5 files changed, 84 insertions, 10 deletions
diff --git a/src/ir/module-utils.cpp b/src/ir/module-utils.cpp
index f2c90dcfd..f3d9201b7 100644
--- a/src/ir/module-utils.cpp
+++ b/src/ir/module-utils.cpp
@@ -84,7 +84,9 @@ struct CodeScanner
       // not written in the binary format, so it doesn't need to be counted, but
       // it does need to be taken into account in the IR (this may be the only
       // place this type appears in the entire binary, and we must scan all
-      // types as the analyses that use us depend on that).
+      // types as the analyses that use us depend on that). TODO: This is kind
+      // of a hack, so it would be nice to remove. If we could remove it, we
+      // could also remove some of the pruning logic in getHeapTypeCounts below.
       counts.include(get->type);
     } else if (auto* set = curr->dynCast<StructSet>()) {
       counts.note(set->ref->type);
@@ -105,7 +107,10 @@ struct CodeScanner
   }
 };
 
-Counts getHeapTypeCounts(Module& wasm) {
+// Count the number of times each heap type that would appear in the binary is
+// referenced. If `prune`, exclude types that are never referenced, even though
+// a binary would be invalid without them.
+Counts getHeapTypeCounts(Module& wasm, bool prune = false) {
   // Collect module-level info.
   Counts counts;
   CodeScanner(wasm, counts).walkModuleCode(&wasm);
@@ -141,6 +146,19 @@ Counts getHeapTypeCounts(Module& wasm) {
     }
   }
 
+  if (prune) {
+    // Remove types that are not actually used.
+    auto it = counts.begin();
+    while (it != counts.end()) {
+      if (it->second == 0) {
+        auto deleted = it++;
+        counts.erase(deleted);
+      } else {
+        ++it;
+      }
+    }
+  }
+
   // Recursively traverse each reference type, which may have a child type that
   // is itself a reference type. This reflects an appearance in the binary
   // format that is in the type section itself. As we do this we may find more
@@ -178,12 +196,14 @@ Counts getHeapTypeCounts(Module& wasm) {
     }
 
     // Make sure we've noted the complete recursion group of each type as well.
-    auto recGroup = ht.getRecGroup();
-    if (includedGroups.insert(recGroup).second) {
-      for (auto type : recGroup) {
-        if (!counts.count(type)) {
-          newTypes.insert(type);
-          counts.include(type);
+    if (!prune) {
+      auto recGroup = ht.getRecGroup();
+      if (includedGroups.insert(recGroup).second) {
+        for (auto type : recGroup) {
+          if (!counts.count(type)) {
+            newTypes.insert(type);
+            counts.include(type);
+          }
         }
       }
     }
@@ -297,11 +317,11 @@ std::vector<HeapType> getPublicHeapTypes(Module& wasm) {
 }
 
 std::vector<HeapType> getPrivateHeapTypes(Module& wasm) {
-  auto allTypes = getHeapTypeCounts(wasm);
+  auto allTypes = getHeapTypeCounts(wasm, true);
   auto publicTypes = getPublicTypeSet(wasm);
   std::vector<HeapType> types;
   types.reserve(allTypes.size() - publicTypes.size());
-  for (auto [type, _] : allTypes) {
+  for (auto& [type, _] : allTypes) {
     if (!publicTypes.count(type)) {
       types.push_back(type);
     }
diff --git a/src/passes/CMakeLists.txt b/src/passes/CMakeLists.txt
index e97f5e388..f29e7db9b 100644
--- a/src/passes/CMakeLists.txt
+++ b/src/passes/CMakeLists.txt
@@ -94,6 +94,7 @@ set(passes_SOURCES
   RemoveUnusedBrs.cpp
   RemoveUnusedNames.cpp
   RemoveUnusedModuleElements.cpp
+  RemoveUnusedTypes.cpp
   ReorderFunctions.cpp
   ReorderGlobals.cpp
   ReorderLocals.cpp
diff --git a/src/passes/RemoveUnusedTypes.cpp b/src/passes/RemoveUnusedTypes.cpp
new file mode 100644
index 000000000..d346bc585
--- /dev/null
+++ b/src/passes/RemoveUnusedTypes.cpp
@@ -0,0 +1,48 @@
+/*
+ * Copyright 2022 WebAssembly Community Group participants
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+//
+// Remove unused types from private rec groups. This is done implicitly by other
+// type optimizations as well, but only if they find anything else they want to
+// optimize.
+//
+
+#include "ir/module-utils.h"
+#include "ir/type-updating.h"
+#include "pass.h"
+
+namespace wasm {
+
+namespace {
+
+struct RemoveUnusedTypes : Pass {
+  void run(Module* module) override {
+    if (!module->features.hasGC()) {
+      // This pass only does anything with GC types.
+      return;
+    }
+    // We're not changing the contents of any of the types, so we just round
+    // trip them throgh GlobalTypeRewriter, which will put all the private types
+    // in a single new rec group and leave out all the unused types.
+    GlobalTypeRewriter(*module).update();
+  }
+};
+
+} // anonymous namespace
+
+Pass* createRemoveUnusedTypesPass() { return new RemoveUnusedTypes(); }
+
+} // namespace wasm
diff --git a/src/passes/pass.cpp b/src/passes/pass.cpp
index 21501c62f..732c2e8e2 100644
--- a/src/passes/pass.cpp
+++ b/src/passes/pass.cpp
@@ -369,6 +369,9 @@ void PassRegistry::registerPasses() {
   registerPass("remove-unused-names",
                "removes names from locations that are never branched to",
                createRemoveUnusedNamesPass);
+  registerPass("remove-unused-types",
+               "remove unused private GC types",
+               createRemoveUnusedTypesPass);
   registerPass("reorder-functions",
                "sorts functions by access frequency",
                createReorderFunctionsPass);
@@ -620,6 +623,7 @@ void PassRunner::addDefaultGlobalOptimizationPrePasses() {
     }
     addIfNoDWARFIssues("remove-unused-module-elements");
     if (options.closedWorld) {
+      addIfNoDWARFIssues("remove-unused-types");
       addIfNoDWARFIssues("cfp");
       addIfNoDWARFIssues("gsi");
     }
diff --git a/src/passes/passes.h b/src/passes/passes.h
index 8133c867e..4d6da0ca9 100644
--- a/src/passes/passes.h
+++ b/src/passes/passes.h
@@ -116,6 +116,7 @@ Pass* createRemoveUnusedBrsPass();
 Pass* createRemoveUnusedModuleElementsPass();
 Pass* createRemoveUnusedNonFunctionModuleElementsPass();
 Pass* createRemoveUnusedNamesPass();
+Pass* createRemoveUnusedTypesPass();
 Pass* createReorderFunctionsPass();
 Pass* createReorderGlobalsPass();
 Pass* createReorderGlobalsAlwaysPass();