[Custom Descriptors] Optimize away unneeded descriptors in GlobalTypeOptimization (#7872)

This adds descriptor support to struct-utils.h as an extra field. It is
reported as index "-1" when an index is used in the API, allowing
passes to skip it where irrelevant.

That new support is then used in a single pass, GTO, where we just
see if a descriptor can be removed, and remove it if so.

Author: kripken

src/ir/struct-utils.h

@@ -44,6 +44,16 @@ template<typename T> struct StructValues : public std::vector<T> {
     assert(index < this->size());
     return std::vector<T>::operator[](index);
   }
+
+  // Store the descriptor as another field. (This could be a std::optional to
+  // indicate that the descriptor's existence depends on the type, but that
+  // would add overhead & code clutter (type checks). If there is no descriptor,
+  // this will just hang around with the default values, not harming anything
+  // except perhaps for looking a little odd during debugging. And whenever we
+  // combine() a non-existent descriptor, we are doing unneeded work, but the
+  // data here is typically just a few bools, so it is simpler and likely
+  // faster to just copy those rather than check if the type has a descriptor.)
+  T desc;
 };
 
 // Maps heap types to a StructValues for that heap type.
@@ -69,6 +79,7 @@ struct StructValuesMap : public std::unordered_map<HeapType, StructValues<T>> {
       for (Index i = 0; i < info.size(); i++) {
         combinedInfos[type][i].combine(info[i]);
       }
+      combinedInfos[type].desc.combine(info.desc);
     }
   }
 
@@ -80,6 +91,8 @@ struct StructValuesMap : public std::unordered_map<HeapType, StructValues<T>> {
         x.dump(o);
         o << " ";
       };
+      o << " desc: ";
+      vec.desc.dump(o);
       o << '\n';
     }
   }
@@ -129,14 +142,17 @@ struct FunctionStructValuesMap
 //
 //   void noteCopy(HeapType type, Index index, T& info);
 //
-// * Note a read
+// * Note a read.
 //
 //   void noteRead(HeapType type, Index index, T& info);
 //
 // We track information from struct.new and struct.set/struct.get separately,
 // because in struct.new we know more about the type - we know the actual exact
 // type being written to, and not just that it is of a subtype of the
 // instruction's type, which helps later.
+//
+// Descriptors are treated as fields in that we call the above functions on
+// them. We pass DescriptorIndex for their index as a fake value.
 template<typename T, typename SubType>
 struct StructScanner
   : public WalkerPass<PostWalker<StructScanner<T, SubType>>> {
@@ -146,6 +162,8 @@ struct StructScanner
 
   SubType& self() { return *static_cast<SubType*>(this); }
 
+  static const Index DescriptorIndex = -1;
+
   StructScanner(FunctionStructValuesMap<T>& functionNewInfos,
                 FunctionStructValuesMap<T>& functionSetGetInfos)
     : functionNewInfos(functionNewInfos),
@@ -168,6 +186,10 @@ struct StructScanner
         noteExpressionOrCopy(curr->operands[i], heapType, i, infos[i]);
       }
     }
+
+    if (curr->desc) {
+      self().noteExpression(curr->desc, heapType, DescriptorIndex, infos.desc);
+    }
   }
 
   void visitStructSet(StructSet* curr) {
@@ -236,6 +258,42 @@ struct StructScanner
     noteExpressionOrCopy(curr->replacement, heapType, index, info);
   }
 
+  void visitRefCast(RefCast* curr) {
+    if (curr->desc) {
+      // We may try to read a descriptor from anything arriving in |curr->ref|,
+      // but the only things that matter are the things we cast to: other types
+      // can lack a descriptor, and are skipped anyhow. So the only effective
+      // read is of the cast type.
+      handleDescRead(curr->getCastType());
+    }
+  }
+
+  void visitBrOn(BrOn* curr) {
+    if (curr->desc &&
+        (curr->op == BrOnCastDesc || curr->op == BrOnCastDescFail)) {
+      handleDescRead(curr->getCastType());
+    }
+  }
+
+  void visitRefGetDesc(RefGetDesc* curr) {
+    // Unlike a cast, anything in |curr->ref| may be read from.
+    handleDescRead(curr->ref->type);
+  }
+
+  void handleDescRead(Type type) {
+    if (type == Type::unreachable) {
+      return;
+    }
+    auto heapType = type.getHeapType();
+    if (heapType.isStruct()) {
+      // Any subtype of the reference here may be read from.
+      self().noteRead(heapType,
+                      DescriptorIndex,
+                      functionSetGetInfos[this->getFunction()][heapType].desc);
+      return;
+    }
+  }
+
   void
   noteExpressionOrCopy(Expression* expr, HeapType type, Index index, T& info) {
     // Look at the value falling through, if it has the exact same type
@@ -268,8 +326,8 @@ struct StructScanner
   FunctionStructValuesMap<T>& functionSetGetInfos;
 };
 
