->op == BrOnNonNull) {
           return;

test/lit/passes/abstract-type-refining-desc.wast

@@ -1020,3 +1020,48 @@
     )
   )
 )
+
+(module
+  ;; $B is never created. When removing it from the result of the function, we
+  ;; must remove the ref.get_desc too (otherwise it would return a non-
+  ;; validating type for the new result; and it traps anyhow).
+  (rec
+    ;; YESTNH:      (rec
+    ;; YESTNH-NEXT:  (type $A (sub (descriptor $B (struct))))
+    ;; NO_TNH:      (rec
+    ;; NO_TNH-NEXT:  (type $A (sub (descriptor $B (struct))))
+    (type $A (sub (descriptor $B (struct))))
+    ;; YESTNH:       (type $B (describes $A (struct)))
+    ;; NO_TNH:       (type $B (describes $A (struct)))
+    (type $B (describes $A (struct)))
+  )
+
+  ;; YESTNH:       (type $2 (func (result (ref none))))
+
+  ;; YESTNH:      (func $test (type $2) (result (ref none))
+  ;; YESTNH-NEXT:  (drop
+  ;; YESTNH-NEXT:   (struct.new_default $A
+  ;; YESTNH-NEXT:    (ref.null none)
+  ;; YESTNH-NEXT:   )
+  ;; YESTNH-NEXT:  )
+  ;; YESTNH-NEXT:  (unreachable)
+  ;; YESTNH-NEXT: )
+  ;; NO_TNH:       (type $2 (func (result (ref none))))
+
+  ;; NO_TNH:      (func $test (type $2) (result (ref none))
+  ;; NO_TNH-NEXT:  (drop
+  ;; NO_TNH-NEXT:   (struct.new_default $A
+  ;; NO_TNH-NEXT:    (ref.null none)
+  ;; NO_TNH-NEXT:   )
+  ;; NO_TNH-NEXT:  )
+  ;; NO_TNH-NEXT:  (unreachable)
+  ;; NO_TNH-NEXT: )
+  (func $test (result (ref (exact $B)))
+    (ref.get_desc $A
+      (struct.new_default $A
+        (ref.null none)
+      )
+    )
+  )
+)
+