[ktpsns]

Conditionals actually destroy information. The way they are written does not even matter; Consider the evaluation of this functional pseudocode:

   min(a,b) := if(a < b, a, b)

Obviously, the evaluation of this function looses information about either a or b. This is of course by intention. But this is incompatible to a closed (quantum) system where information must be preserved.

Interestingly (but I think there is no deeper connection), vectorization uses masks to avoid branching. Programmatically, one implements then both branches at the same time, and on a per-data level different operations are performed. A simple pseudo-numpy example would be

   B[where(A < B)] = A
   which should actually read component by component, for
   instance, having A and B being matrices,

      B_{ij} = A_{ij}  if A_{ij} < B_{ij}
               B_{ij}  else  ("no-op")

One can write long programs following this paradigm which frequently excel at performance at GPUs and modern day CPUs (which are actually vector machines).

[q_eng_anon]

thanks for tying it back into quantum - also for the clarification that the way you write them doesn't matter - whether that be in terms of if/else, switch statements or actual CPU-level branch instructions.