[Wasm GC][NFC] Optimize getStrictSubTypes() (#5130)

Avoid allocating there. This is both faster and also it ensures we never modify
our internal data structure after our constructor.

1 files changed, 11 insertions, 1 deletions

diff --git a/src/ir/subtypes.h b/src/ir/subtypes.h
index 74c95032d..5a4ff87fc 100644
--- a/src/ir/subtypes.h
+++ b/src/ir/subtypes.h
@@ -37,7 +37,14 @@ struct SubTypes {
   }
 
   const std::vector<HeapType>& getStrictSubTypes(HeapType type) {
-    return typeSubTypes[type];
+    if (auto iter = typeSubTypes.find(type); iter != typeSubTypes.end()) {
+      return iter->second;
+    }
+
+    // No entry exists. Return a canonical constant empty vec, to avoid
+    // allocation.
+    static const std::vector<HeapType> empty;
+    return empty;
   }
 
   // Get all subtypes of a type, and their subtypes and so forth, recursively.
@@ -75,6 +82,9 @@ private:
   }
 
   // Maps a type to its subtypes.
+  //
+  // After our constructor we never modify this data structure, so we can take
+  // references to the vectors here safely.
   std::unordered_map<HeapType, std::vector<HeapType>> typeSubTypes;
 };