[Wasm GC] GlobalTypeOptimization: Remove fields from end based on subtypes (#4553)

Previously we'd remove a field from a type if that field has no uses in any
sub- or super-type. In that case we'd remove it from all the types at once.
However, there is a case where we can remove a field only from a parent
but not from its children, if the field is at the end: if A has fields {x, y, z}
and its subtype B has fields {x, y, z, w}, and A pointers only access
field y while B pointers access all the fields, then we can remove z
from A. Removing it from the end is safe, and then B will not only add
w as it did before but also add z. Note that we cannot remove x,
because it is not at the end: removing it from just A but not B would
shift the indexes, making them incompatible.

1 files changed, 18 insertions, 10 deletions

diff --git a/src/ir/struct-utils.h b/src/ir/struct-utils.h
index f68692363..bc2eb5485 100644
--- a/src/ir/struct-utils.h
+++ b/src/ir/struct-utils.h
@@ -221,15 +221,21 @@ public:
   SubTypes subTypes;
 
   void propagateToSuperTypes(StructValuesMap<T>& infos) {
-    propagate(infos, false);
+    propagate(infos, false, true);
+  }
+
+  void propagateToSubTypes(StructValuesMap<T>& infos) {
+    propagate(infos, true, false);
   }
 
   void propagateToSuperAndSubTypes(StructValuesMap<T>& infos) {
-    propagate(infos, true);
+    propagate(infos, true, true);
   }
 
 private:
-  void propagate(StructValuesMap<T>& combinedInfos, bool toSubTypes) {
+  void propagate(StructValuesMap<T>& combinedInfos,
+                 bool toSubTypes,
+                 bool toSuperTypes) {
     UniqueDeferredQueue<HeapType> work;
     for (auto& [type, _] : combinedInfos) {
       work.push(type);
@@ -238,13 +244,15 @@ private:
       auto type = work.pop();
       auto& infos = combinedInfos[type];
 
-      // Propagate shared fields to the supertype.
-      if (auto superType = type.getSuperType()) {
-        auto& superInfos = combinedInfos[*superType];
-        auto& superFields = superType->getStruct().fields;
-        for (Index i = 0; i < superFields.size(); i++) {
-          if (superInfos[i].combine(infos[i])) {
-            work.push(*superType);
+      if (toSuperTypes) {
+        // Propagate shared fields to the supertype.
+        if (auto superType = type.getSuperType()) {
+          auto& superInfos = combinedInfos[*superType];
+          auto& superFields = superType->getStruct().fields;
+          for (Index i = 0; i < superFields.size(); i++) {
+            if (superInfos[i].combine(infos[i])) {
+              work.push(*superType);
+            }
           }
         }
       }