value range propagation for _bitwise_ OR

[Andrei Alexandrescu]

On 04/10/2010 06:58 PM, Adam D. Ruppe wrote:
> On Sat, Apr 10, 2010 at 07:38:07PM -0400, Adam D. Ruppe wrote:
>> I *might* have it, but I'm not 100% confident in my test program.
>
> I think my test program is telling me something: a non-zero min_c subtracts from
> max_c. If I take my brute-force empirically calculated max_c and compare it to
> what my max_c function returns, it doesn't always match if the minimums change.
>
> This might be a bug; it is unintuitive that it would do this, but it actually
> makes sense. Consider if min == max, you only have one value to work with.
>
> let max_a = 4, min_a = 0;
>
> max_b = 4
>
> You can get 7 out of it, since 3<  4, so it is a possible number and: 100 | 011 == 7
>
> But, if min_b == 4, the only possible value for b is 4. And: 100 | 100 == 4.
>
> Increasing min_b does indeed decrease the max you can get.
>
>
> Nice fact, but I've spent too much time on this already, so I'll call it done with
> this:
>
> My max_c included some unnecessary code: my reusable mask obsoletes all of
> my special case code! So, we can bring the program down to three lines:
>
> import std.intrinsic;
> uint numbits(uint a) { return a == 0 ? a : (bsr(a) + 1); }
> uint max_c(uint a, uint b) { return a | ((1<<  numbits(a&b)) - 1) | b; }
>
> It passes my test, though since I bugged it up the first time, I might have done
> it again.

What results does it yield with my main() test harness?

Andrei