From 42dd0e1f84c1aa4c0b4331bf0a99dea8c0cbe5b7 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Cezary=20Skrzy=C5=84ski?=
 <cezary.skrzynski@ng-analytics.com>
Date: Thu, 25 Feb 2021 13:38:40 +0100
Subject: [PATCH] #1258: improve footprinting of CallbackUnion

---
 src/vt/pipe/callback/cb_union/cb_raw.h | 31 ++++++++++++++------------
 1 file changed, 17 insertions(+), 14 deletions(-)

diff --git a/src/vt/pipe/callback/cb_union/cb_raw.h b/src/vt/pipe/callback/cb_union/cb_raw.h
index 912f505211..810d157388 100644
--- a/src/vt/pipe/callback/cb_union/cb_raw.h
+++ b/src/vt/pipe/callback/cb_union/cb_raw.h
@@ -219,40 +219,43 @@ struct GeneralCallback {
 
   template <typename SerializerT>
   void serialize(SerializerT& s) {
-    using EnumDataType = typename std::underlying_type<CallbackEnum>::type;
-    EnumDataType val = static_cast<EnumDataType>(active_);
-    s | val;
-    s.skip(active_);
+    s | active_;
 
-    active_ = static_cast<CallbackEnum>(val);
-    s.skip(u_); // only serialize actual content of the union
+    // serialize actual content of the union and account for leftovers and
+    // padding bytes when footprinting
+    auto ser = [&](auto& cb){
+      s | cb;
+      s.addBytes(sizeof(this->u_) - sizeof(cb));
+    };
+    s.skip(u_);
     switch (active_) {
     case CallbackEnum::AnonCB:
-      s | u_.anon_cb_;
+      ser(u_.anon_cb_);
       break;
     case CallbackEnum::SendMsgCB:
-      s | u_.send_msg_cb_;
+      ser(u_.send_msg_cb_);
       break;
     case CallbackEnum::SendColMsgCB:
-      s | u_.send_col_msg_cb_;
+      ser(u_.send_col_msg_cb_);
       break;
     case CallbackEnum::BcastMsgCB:
-      s | u_.bcast_msg_cb_;
+      ser(u_.bcast_msg_cb_);
       break;
     case CallbackEnum::BcastColMsgCB:
-      s | u_.bcast_col_msg_cb_;
+      ser(u_.bcast_col_msg_cb_);
       break;
     case CallbackEnum::BcastColDirCB:
-      s | u_.bcast_col_dir_cb_;
+      ser(u_.bcast_col_dir_cb_);
       break;
     case CallbackEnum::BcastObjGrpCB:
-      s | u_.bcast_obj_cb_;
+      ser(u_.bcast_obj_cb_);
       break;
     case CallbackEnum::SendObjGrpCB:
-      s | u_.send_obj_cb_;
+      ser(u_.send_obj_cb_);
       break;
     case CallbackEnum::NoCB:
       // Serializing empty callback!
+      s.addBytes(sizeof(u_) - sizeof(active_));
       break;
     default:
       vtAssert(0, "Should be unreachable");