[nikic]

The main performance-critical undefined behavior in C is provenance. The rest can be removed without major performance impact. (Which is not to say they can't give you 10% on specific workloads, just they aren't what is taking you from -O0 to -O3.)

A related student poster from EuroLLVM 2023: https://llvm.org/devmtg/2023-05/slides/Posters/05-Popescu-Pe... It tests the performance impact of some of the secondary undefined behaviors, and the result is basically what you'd expect. They do have impact, but if you average over all benchmarks the improvement is, at best, in the low single digits.

[jules]

Yes, I do agree that you basically only need provenance, and that C has more UB than necessary. You can indeed reduce UB without any major performance impact (e.g., shifts by a large n, signed overflow). I think that would be a good idea.