What a fun problem to think about.

My first instinct, knowing less about this domain than maybe I should, would be to abuse the return address predictor. I believe CPUs will generally predict the target of a “ret” instruction using an internal stack of return addresses; some ARM flavours even make this explicit (https://developer.arm.com/documentation/den0042/0100/Unified...).

The way to abuse this would be to put send() on the normal return path and call abandon() by rewriting the return address. In code:

  void resolve(Transaction *t) {
    predict(t);
    send(t);
  }

  void predict(Transaction *t) {
    if (!should_send(t)) *(void **)__builtin_return_address(0) = &abandon;
  }

The if in predict() is implementable as a branchless conditional move. The return address predictor should guess that predict() will return to send(), but in most cases you’ll smash the return address to point at abandon() instead.