From: Vincent Bernat <vincent@bernat.ch>
Date: Sun, 26 Jan 2020 14:06:54 +0000 (+0100)
Subject: netlink: use an optimized version of num_bits_set
X-Git-Tag: 1.0.5~17^2~1
X-Git-Url: http://git.ipfire.org/?a=commitdiff_plain;h=b2e80798d1fcdb75aefa60fe5b64dafd4a78e18b;p=thirdparty%2Flldpd.git

netlink: use an optimized version of num_bits_set

This is a classic problem. See
<https://stackoverflow.com/questions/109023/how-to-count-the-number-of-set-bits-in-a-32-bit-integer>
---

diff --git a/src/daemon/netlink.c b/src/daemon/netlink.c
index a9a7d54e..fcb30c8b 100644
--- a/src/daemon/netlink.c
+++ b/src/daemon/netlink.c
@@ -75,19 +75,16 @@ is_bitmap_empty(uint32_t *bmap)
  * Calculate the number of bits set in the bitmap to get total
  * number of VLANs
  */
-static int
+static unsigned int
 num_bits_set(uint32_t *bmap)
 {
-	int num = 0;
-	int i, bit;
-
-	for (i = 0; (i < VLAN_BITMAP_LEN); i++) {
-		if (!bmap[i])
-			continue;
-		for (bit = 0; bit < 32; bit++) {
-			if (bmap[i] & (1 << bit))
-				num++;
-		}
+	unsigned int num = 0;
+
+	for (int i = 0; (i < VLAN_BITMAP_LEN); i++) {
+		uint32_t v = bmap[i];
+		v = v - ((v >> 1) & 0x55555555);
+		v = (v & 0x33333333) + ((v >> 2) & 0x33333333);
+		num += (((v + (v >> 4)) & 0x0F0F0F0F) * 0x01010101) >> 24;
 	}
 
 	return num;