Comment by shakna
8 hours ago
I've mentioned elsewhere the standards, and compilers as well, disagreeing with you here.
But feel free to run against the various compilers through godbolt. [0] They won't optimise the branch away. Access to a volatile, must be preserved, in the order that they exist. No optimisation, UB or otherwise, is allowed to impede that. Because an access is a side-effect.
Compilers not doing something is not a demonstration that they are not actually allowed to do that thing.
That they won’t is as most a courtesy to you but they are not required to do this.
> Furthermore, at every sequence point the value last stored in the object shall agree with that prescribed by the abstract machine, except as modified by the unknown factors mentioned previously.
I quoted the C standard, first. Not compiler behaviour.
I showed where it requires the compiler not to optimise this.
How about, instead of one-line throwaway disagreements, you point out where they are permitted to do this, instead?
The compiler is required to not optimise out reads/writes through volatile. That's unrelated to code also having UB: you can't sprinkle volatile through arbitrary UB and suddenly have it be defined.
> A compliant compiler is only free to optimise away, where it can determine there are no side-effects
A compliant compiler is also allowed to assume UB cannot occur.