The code is not optimal, because there are implicit conversions to int (32-bit values) which are quite expansive. There should be explicit casts to uint16_t, like this: dest.r = uint16_t(uint16_t(dest.r * invAlpha) + uint16_t(c.r * c.a)) >> 8;
dest.r = uint16_t(dest.r * invAlpha + c.r * c.a) >> 8;
Amazingly, switching from gcc 8.2 to gcc trunk, reduces the instructions in the loop to 18. See https://godbolt.org/z/sEvz2w