[quietbritishjim]

> The above example will print the value of a, but it won’t be initialized to 123!

It certainly could do though. In C, using an uninitialised variable does not mean "whatever that memory happened to have in it before" (although that is a potential result). Instead, it's undefined behaviour, so the compiler can do what it likes.

For example, it could well unconditionally initialise that memory to 123. Alternatively, it could notice that the whole snippet has undefined behaviour so simply replace it with no instructions, so it doesn't print anything at all. It could even optimise away the return that presumably follows that code in a function, so it ends up crashing or doing something random. It could even optimise away the instructions before that snippet, if it can prove that they would only be executed if followed by undefined behaviour – essentially the undefined behaviour can travel back in time!

[uecker]

UB can not travel back in time in C. Although it is true that it can affect previous instructions, but that code is reordered or transformed in complicated ways is true even without UB.

[emmericp]

The time-travelling UB interpretation was popularized by this blog post about 10 years ago [1].

I'm not enough of a specification lawyer to say that this is definitely true, but the reasoning and example given there seems sound to me.

[1] https://devblogs.microsoft.com/oldnewthing/20140627-00/?p=63...

[uecker]

Yes, random blog posts did a lot of damage here. Also broken compilers [1]. Note that blog post is correct about C++ but incorrectly assumes this is true for C as well.

[1]. https://developercommunity.visualstudio.com/t/Invalid-optimi...

[kibwen]

I'm inclined to trust Raymond Chen and John Regehr on these matters, so if you assert that they're incorrect here then a source to back up your assertion would help your argument.

[uecker]

I am a member of WG14. You should check the C standard. I do not see how "time-travel" is a possible reading of the definition of UB in C. We added another footnote to C23 to counter this idea:

https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3220.pdf "Any other behavior during execution of a program is only affected as a direct consequence of the concrete behavior that occurs when encountering the erroneous or non portable program construct or data. In particular, all observable behavior (5.1.2.4) appears as specified in this document when it happens before an operation with undefined behavior in the execution of the program."

I should point out that compilers also generally do not do true time-travel: Consider this example: https://godbolt.org/z/rPG14rrbj

[grumbelbart]

So maybe we have different definitions of "time travel". But I recall that

- if a compiler finds that condition A would lead to UB, it can assume that A is never true - that fact can "backpropagate" to, for example, eliminate comparisons long before the UB.

Here is an older discussion: https://softwareengineering.stackexchange.com/q/291548

Is that / will that no longer be true for C23? Or does "time-travel" mean something else in this context?

[lmm]

While interactions with volatile and interactive streams cannot time-travel, anything else is free to - the standard only imposes requirements on a conforming implementation in terms of the contents of files at program termination, and programs with undefined behaviour are not required to terminate, so there are approximately no requirements on a program that invokes undefined behaviour.

[quietbritishjim]

> Also broken compilers [1].

The issue you linked to is not a counter example because, as the poster said, g may terminate the program in which case that snippet does not have undefined behaviour even if b is zero. The fact that they bothered to mention that g may terminate the program seems like an acknowledgement that it would be valid to do that time travelling if it didn't.

> Note that blog post is correct about C++ but incorrectly assumes this is true for C as well.

Presumably you're referring to this line of the C++ standard, which does not appear in the C standard:

> However, if any such execution contains an undefined operation, this International Standard places no requirement on the implementation executing that program with that input (not even with regard to operations preceding the first undefined operation).

I looked at every instance of the word "undefined" in the C standard and, granted, it definitely didn't have anything quite so clear about time travel as that. But it also didn't make any counter claims that operations before are valid. It pretty much just said that undefined behaviour causes behaviour that is undefined! So, without strong evidence, it seem presumptuous to assume that operations provably before undefined behaviour are well defined.

[uecker]

The poster is me. You are right that this is not an example for time-travel. There aren't really good examples for true time travel because compilers generally do not do this. But my point is that with compilers behaving like this, people might confuse this for time-traveling UB. I have certainly met some who did and the blog posts seems to have similar examples (but I haven't looked closely now).

Note that I am a member of WG14. We added more clarification to C23 to make clear that this is not a valid interpretation of UB, see here: https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3220.pdf

[AlotOfReading]

I didn't notice that section when I last read through C23, but I'm very glad to see it. Reining in UB is one of the hardest problems I've had to deal with, and being able to say operations are defined up to the point of UB makes my job so much easier.

The lack of clarity in earlier standards made it impossible to deal with code incrementally, since all the unknown execution paths could potentially breach back in time and smash your semantics.

[quietbritishjim]

Ok, fair enough. I must admit I was looking at C99 as I thought that was most generally relevant, I don't follow recent C standards (as much as I do those for C++) and C23 hasn't been ratified yet. I've found your new snippet:

> In particular, all observable behavior (5.1.2.4) appears as specified in this document when it happens before an operation with undefined behavior in the execution of the program.

I consider that a change in the standard but, of course, that's allowed, especially as it's backwards compatible for well defined programs.

