Add bytearray checking to TclCheckEmptyString(), and then use

TclCheckEmptyString() in Tcl_AppendObjToObj and TclStringCat() to reduce string
generation.

doc/StringObj.3

@@ -115,7 +115,7 @@ The index of the last Unicode character in the Unicode range to be
 returned as a new value. If negative, take all characters up to
 the last one available.
 .AP Tcl_Obj *objPtr in/out
-Points to a value to manipulate.
+A pointer to a value to read, or to an unshared value to modify.
 .AP Tcl_Obj *appendObjPtr in
 The value to append to \fIobjPtr\fR in \fBTcl_AppendObjToObj\fR.
 .AP "Tcl_Size \&| int" *lengthPtr out

doc/encoding.n

@@ -1,5 +1,6 @@
 '\"
 '\" Copyright (c) 1998 Scriptics Corporation.
+'\" Copyright (c) 2023 Nathan Coulter
 '\"
 '\" See the file "license.terms" for information on usage and redistribution
 '\" of this file, and for a DISCLAIMER OF ALL WARRANTIES.

generic/tclStringObj.c

@@ -515,6 +515,11 @@ TclCheckEmptyString(
 	return TCL_EMPTYSTRING_YES;
     }
 
+    if (TclIsPureByteArray(objPtr)
+	&& Tcl_GetCharLength(objPtr) == 0) {
+	return TCL_EMPTYSTRING_YES;
+    }
+
     if (TclListObjIsCanonical(objPtr)) {
 	TclListObjLengthM(NULL, objPtr, &length);
 	return length == 0;
@@ -1431,26 +1436,26 @@ Tcl_AppendObjToObj(
     Tcl_Size appendNumChars = TCL_INDEX_NONE;
     const char *bytes;
 
-    /*
-     * Special case: second object is standard-empty is fast case. We know
-     * that appending nothing to anything leaves that starting anything...
-     */
+    if (TclCheckEmptyString(appendObjPtr) == TCL_EMPTYSTRING_YES) {
+	return;
+    }
 
-    if (appendObjPtr->bytes == &tclEmptyString) {
+    if (TclCheckEmptyString(objPtr) == TCL_EMPTYSTRING_YES) {
+	TclSetDuplicateObj(objPtr, appendObjPtr);
 	return;
     }
 
-    /*
-     * Handle append of one ByteArray object to another as a special case.
-     * Note that we only do this when the objects are pure so that the
-     * bytearray faithfully represent the true value; Otherwise appending the
-     * byte arrays together could lose information;
-     */
+    if (
+	TclIsPureByteArray(appendObjPtr)
+	&& (TclIsPureByteArray(objPtr) || objPtr->bytes == &tclEmptyString)
+    ) {
+	/*
+	 * Both bytearray objects are pure, so the second internal bytearray value
+	 * can be appended to the first, with no need to modify the "bytes" field.
+	 */
 
-    if ((TclIsPureByteArray(objPtr) || objPtr->bytes == &tclEmptyString)
-	    && TclIsPureByteArray(appendObjPtr)) {
 	/*
-	 * You might expect the code here to be
+	 * One might expect the code here to be
 	 *
 	 *  bytes = Tcl_GetByteArrayFromObj(appendObjPtr, &length);
 	 *  TclAppendBytesToByteArray(objPtr, bytes, length);
@@ -3126,7 +3131,7 @@ TclStringCat(
     int allowUniChar = 1, requestUniChar = 0, forceUniChar = 0;
     Tcl_Size first = objc - 1;	/* Index of first value possibly not empty */
     Tcl_Size last = 0;		/* Index of last value possibly not empty */
-    int inPlace = flags & TCL_STRING_IN_PLACE;
+    int inPlace = (flags & TCL_STRING_IN_PLACE) && !Tcl_IsShared(*objv);
 
     /* assert ( objc >= 0 ) */
 
@@ -3254,7 +3259,8 @@ TclStringCat(
 
 		Tcl_Obj *objPtr = *ov++;
 
-		if (objPtr->bytes == NULL) {
+		if (objPtr->bytes == NULL
+		    && TclCheckEmptyString(objPtr) != TCL_EMPTYSTRING_YES) {
 		    /* No string rep; Take the chance we can avoid making it */
 		    pendingPtr = objPtr;
 		} else {
@@ -3330,6 +3336,7 @@ TclStringCat(
     }
 
     objv += first; objc = (last - first + 1);
+    inPlace = (flags & TCL_STRING_IN_PLACE) && !Tcl_IsShared(*objv);
 
     if (binary) {
 	/* Efficiently produce a pure byte array result */
@@ -3340,7 +3347,7 @@ TclStringCat(
 	 * failure to allocate enough space. Following stanza may panic.
 	 */
 
-	if (inPlace && !Tcl_IsShared(*objv)) {
+	if (inPlace) {
 	    Tcl_Size start = 0;
 
 	    objResultPtr = *objv++; objc--;
@@ -3370,7 +3377,7 @@ TclStringCat(
 	/* Efficiently produce a pure Tcl_UniChar array result */
 	Tcl_UniChar *dst;
 
-	if (inPlace && !Tcl_IsShared(*objv)) {
+	if (inPlace) {
 	    Tcl_Size start;
 
 	    objResultPtr = *objv++; objc--;
@@ -3421,7 +3428,7 @@ TclStringCat(
 	/* Efficiently concatenate string reps */
 	char *dst;
 
-	if (inPlace && !Tcl_IsShared(*objv)) {
+	if (inPlace) {
 	    Tcl_Size start;
 
 	    objResultPtr = *objv++; objc--;

tests/string.test

@@ -2433,11 +2433,11 @@ test string-29.11.$noComp {string cat, efficiency} -body {
 test string-29.12.$noComp {string cat, efficiency} -body {
     tcl::unsupported::representation \
 	[run {string cat [encoding convertto utf-8 {}] [list x]}]
-} -match glob -result {*, string representation "x"}
+} -match glob -result {*, no string representation}
 test string-29.13.$noComp {string cat, efficiency} -body {
     tcl::unsupported::representation [run {string cat \
 	[encoding convertto utf-8 {}] [encoding convertto utf-8 {}] [list x]}]
-} -match glob -result {*, string representation "x"}
+} -match glob -result {*, no string representation}
 test string-29.14.$noComp {string cat, efficiency} -setup {
     set e [encoding convertto utf-8 {}]
 } -cleanup {