ClickHouse/dbms/include/DB/Common/BitHelpers.h

33 lines
632 B
C++
Raw Normal View History

2016-03-07 06:18:06 +00:00
#pragma once
#include <cstddef>
/** Returns log2 of number, rounded down.
* Compiles to single 'bsr' instruction on x86.
* For zero argument, result is unspecified.
*/
inline unsigned int bitScanReverse(unsigned int x)
2016-03-07 06:18:06 +00:00
{
return sizeof(unsigned int) * 8 - 1 - __builtin_clz(x);
}
/** For zero argument, result is zero.
* For arguments with most significand bit set, result is zero.
* For other arguments, returns value, rounded up to power of two.
*/
inline size_t roundUpToPowerOfTwoOrZero(size_t n)
{
--n;
n |= n >> 1;
n |= n >> 2;
n |= n >> 4;
n |= n >> 8;
n |= n >> 16;
n |= n >> 32;
++n;
return n;
}