On 2020-06-13, Theo Buehler <[email protected]> wrote:
> Others have pointed out off-list that one can use __builtin_popcount(),
> but __builtin_parity() is exactly what I want. Is it available on all
> architectures?
The standard implementation will be perfectly fine, no need to resort
to magic compiler built-ins.
int
popcount(uint16_t x)
{
x = (x & 0x5555) + ((x & 0xAAAA) >> 1);
x = (x & 0x3333) + ((x & 0xCCCC) >> 2);
x = (x & 0x0F0F) + ((x & 0xF0F0) >> 4);
x = (x & 0x00FF) + ((x & 0xFF00) >> 8);
return (x);
}
... which immediately lends itself to:
int
parity(uint16_t x)
{
x = (x & 0x5555) ^ ((x & 0xAAAA) >> 1);
x = (x & 0x3333) ^ ((x & 0xCCCC) >> 2);
x = (x & 0x0F0F) ^ ((x & 0xF0F0) >> 4);
x = (x & 0x00FF) ^ ((x & 0xFF00) >> 8);
return (x);
}
--
Christian "naddy" Weisgerber [email protected]
