Bit mask in C

Question

What is the best way to construct a bit mask in C with m set bits preceded by k unset bits, and followed by n unset bits:

Answer 1

(Only) For those who are interested in a slightly more efficient solution on x86 systems with BMI2 support (Intel Haswell or newer, AMD Excavator or newer):

mask = _bzhi_u32(-1,m)<

The bzhi instruction zeros the high bits starting with specified bit position. The _bzhi_u32 intrinsic compiles to this instruction. Test code:

#include 
#include 
/*  gcc -O3 -Wall -m64 -march=haswell bitmsk_mn.c   */

unsigned int bitmsk(unsigned int m, unsigned int n)
{
    return _bzhi_u32(-1,m)<

Output:

$./a.out
k= 000FE000

The code fragment _bzhi_u32(-1,m)< compiles to three instructions

movl    $-1, %edx
bzhi    %edi, %edx, %edi
shlx    %esi, %edi, %eax

Which is one instruction less than the codes by @Jonathan Leffler and @Darius Bacon. On Intel Haswell processors or newer, both bzhi and shlx have a latency of 1 cycle and a throughput of 2 per cycle. On AMD Ryzen these two instructions even have a throughput of 4 per cycle.