net: mqtt: improve decoder buffer handling

Improve buffer handling logic to use local variables extensively.

This change reduces the number of pointer dereferences, which leads
to more efficient runtime and helps reduce the code size.

(cherry picked from commit 6211de8)

Original-Signed-off-by: Pisit Sawangvonganan <pisit@ndrsolution.com>
GitOrigin-RevId: 6211de8
Change-Id: If7073516c999c0e7a1af49eca5c7b863d6cb7f6f
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/zephyr/+/5753582
Tested-by: ChromeOS Prod (Robot) <chromeos-ci-prod@chromeos-bot.iam.gserviceaccount.com>
Commit-Queue: Fabio Baltieri <fabiobaltieri@google.com>
Reviewed-by: Fabio Baltieri <fabiobaltieri@google.com>
Tested-by: Fabio Baltieri <fabiobaltieri@google.com>

subsys/net/lib/mqtt/mqtt_decoder.c

@@ -29,13 +29,17 @@ LOG_MODULE_REGISTER(net_mqtt_dec, CONFIG_MQTT_LOG_LEVEL);
  */
 static int unpack_uint8(struct buf_ctx *buf, uint8_t *val)
 {
-	NET_DBG(">> cur:%p, end:%p", (void *)buf->cur, (void *)buf->end);
+	uint8_t *cur = buf->cur;
+	uint8_t *end = buf->end;
+
+	NET_DBG(">> cur:%p, end:%p", (void *)cur, (void *)end);
 
-	if ((buf->end - buf->cur) < sizeof(uint8_t)) {
+	if ((end - cur) < sizeof(uint8_t)) {
 		return -EINVAL;
 	}
 
-	*val = *(buf->cur++);
+	*val = cur[0];
+	buf->cur = (cur + sizeof(uint8_t));
 
 	NET_DBG("<< val:%02x", *val);
 
@@ -55,14 +59,17 @@ static int unpack_uint8(struct buf_ctx *buf, uint8_t *val)
  */
 static int unpack_uint16(struct buf_ctx *buf, uint16_t *val)
 {
-	NET_DBG(">> cur:%p, end:%p", (void *)buf->cur, (void *)buf->end);
+	uint8_t *cur = buf->cur;
+	uint8_t *end = buf->end;
+
+	NET_DBG(">> cur:%p, end:%p", (void *)cur, (void *)end);
 
-	if ((buf->end - buf->cur) < sizeof(uint16_t)) {
+	if ((end - cur) < sizeof(uint16_t)) {
 		return -EINVAL;
 	}
 
-	*val = *(buf->cur++) << 8; /* MSB */
-	*val |= *(buf->cur++); /* LSB */
+	*val = sys_get_be16(cur);
+	buf->cur = (cur + sizeof(uint16_t));
 
 	NET_DBG("<< val:%04x", *val);