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%.

Reflects ceph.git commit 110de33ca497d94fc4737e5154d3fe781fa84a0a.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>

1 files changed, 9 insertions, 3 deletions

diff --git a/net/ceph/crush/mapper.c b/net/ceph/crush/mapper.c
index 5fcfb98f309e..511ade95339a 100644
--- a/net/ceph/crush/mapper.c
+++ b/net/ceph/crush/mapper.c
@@ -253,9 +253,15 @@ 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;