Thank you. It appears Gcc won't put two cmov writes to memory in a block. Thus,

  void swap_if(bool c, int& a, int& b) {
    int ta = a, tb = b;
    a = c ? tb : ta;
    b = c ? ta : tb;
  }

is very slow, under Gcc, when c is poorly predicted, as is typical when e.g. partitioning for quicksort. But how well it will be predicted depends on input data.

[0] https://godbolt.org/z/j5W9dMjYE