On 09/10/20 18:09 +0100, Jonathan Wakely wrote:
This removes the __detail::_Shift class template, replacing it with a
constexpr function template __pow2m1. Instead of using the _Mod class
template to calculate a modulus just perform a bitwise AND with the
result of __pow2m1. This works because the places that change all
perform a modulus operation with a power of two, x mod 2^w, which can be
replaced with x & (2^w - 1).
Actually that's not quite true, there are two kinds of usage of
_Shift. The first is used with _Mod to do x mod 2^w as mentioned
above.
The second is to calculate the factor to multiply a 32-bit value by
when shifting it into the higher bits of a 64-bit or 128-bit value:
- _UIntType __factor = 1u;
_UIntType __sum = 0u;
for (size_t __j = 0; __j < __k; ++__j)
{
- __sum += __arr[__j + 3] * __factor;
- __factor *= __detail::_Shift<_UIntType, 32>::__value;
+ __sum += __arr[__j + 3] << (32 * __j);
}
But we don't need the _Shift template here either. Instead of
multiplying by (j+1) * 2^32 we can just shift by j * 32 instead.
I'll update the commit msg to mention that usage too.
