Kim Walisch
Kim Walisch,
a software engineer from Luxembourg, and avocational expert in prime number generation [1] [2], applying the Sieve of Eratosthenes [3]. He is knowledgeable about processor architectures, software optimization and bit-twiddling.
Bitscan
In October 2012, Kim Walisch proposed an improved bitscan routine, multiplying the De Bruijn Sequence with a 0-1 mask separated by the least significant one bit of a scanned integer or bitboard. The mask separation is cheaper than the bit isolation due to the x86 lea instruction, performing the decrement of a source register into another target register, not affecting processor flags. This is a 32-bit bitscan forward routine:
const int index32[32] = {
0, 9, 1, 10, 13, 21, 2, 29,
11, 14, 16, 18, 22, 25, 3, 30,
8, 12, 20, 28, 15, 17, 24, 7,
19, 27, 23, 6, 26, 5, 4, 31
};
/**
* bitScanForward
* @author Kim Walisch (2012)
* @param v 32-bit set
* @precondition v != 0
* @return index (0..31) of least significant one bit
*/
int bitScanForward(unsigned int v) {
return index32[((v ^ (v - 1)) * 0x07C4ACDDU) >> 27];
}
Population Count
Kim Walisch provides a header-only C/C++ library for counting the number of 1 bits using specialized instructions of various processor architectures [4], performing algorithms as described by Wojciech Muła et al. [5].
External Links
References
- ↑ Referent Kim Walisch www.primzahlen.de (German)
- ↑ Formula for primes from Wikipedia
- ↑ GitHub - kimwalisch/primesieve: Fast C/C++ prime number generator
- ↑ GitHub - kimwalisch/libpopcnt: Fast C/C++ bit population count library
- ↑ Wojciech Muła, Nathan Kurz, Daniel Lemire (2016). Faster Population Counts Using AVX2 Instructions. arXiv:1611.07612
