AcoSail/kernel_optimize_test
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crush: don't normalize input of crush_ln iteratively

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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>
This commit is contained in:
Ilya Dryomov 2016-09-27 12:30:09 +02:00
parent 464691bd52
commit 74a5293832
1 changed files with 9 additions and 3 deletions
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12
net/ceph/crush/mapper.c
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@ -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;