Hello,
> - m = 1UL << (BITS_PER_LONG - 2);
> + if (x <= 0xffff) {
> + if (m <= 0xff)
> + m = 1UL << (8 - 2);
> + else
> + m = 1UL << (16 - 2);
> + } else if (x <= 0xffffffff)
> + m = 1UL << (32 - 2);
> + else
> + m = 1UL << (BITS_PER_LONG - 2);
> while (m != 0) {
> b = y + m;
> y >>= 1;
>
I think, m can be initialized with
1 << (greatest multiple of 2 less than or equal to (position of most
significant bit of x))
i.e. 1 << ((position of most significant bit of x) & 62)
without changing the outcome of the original algorithm (as long as x<m the loop does
just m >>= 2).
I believe, that for (position of most significant bit of x) there is an
efficient macro, and
some processors directly have an instruction for it. So this would probably be
faster than your suggestion
for an initial starting value and give an even better starting value (cutting
in some cases further on the number of
while loop interations).
If one just wants to achieve a result with a certain relative error in terms of
the fraction of the input, one can
probably only look at the most significant bit and a few following bits of x.
Sincerely,
Thomas
