value range propagation for _bitwise_ OR

[Steven Schveighoffer]

On Mon, 12 Apr 2010 13:56:23 -0400, Jérôme M. Berger <jeberger at free.fr>  
wrote:

> 	Here's a fast 100% precise solution:
>
> ==============================8<------------------------------
> uint32_t maxOr (uint32_t minA, uint32_t minB,
>                 uint32_t maxA, uint32_t maxB)
> {
>    assert (minA <= maxA);
>    assert (minB <= maxB);
>
>    if (maxA == 0) return maxB;
>    if (maxB == 0) return maxA;
>
>    uint32_t a = maxA ^ minA;
>    if (a != 0) {
>       a = ((1 << (highbit (a)+1)) - 1) & maxA & maxB;
>       if (a != 0)
>          a = (1 << highbit (a)) - 1;
>    }
>
>    uint32_t b = maxB ^ minB;
>    if (b != 0) {
>       b = ((1 << (highbit (b)+1)) - 1) & maxA & maxB;
>       if (b != 0)
>          b = (1 << highbit (b)) - 1;
>    }
>
>    return maxA|maxB|a|b;
> }
> ------------------------------>8==============================


Fails for test case:

minA = 4, maxA = 4, minB = 4, maxB = 6 (outputs 7, accurate result is 6).

I'm not quite sure what you are trying to do, but I think you are on the  
right path to getting a faster solution.

-Steve