-// Helper class to propagate information about fields to sub- and/or super-
-// classes in the type hierarchy. While propagating it calls a method
+// Helper class to propagate information to sub- and/or super- classes in the
+// type hierarchy. While propagating it calls a method
 //
 //  to.combine(from)
 //
@@ -286,18 +344,32 @@ template<typename T> class TypeHierarchyPropagator {
 
   SubTypes subTypes;
 
+  // Propagate given a StructValuesMap, which means we need to take into
+  // account fields.
   void propagateToSuperTypes(StructValuesMap<T>& infos) {
     propagate(infos, false, true);
   }
-
   void propagateToSubTypes(StructValuesMap<T>& infos) {
     propagate(infos, true, false);
   }
-
   void propagateToSuperAndSubTypes(StructValuesMap<T>& infos) {
     propagate(infos, true, true);
   }
 
+  // Propagate on a simpler map of structs and infos (that is, not using
+  // separate values for the fields, as StructValuesMap does). This is useful
+  // when not tracking individual fields, but something more general about
+  // types.
+  using StructMap = std::unordered_map<HeapType, T>;
+
+  void propagateToSuperTypes(StructMap& infos) {
+    propagate(infos, false, true);
+  }
+  void propagateToSubTypes(StructMap& infos) { propagate(infos, true, false); }
+  void propagateToSuperAndSubTypes(StructMap& infos) {
+    propagate(infos, true, true);
+  }
+
 private:
   void propagate(StructValuesMap<T>& combinedInfos,
                  bool toSubTypes,
@@ -320,6 +392,11 @@ template<typename T> class TypeHierarchyPropagator {
               work.push(*superType);
             }
           }
+          // Propagate the descriptor to the super, if the super has one.
+          if (superType->getDescriptorType() &&
+              superInfos.desc.combine(infos.desc)) {
+            work.push(*superType);
+          }
         }
       }
 
@@ -333,6 +410,41 @@ template<typename T> class TypeHierarchyPropagator {
               work.push(subType);
             }
           }
+          // Propagate the descriptor.
+          if (subInfos.desc.combine(infos.desc)) {
+            work.push(subType);
+          }
+        }
+      }
+    }
+  }
+
+  void propagate(StructMap& combinedInfos, bool toSubTypes, bool toSuperTypes) {
+    UniqueDeferredQueue<HeapType> work;
+    for (auto& [type, _] : combinedInfos) {
+      work.push(type);
+    }
+    while (!work.empty()) {
+      auto type = work.pop();
+      auto& info = combinedInfos[type];
+
+      if (toSuperTypes) {
+        // Propagate to the supertype.
+        if (auto superType = type.getDeclaredSuperType()) {
+          auto& superInfo = combinedInfos[*superType];
+          if (superInfo.combine(info)) {
+            work.push(*superType);
+          }
+        }
+      }
+
+      if (toSubTypes) {
+        // Propagate shared fields to the subtypes.
+        for (auto subType : subTypes.getImmediateSubTypes(type)) {
+          auto& subInfo = combinedInfos[subType];
+          if (subInfo.combine(info)) {
+            work.push(subType);
+          }
         }
       }
     }

src/passes/ConstantFieldPropagation.cpp

@@ -443,6 +443,11 @@ struct PCVScanner
   }
 
   void noteCopy(HeapType type, Index index, PossibleConstantValues& info) {
+    if (index == DescriptorIndex) {
+      // We cannot continue on below, where we index into the vector of values.
+      return;
+    }
+
     // Note copies, as they must be considered later. See the comment on the
     // propagation of values below.
     functionCopyInfos[getFunction()][type][index] = true;