Avoid computing strlen() inside loops

Compiling with -O0 (no proper optimizations), strlen() call
in loops for comparing the size, isn't being called/initialized
before the actual loop gets started, which causes n-numbers of
strlen() calls (as long as the string is). Keeping the length
before entering in the loop is a good idea.

On some places, even with -O2, both GCC and Clang can't
recognize this pattern, which seem to happen in an array
of char pointer.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Signed-off-by: rilysh <nightquick@proton.me>
Closes #16584

cmd/zdb/zdb.c

@@ -8661,7 +8661,7 @@ zdb_read_block(char *thing, spa_t *spa)
 	void *lbuf, *buf;
 	char *s, *p, *dup, *flagstr, *sizes, *tmp = NULL;
 	const char *vdev, *errmsg = NULL;
-	int i, error;
+	int i, len, error;
 	boolean_t borrowed = B_FALSE, found = B_FALSE;
 
 	dup = strdup(thing);
@@ -8689,7 +8689,8 @@ zdb_read_block(char *thing, spa_t *spa)
 	for (s = strtok_r(flagstr, ":", &tmp);
 	    s != NULL;
 	    s = strtok_r(NULL, ":", &tmp)) {
-		for (i = 0; i < strlen(flagstr); i++) {
+		len = strlen(flagstr);
+		for (i = 0; i < len; i++) {
 			int bit = flagbits[(uchar_t)flagstr[i]];
 
 			if (bit == 0) {
@@ -8960,13 +8961,14 @@ zdb_embedded_block(char *thing)
 static boolean_t
 zdb_numeric(char *str)
 {
-	int i = 0;
+	int i = 0, len;
 
-	if (strlen(str) == 0)
+	len = strlen(str);
+	if (len == 0)
 		return (B_FALSE);
 	if (strncmp(str, "0x", 2) == 0 || strncmp(str, "0X", 2) == 0)
 		i = 2;
-	for (; i < strlen(str); i++) {
+	for (; i < len; i++) {
 		if (!isxdigit(str[i]))
 			return (B_FALSE);
 	}

cmd/ztest.c

@@ -663,15 +663,17 @@ static int
 str2shift(const char *buf)
 {
 	const char *ends = "BKMGTPEZ";
-	int i;
+	int i, len;
 
 	if (buf[0] == '\0')
 		return (0);
-	for (i = 0; i < strlen(ends); i++) {
+
+	len = strlen(ends);
+	for (i = 0; i < len; i++) {
 		if (toupper(buf[0]) == ends[i])
 			break;
 	}
-	if (i == strlen(ends)) {
+	if (i == len) {
 		(void) fprintf(stderr, "ztest: invalid bytes suffix: %s\n",
 		    buf);
 		usage(B_FALSE);