From what I can tell this achevies nothing. (not that it'll do any harm)

The constexpr specifier declares that it is possible to evaluate the value of the function or variable at compile time.

The only usage currently:

const xcb_render_directformat_t *direct = XRenderUtils::findPictFormatInfo(format);
const int red_bits   = bitCount(direct->red_mask);

There's no way that can be calculated at compile time.