[clang][AST] avoid unnecessary FunctionProtoTypes.FindNodeOrInsertPos call

[troyj]

Summary

getFunctionTypeInternal forms a FoldingSetNodeID and then calls FunctionProtoTypes.FindNodeOrInsertPos(). Later, upon returning from recursion, it calls FunctionProtoTypes.FindNodeOrInsertPos() again with the same ID. This second call will yield the same InsertPos unless the recursion caused FunctionProtoTypes to rebucket. The rebucketing can be detected by comparing the capacity of the set before and after the recursion, which allows us to skip the redundant call.

[bruno]

Nice catch, thanks for improving this.

Any chance you've got any small enough example that trigger this that could be transformed into a testcase? It would be nice to have a unittest for this, see others in clang/unittests/AST.

Nitpick: please capitalize avoid in the title.

clang/lib/AST/ASTContext.cpp

         llvm_unreachable("dependent noexcept is already canonical");
       }
     } else {
       CanonicalEPI.ExceptionSpec = FunctionProtoType::ExceptionSpecInfo();
     }
 
     // Adjust the canonical function result type.
     CanQualType CanResultTy = getCanonicalFunctionResultType(ResultTy);
+    const auto OrigCapacity = FunctionProtoTypes.capacity();
     Canonical =
         getFunctionTypeInternal(CanResultTy, CanonicalArgs, CanonicalEPI, true);
 
-    // Get the new insert position for the node we care about.
-    FunctionProtoType *NewIP =
-      FunctionProtoTypes.FindNodeOrInsertPos(ID, InsertPos);
-    assert(!NewIP && "Shouldn't be in the map!"); (void)NewIP;
+    // Get the new insert position for the node we care about, if the recursive
+    // call invalidated InsertPos.
+    if (FunctionProtoTypes.capacity() != OrigCapacity) {
+      FunctionProtoType *NewIP =
+        FunctionProtoTypes.FindNodeOrInsertPos(ID, InsertPos);
+      assert(!NewIP && "Shouldn't be in the map!"); (void)NewIP;
+    }
   }
 
   // Compute the needed size to hold this FunctionProtoType and the
   // various trailing objects.
   auto ESH = FunctionProtoType::getExceptionSpecSize(
       EPI.ExceptionSpec.Type, EPI.ExceptionSpec.Exceptions.size());
   size_t Size = FunctionProtoType::totalSizeToAlloc<
       QualType, SourceLocation, FunctionType::FunctionTypeExtraBitfields,