Comment by a_e_k

Yes, and back when hand-writing vectorized kernels via intrinsics, one learned to do the equivalent of (pseudocode here - picture SSE, AltiVec, NEON, etc.):

    vector conditionmask = <some computation...>; // E.g., 11111111 00000000 00000000 11111111
    vector truebranch = <some computation...>;
    vector falsebranch = <some computation...>;
    vector result = (truebranch & conditionmask) | (falsebranch & ~conditionmask);

where each lane of the conditionmask has either all bits set or all bits clear, depending on the outcome of the conditional test for that lane.

The processor obviously does execute both branches here, so there's going to be wasted work. But since it's just a linear sequence of operations it can often schedule them independently and run them out-of-order and in parallel. And of course, if there's any shared computation between the two branches, the compiler can do common subexpression elimination.

That said, that sort of approach where you go ahead and do both and then blend them was definitely the kind of optimization where you'd want to profile rather than doing it blindly. But it was a pretty common thing to do when hand-vectorizing code. (Thankfully, auto-vectorizers are pretty good at doing this sort of optimization for you these days. It's been a very long time now since I've had to hand-write vector intrinsics.)