The xxh* checksums don't need to be reversed on output.

checksum.c

@@ -162,9 +162,29 @@ int csum_len_for_type(int cst, BOOL flist_csum)
 	return 0;
 }
 
+/* Returns 0 if the checksum is not canonical (i.e. it includes a seed value).
+ * Returns 1 if the public sum order matches our internal sum order.
+ * Returns -1 if the public sum order is the reverse of our internal sum order.
+ */
 int canonical_checksum(int csum_type)
 {
-	return csum_type >= CSUM_MD4 ? 1 : 0;
+	switch (csum_type) {
+	  case CSUM_NONE:
+	  case CSUM_MD4_ARCHAIC:
+	  case CSUM_MD4_OLD:
+	  case CSUM_MD4_BUSTED:
+		break;
+	  case CSUM_MD4:
+	  case CSUM_MD5:
+		return -1;
+#ifdef SUPPORT_XXHASH
+	  case CSUM_XXH64:
+		return 1;
+#endif
+	  default: /* paranoia to prevent missing case values */
+		exit_cleanup(RERR_UNSUPPORTED);
+	}
+	return 0;
 }
 
 #ifndef HAVE_SIMD /* See simd-checksum-*.cpp. */

log.c

@@ -679,9 +679,9 @@ static void log_formatted(enum logcode code, const char *format, const char *op,
 		case 'C':
 			n = NULL;
 			if (S_ISREG(file->mode)) {
-				if (always_checksum && canonical_checksum(checksum_type))
+				if (always_checksum)
 					n = sum_as_hex(checksum_type, F_SUM(file), 1);
-				else if (iflags & ITEM_TRANSFER && canonical_checksum(xfersum_type))
+				else if (iflags & ITEM_TRANSFER)
 					n = sum_as_hex(xfersum_type, sender_file_sum, 0);
 			}
 			if (!n) {

util2.c

@@ -89,21 +89,26 @@ const char *sum_as_hex(int csum_type, const char *sum, int flist_csum)
 {
 	static char buf[MAX_DIGEST_LEN*2+1];
 	int i, x1, x2;
+	int canonical = canonical_checksum(csum_type);
 	int sum_len = csum_len_for_type(csum_type, flist_csum);
-	char *c = buf + sum_len*2;
+	char *c;
 
-	assert(c - buf < (int)sizeof buf);
+	if (!canonical)
+		return NULL;
 
-	*c = '\0';
+	assert(sum_len*2 < (int)sizeof buf);
 
-	for (i = sum_len; --i >= 0; ) {
-		x1 = CVAL(sum, i);
-		x2 = x1 >> 4;
-		x1 &= 0xF;
-		*--c = x1 <= 9 ? x1 + '0' : x1 + 'a' - 10;
-		*--c = x2 <= 9 ? x2 + '0' : x2 + 'a' - 10;
+	for (i = sum_len, c = buf; --i >= 0; ) {
+		int ndx = canonical < 0 ? sum_len - i - 1 : i;
+		x2 = CVAL(sum, ndx);
+		x1 = x2 >> 4;
+		x2 &= 0xF;
+		*c++ = x1 <= 9 ? x1 + '0' : x1 + 'a' - 10;
+		*c++ = x2 <= 9 ? x2 + '0' : x2 + 'a' - 10;
 	}
 
+	*c = '\0';
+
 	return buf;
 }