Bluetooth: controller: Optimize the loop in nRF5 hal rand get()

Optimised the get() function in nRF5 hal rand implementation
to reduce number of probable branching operations.

This is needed to reduce nRF51 platform's CPU use in radio
ISR when using the fast encryption setup implementation.

Signed-off-by: Vinayak Kariappa Chettimada <[email protected]>

Author: cvinayak

subsys/bluetooth/controller/hal/nrf5/rand.c

@@ -57,32 +57,81 @@ void rand_isr_init(u8_t *context, u8_t context_len, u8_t threshold)
 
 static size_t get(struct rand *rng, size_t octets, u8_t *rand)
 {
-	u8_t first;
-	u8_t avail;
+	u8_t first, last, remaining;
 
 	LL_ASSERT(rng);
 
-	while (octets) {
-		if (rng->first == rng->last) {
-			break;
+	first = rng->first;
+	last = rng->last;
+
+	if (first <= last) {
+		u8_t *d, *s;
+		u8_t avail;
+
+		d = &rand[octets];
+		s = &rng->rand[first];
+
+		avail = last - first;
+		if (octets < avail) {
+			remaining = avail - octets;
+			avail = octets;
+		} else {
+			remaining = 0;
 		}
 
-		rand[--octets] = rng->rand[rng->first];
+		first += avail;
+		octets -= avail;
 
-		first = rng->first + 1;
-		if (first == rng->count) {
-			first = 0;
+		while (avail--) {
+			*(--d) = *s++;
 		}
-		rng->first = first;
-	}
 
-	if (rng->first <= rng->last) {
-		avail = rng->last - rng->first;
+		rng->first = first;
 	} else {
-		avail = rng->count - rng->first + rng->last;
+		u8_t *d, *s;
+		u8_t avail;
+
+		d = &rand[octets];
+		s = &rng->rand[first];
+
+		avail = rng->count - first;
+		if (octets < avail) {
+			remaining = avail + last - octets;
+			avail = octets;
+			first += avail;
+		} else {
+			remaining = last;
+			first = 0;
+		}
+
+		octets -= avail;
+
+		while (avail--) {
+			*(--d) = *s++;
+		}
+
+		if (octets && last) {
+			s = &rng->rand[0];
+
+			if (octets < last) {
+				remaining = last - octets;
+				last = octets;
+			} else {
+				remaining = 0;
+			}
+
+			first = last;
+			octets -= last;
+
+			while (last--) {
+				*(--d) = *s++;
+			}
+		}
+
+		rng->first = first;
 	}
 
-	if (avail < rng->threshold) {
+	if (remaining < rng->threshold) {
 		NRF_RNG->TASKS_START = 1;
 	}