From dff4ed85a2e6fac50d5e9d7caee4a7f1dffcd835 Mon Sep 17 00:00:00 2001
From: dgp <dgp@users.sourceforge.net>
Date: Fri, 28 Oct 2016 15:46:49 +0000
Subject: Add optimization to [dict append].

---
 generic/tclDictObj.c | 25 +++++++++++++++++--------
 1 file changed, 17 insertions(+), 8 deletions(-)

diff --git a/generic/tclDictObj.c b/generic/tclDictObj.c
index 428173d..f99f984 100644
--- a/generic/tclDictObj.c
+++ b/generic/tclDictObj.c
@@ -2305,17 +2305,26 @@ DictAppendCmd(
 	return TCL_ERROR;
     }
 
-    if (valuePtr == NULL) {
-	TclNewObj(valuePtr);
-    } else if (Tcl_IsShared(valuePtr)) {
-	valuePtr = Tcl_DuplicateObj(valuePtr);
-    }
+    if ((objc > 3) || (valuePtr == NULL)) {
+	/* Only go through append activites when something will change. */
+
+	if (valuePtr == NULL) {
+	    TclNewObj(valuePtr);
+	} else if (Tcl_IsShared(valuePtr)) {
+	    valuePtr = Tcl_DuplicateObj(valuePtr);
+	}
 
-    for (i=3 ; i<objc ; i++) {
-	Tcl_AppendObjToObj(valuePtr, objv[i]);
+	for (i=3 ; i<objc ; i++) {
+	    Tcl_AppendObjToObj(valuePtr, objv[i]);
+	}
+
+	Tcl_DictObjPut(NULL, dictPtr, objv[2], valuePtr);
     }
 
-    Tcl_DictObjPut(NULL, dictPtr, objv[2], valuePtr);
+    /*
+     * Even if nothing changed, we still overwrite so that variable
+     * trace expectations are met.
+     */
 
     resultPtr = Tcl_ObjSetVar2(interp, objv[1], NULL, dictPtr,
 	    TCL_LEAVE_ERR_MSG);
-- 
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