Comment by 1718627440
8 hours ago
> They could easily say something like "non-atomic misaligned accesses either succeed or trap" or something like that.
That means that the compiler must emit the read, even if the value is already known or never used, as it might trap. There is a reason for the UB!
No it doesn't. Compilers are only required to emit the read for volatile types. If the type is non-volatile, misaligned, and can be optimised out then it would be perfectly fine to omit it (that would be the "succeed" option).
If a trap is observable behaviour, then the compiler either needs to add code, that checks for the condition and then traps explicitly or it needs to actually perform the read. Currently it can be optimized out, because it is UB.
I think you misunderstood my suggestion. It isn't that misaligned accesses must either all succeed or all fail. That's not possible in general because of MMIO devices.
The suggestion is that each individual access must either succeed or trap. Those are the only possible outcomes, but different accesses can result in different outcomes.
You're merely attacking his particular suggestion and using this as an argument to defend UB, when those are completely independent concerns.
What people want is for a compiler that assumes that all pointers are aligned to use an aligned store or load instruction whenever the compiler wants to issue such an instruction. There is no need for UB here.
In other words, they want the compiler to stick with the decision it made and not randomly say "I can't do the thing I've been doing correctly for decades, because that's UB, my hands are tied, I must ruin the code, there's no other way."