That, plus a modicum of reasoning like "if this were to be evaluated, it would be UB" (therefore, let's assume that is not evaluated).
Fact is that in this instance, the compiler did not remove a basic black of code (including or excluding "observeable side-effects" leading up to the point of UB happening). It would not be valid for the compiler to assume that the path is never taken in this case, even assuming that UB never happens, because depending on the the value of the variables, there are possible paths through the code that do not exhibit UB. In other words, "the compiler wasn't able to prove UB".
So this is not an instance of the situation that we are discussing. The emitted code is just fine, unless a division by zero occurs. Handling division by zero is responsibility of the programmer.
Nobody is arguing that UB can lead to weird runtime effects -- just dereference an invalid pointer or whatever.
The issue discussed is that based on assumptions about UB, the compiler emits code that does not correspond to the source in an intuitive way, for example a branch of code is entirely removed, including any observeable side-effects that logically happened before the UB.
Now the point of the GGP poster is probably that the observeable side-effect (the volatile access) does not happen at all because the UB happens first. But I would classify this case differently -- the volatile access is not elided from the branch.
Further more, it might well be that (and let me assume so) the order of the volatile access and the division operation that causes the UB are probably not defined as happening in a strict sequence (because, I'm assuming again as any reasonable standards layman would, UB is not considered a side-effect (that would kinda defeat the point, disallowing optimizations)). So it's entirely valid for the compiler to order the operation that causes the (potential) UB before the volatile access.
That's literally what happens in my example: the div is hoisted above the volatile read which is an observable side effect. The practical effect is that the expected side effect is not executed even if it should have happened-before the SIGFPE.
uecker claims that the UB should still respect happens-before, and I'm inclined to agree that's an useful property to preserve.
And I don't see any significant difference between my example and what you are arguing.
If you want an example of GCC removing a side effect that happens-before provable subsequent UB: https://godbolt.org/z/PfoT8E8PP but I don't find it terribly interesting as the compiler warns here.
extern volatile int x;
int ub() {
int r = x;
r += 1/0;
return r;
}
ub:
mov eax, DWORD PTR x[rip]
ud2
As for the earlier example (hoisting the division out of the loop), I was going to write a wall of text explaining why I find the behaviour totally intuitive and in line with what I'd expect.
But we can make it simpler: The code doesn't even have any observeable side effect (at least I think so), because it only reads the volatile, never writes it! The observeable behaviour is exactly the same as if the hoist hadn't happened. I believe it's a totally valid transformation, at least I don't have any concerns with it.
Here I've inserted an increment of the volatile (i.e. a write access) at the start of the loop. If the divisor is 0, in the optimized version with the division hoisted out of the loop, the increment will never actually happen, not even once. Whereas it should in fact happen 1x at the beginning of the first loop iteration with "unoptimized" code.
I don't find this offputting: First, the incrementing code is still in the output binary. I think what is understood by "time travel", and what would be offputting to most programmers, is if the compiler was making static inferences and was removing entire code branches based on that -- without telling the user. If that was the case, I would consider it a compiler usability bug. But that's not what's happening here.
Second, I think everybody can agree that the compiler should be able to reorder a division operation before a write access, especially when hoisting the division out of a loop. So while maybe an interesting study, I think the behaviour here is entirely reasonable -- irrespective of standards. (But again, I don't think uecker, nor anyone else, said that the compiler may never reorder divisions around side-effecting operations just because the division "could" be UB).
I think that discussing about omitting branches is a red herring, there is no expectation that the compiler should emit branches or basic blocks that match the source code even in the boring, non-ub case.
The only constraint to compiler optimizations is the as-if rule and the as-if rule only requires that side effects and their order be preserved for conforming programs. Uecker says that in addition to conforming programs, side effects and their ordering also need to be preserved up to the UB.
I do of course also find it unsurprising that the idiv is hoisted, but, as the only way that the standard can constraint compilers is through observable behaviour, I don't see how you can standardize rules where that form of hoisting is allowed while the other are not.
In fact the compiler could easily optimize that loop while preserving the ordering by transforming it to this:
extern volatile int x;
int ub(int d, int c) {
int r;
x += 3;
r += x;
int _div = d / c;
r += _div;
for (int 2 = 0; i < 100; ++i) {
x += 3;
r += x;
r += _div;
}
return r;
}
Now I'm not able to reproduce the issue with a guaranteed UB. I still think the loop variant shows the same problem though.
In any case, yes, according to the C standard a volatile read counts as an observable side effect.