[jcalvinowens]

If they somehow magically didn't, how much could be gained?

I've never seen an attempt to answer that question. Maybe it's unanswerable in practice. But the examples of aliasing optimizations always seem to be eliminating a load, which in my experience is not an especially impactful thing in the average userspace widget written in C++.

The closest example of a more sophisticated aliasing optimization I've seen is example 18 in this paper: https://dl.acm.org/doi/pdf/10.1145/3735592

...but that specific example with a pointer passed to a function seems analogous to what is possible with 'restrict' in C. Maybe I misunderstood it.

This is an interesting viewpoint, but is unfortunately light on details: https://lobste.rs/s/yubalv/pointers_are_complicated_ii_we_ne...

Don't get me wrong, I'm not saying aliasing is a big conspiracy :) But it seems to have one of the higher hype-to-reality disconnects for compiler optimizations, in my limited experience.

[Measter]

Back in 2015 when the Rust project first had to disable use of LLVM's `noalias` they found that performance dropped by up to 5% (depending on the program). The big caveat here is that it was miscompiling, so some of that apparent performance could have been incorrect.

Of course, that was also 10 years ago, so things may be different now. There'll have been interest from the Rust project for improving the optimisations `noalias` performs, as well as improvements from Clang to improve optimisations under C and C++'s aliasing model.

[jcalvinowens]

Thanks! I've heard a lot of anecdotes like this, but I've never found anyone presenting anything I can repeoduce myself.