util: Add ALIGN_POWER2
Add static inline function to align a value to it's next power of 2.
This is commonly done by a SWAR like the one in:
http://aggregate.org/MAGIC/#Next Largest Power of 2
However a microbench shows that the implementation herer is a faster.
It doesn't really impact the possible user of this function, but it's
interesting nonetheless.
Using a x86_64 i7 Ivy Bridge it shows a ~4% advantage by using clz
instead instead of the OR and SHL chain. And this is by using a BSR
since Ivy Bridge doesn't have LZCNT. New Haswell processors have the
LZCNT instruction which can make this even better. ARM also has a CLZ
instruction so it should be better, too.
Code used to test:
...
v = val[i];
t1 = get_cycles(0);
a = ALIGN_POWER2(v);
t1 = get_cycles(t1);
t2 = get_cycles(0);
v = nlpo2(v);
t2 = get_cycles(t2);
printf("%u\t%llu\t%llu\t%d\n", v, t1, t2, v == a);
...
In which val is an array of 20 random unsigned int, nlop2 is the SWAR
implementation and get_cycles uses RDTSC to measure the performance.
Averages:
ALIGN_POWER2: 30 cycles
nlop2: 31.4 cycles