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>

2 files changed, 14 insertions, 2 deletions

diff --git a/shell/hush.c b/shell/hush.c
index 6b910569f..179155f66 100644
--- a/shell/hush.c
+++ b/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);
diff --git a/shell/match.c b/shell/match.c
index ee8abb2db..90f77546d 100644
--- a/shell/match.c
+++ b/shell/match.c
@@ -64,11 +64,10 @@ char* FAST_FUNC scan_and_match(char *string, const char *pattern, unsigned flags
         }
 
         while (loc != end) {
-                char c;
                 int r;
 
-                c = *loc;
                 if (flags & SCAN_MATCH_LEFT_HALF) {
+                        char c = *loc;
                         *loc = '\0';
                         r = fnmatch(pattern, string, 0);
                         //bb_error_msg("fnmatch('%s','%s',0):%d", pattern, string, r);