The point is there is no advantage to throwing an exception VS. using sum type return values (e.g. Result types in Rust)
Re:The Year of C++ Successor Languages
by kllrnohj on Wednesday January 04, 2023 @08:09AM
(#34245093)
That's a bad point? Exceptions are not normal control flow. They are rare or, as one might say, exceptional. The performance of them when thrown isn't of key concern, it's the performance when they are not thrown that matters since that's the >90% case. And in that case, code using exceptions is faster than code using sum type return values, especially if those errors propagate deeply across the call stack which they very often do.
Here's a simple example showing exceptions vs. sum types: https://godbolt.org/z/9Er6eKxs9
In the non-throwing exception path, there's literally no error or exception handling code executed at all. Whereas in the sum-type error-returning version, you have a branch at every call site that's always executed regardless of if there's an error or not.
Now the exception handler generates ".cold" clones of the function, so the total assembly for the exception handling one is larger. However, that assembly isn't every executed if an exception isn't thrown, which is the broader point. So it's not taking up CPU cache space & it's not taking up branch predictor slots.