value range propagation for _bitwise_ OR

[Andrei Alexandrescu]

On 04/10/2010 05:33 PM, Andrei Alexandrescu wrote:
[snip]

One more interesting detail. I simplified the routine to:

uint maxOR(uint maxa, uint maxb) {
     uint candidate = 0;
     uint mask = 1u << 31;
     for (; mask; mask >>= 1) {
         if (maxa & mask) continue;
         auto t = candidate | mask;
         if (t <= maxb) candidate = t;
     }
     return maxa | candidate;
}

Among other things I remove the test whether maxa >= maxb at the 
beginning of the function. I now obtain:

total=100000000; equal=14585400 (14.5854%)

so this function is better than all others so far. But I don't 
understand why it beats this one:

uint maxOR(uint maxa, uint maxb) {
     if (maxa < maxb) return maxOR(maxb, maxa); // added
     uint candidate = 0;
     uint mask = 1u << 31;
     for (; mask; mask >>= 1) {
         if (maxa & mask) continue;
         auto t = candidate | mask;
         if (t <= maxb) candidate = t;
     }
     return maxa | candidate;
}


Andrei