Re: value range propagation for _bitwise_ OR

[Ellery Newcomer]

On 12/04/2010 01:21 PM, Ellery Newcomer wrote:

is it just me, or does this fail for

minA = 0
minB = 0
maxA = 0x80_00_00_00
maxB = 0x80_00_00_00

I'm pretty sure you need to handle 1<<  32 specially

i'm thinking this is better:

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;
        a = a & maxA & maxB;
        if (a != 0)
            a = (1 << highbit (a)) - 1;
    }

    uint32_t b = maxB ^ minB;
    if (b != 0) {
        b |= (1 << highbit (b)) - 1;
        b = b & maxA & maxB;
        if (b != 0)
            b = (1 << highbit (b)) - 1;
    }

    return maxA|maxB|a|b;
}