From: Piotr Dałek <git@predictor.org.pl>
Date: Wed, 31 Aug 2016 15:24:31 +0000 (+0200)
Subject: crush: don't normalize input of crush_ln iteratively
X-Git-Tag: v11.0.1~270^2~1
X-Git-Url: http://git-server-git.apps.pok.os.sepia.ceph.com/?a=commitdiff_plain;h=110de33ca497d94fc4737e5154d3fe781fa84a0a;p=ceph.git

crush: don't normalize input of crush_ln iteratively

Use __builtin_clz() supported by GCC and Clang to figure out
how many bits we should shift instead of shifting by a bit
in a loop until the value gets normalized. Improves performance
of this function by up to 3x in worst-case scenario and overall
straw2 performance by ~10%.

Signed-off-by: Piotr Dałek <git@predictor.org.pl>
---

diff --git a/src/crush/mapper.c b/src/crush/mapper.c
index 832900f6821b..6f6b09e5807e 100644
--- a/src/crush/mapper.c
+++ b/src/crush/mapper.c
@@ -253,9 +253,13 @@ static __u64 crush_ln(unsigned int xin)
 
 	/* normalize input */
 	iexpon = 15;
-	while (!(x & 0x18000)) {
-		x <<= 1;
-		iexpon--;
+
+	// figure out number of bits we need to shift and
+	// do it in one step instead of iteratively	
+	if (!(x & 0x18000)) {
+	  int bits = __builtin_clz(x & 0x1FFFF) - 16;
+	  x <<= bits;
+	  iexpon = 15 - bits;
 	}
 
 	index1 = (x >> 8) << 1;