On 09/03/2015 08:02 AM, Eric Blake wrote:
Is it any more efficient to use multiplies instead of looping, as in:
uint64_t m;
b &= 7;
m = 0x0101010101010101ULL * ((1 << (8 - b)) - 1);
return (a & m) << b;
Or if multiplies are bad, what about straight-line expansion of the
mask, as in:
uint64_t m;
b &= 7;
m = (1 << (8 - b)) - 1;
m |= m << 32;
m |= m << 16;
m |= m << 8;
return (a & m) << b;
The multiply will (likely) be expanded by the compiler to the shift, but yes, I
should have played the mask game after Peter pointed out that this wasn't a
per-element variable shift in v14.
r~
