hush: optimize ${var/pattern/repl} for trivial patterns

function                                             old     new   delta
expand_one_var                                      2353    2507    +154

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

shell/hush.c

@@ -6466,6 +6466,19 @@ static arith_t expand_and_evaluate_arith(const char *arg, const char **errmsg_p)
 /* ${var/[/]pattern[/repl]} helpers */
 static char *strstr_pattern(char *val, const char *pattern, int *size)
 {
+	if (!strpbrk(pattern, "*?[\\")) {
+		/* Optimization for trivial patterns.
+		 * Testcase for very slow replace (performs about 22k replaces):
+		 * x=::::::::::::::::::::::
+		 * x=$x$x;x=$x$x;x=$x$x;x=$x$x;x=$x$x;x=$x$x;x=$x$x;x=$x$x;x=$x$x;x=$x$x;echo ${#x}
+		 * echo "${x//:/|}"
+		 */
+		char *found = strstr(val, pattern);
+		if (found)
+			*size = strlen(pattern);
+		return found;
+	}
+
 	while (1) {
 		char *end = scan_and_match(val, pattern, SCAN_MOVE_FROM_RIGHT + SCAN_MATCH_LEFT_HALF);
 		debug_printf_varexp("val:'%s' pattern:'%s' end:'%s'\n", val, pattern, end);