[hinkley]

    x |= x >> 1;
    x |= x >> 2;
    x |= x >> 4;
    x |= x >> 8;
    x |= x >> 16;
    x |= x >> 32;

Isn't there another way to do this without so many data races?

I feel like this should be

   x |= x >> 1 | x >> ??? ...

[gpderetta]

By data races I assume you actually mean data dependencies?

[hinkley]

Oops. Yes.