I was writing code that looked like the following...
if(denominator == 0){
return false;
}
int result = value / denominator;
... when I thought about branching behavior in the CPU.
https://stackoverflow.com/a/11227902/620863 This answer says that the CPU will try to correctly guess which way a branch will go, and head down that branch only to stop if it discovers it guessed the branch incorrectly.
But if the CPU predicts the branch above incorrectly, it would divide by zero in the following instructions. This doesn't happen though, and I was wondering why? Does the CPU actually execute a division by zero and wait to see if the branch is correct before doing anything, or can it tell that it shouldn't continue in these situations? What's going on?
The CPU is free to do whatever it wants, when speculatively executing a branch based on a prediction. But it needs to do so in a way that's transparent to the user. So it may stage a "division by zero" fault, but this should be invisible if the branch prediction turns out wrong. By the same logic, it may stage writes to memory, but it may not actually commit them.
As a CPU designer, I wouldn't bother predicting past such a fault. That's probably not worth it. The fault probably means a bad prediction, and that will resolve itself soon enough.
This freedom is a good thing. Consider a simple std::accumulate loop. The branch predictor will correctly predict a lot of jumps (for (auto current = begin, current != end; ++current) which usually jumps back to the begin of loop), and there are a lot of memory reads which may potentially fault (sum += *current). But a CPU that would refuse to read a memory value until the previous branch has been resolved would be a lot slower. And yet a mispredicted jump at the end of the loop might very well cause a harmless memory fault, as the predicted branch tries to read past the buffer. This needs to be resolved without a visible fault.