The wording is a little odd: it makes it sound a like you need some undefined behaviour in order to make the operations beforehand work, and, taken very literally, that operations between two undefined behaviours will work (because they're still "before an operation with undefined behavior"). But I suppose the intention is clear.

[ant6n]

> but that code is reordered or transformed in complicated ways is true even without UB.

Without undefined behavior, the compiler emits code that has the behavior defined by the code —- the ordering may be altered, but not the behavior.

[uecker]

Yes, and with undefined behavior, the compiler has to emit code that has the behavior defined by the code up to the operation that has undefined behavior.

[kazinator]

That is false. If a compiler determines that some statement has undefined behavior, it can treat it as unreachable, and, transitively, other code before it as unreachable.

  printf("hello\n"); // this doesn't have to print
  x = x / 0;         // because this is effectively a notreached() assertion

[jstimpfle]

This is in direct contradiction to what uecker says. Can you back up your claim -- for both C and C++? Putting your code in godbolt with -O3 did not remove the print statement for me in either C or C++. But I didn't experiment with different compilers or compiler flags, or more complicated program constructions.

https://godbolt.org/z/8nbbd3jPW

I've often said that I've never noticed any surprising consequences from UB personally. I know I'm on thin ice here and running risk of looking very ignorant. There are a lot of blogposts and comments that spread what seems like FUD from my tiny personal lookout. It just seems hard to come across measureable evidence of actual miscompilations happening in the wild that show crazy unpredictable behaviour -- I would really like to have some of it to even be able to start tallying the practical impact.

And disregarding whatever formulations there are in the standard -- I think we can all agree that insofar compilers don't already do this, they should be fixed to reject programs with an error message should they be able to prove UB statically -- instead of silently producing something else or acting like the code wouldn't exist.

Is there an error in my logic -- is there a reason why this shouldn't be practically possible for compilers to do, just based on how UB is defined? With all the flaws that C has, UB seems like a relatively minor one to me in practice.

Another example: https://godbolt.org/z/b5j99enTn

This is an adaption from the Raymond Chen post, and it seems to actually compile to a "return 1" when compiling with C++ (not with C), at least with the settings I tried. And even the "return 1" for me is understandable given that we actually hit a bug and there are no observeable side-effects before the UB happens. (But again, the compiler should instead be so friendly and emit a diagnostic about what it's doing here, or better return an error).

Un-comment the printf statement and you'll see that the code totally changes. The printf actually happens now. So again, what uecker says about observable effects seems to apply.

[gpderetta]

In this [1] example GCC hoists, even in C mode, a potentially trapping division above a volatile store. If c=0 you get one less side effect than expected before UB (i.e. the division by zero trap). This is arguably a GCC bug if we agree on the new standard interpretation, but it does show that compilers do some unsafe time travelling transformations.

Hoisting the loop invariant div is an important optimization, but in this case I think the compiler could preserve both the optimization and the ordering of the side effects by loop-peeling.

[1] https://godbolt.org/z/ecsdrPa94

[kazinator]

The implementation can assume that the program does not perpetrate undefined behavior (other than undefined behavior which the implementation itself defines as a documented extension).

The only way the program can avoid perpetrating undefined behavior in the statement "x = x / 0" is if it does not execute that statement.

Thus, to assume that the program does not invoke undefined behavior is tantamount to assuming that the program does not execute "x = x / 0".

But "x = x / 0" follows printf("hello\n") unconditionally. If the printf is executed, then x = x / 0 will be executed. Therefore if the program does not invoke undefined behavior, it does not execute printf("hello\n") either.

If the program can be assumed not to execute printf("hello\n"), there is no need to generate code for it.

Look at the documentation for GCC's __builtin_unreachable:

> Built-in Function: void __builtin_unreachable (void)

> If control flow reaches the point of the __builtin_unreachable, the program is undefined. It is useful in situations where the compiler cannot deduce the unreachability of the code.

The unreachable code assertion works by invoking undefined behavior!

[ant6n]

Mmmh, how about

    #include <stdio.h>


    int f(int y, int a) {
        int x, z;
        printf("hello ");
        x = y / a;
        printf("world!");
        z = y / a;
        return x+y;
    }

In godbolt, it seems the compiler tends to combine the two printfs together. So if a=0, it leads to UB between the printfs, but that wont happen until after the two printfs. Here the UB is delayed. But will the compiler actually make sure that in some other case, the x/a won't be moved earlier somehow? Does the compiler take any potentially undefined behavior and force ordering constraints around them? ...The whole point of UB is to be able to optimize the code as if it doesn't have undefined behavior, so that we all get the maximum optimization and correct behavior as long as there's no UB in the code.

[kazinator]

If a compiler can determine that some statement is UB, it can treat that as an assertion that the code is unreachable. All other statements which reach only that code and no other are also unreachable.

A compiler's analysis can go backward in time. That is to say, the compiler can build a model of what happens in some section of code over time, and analyze it whichever way it wants.

You cannot go back in time from execution time to translation time, but the translator can follow the code as if it were executing it at translation time.

[kazinator]

In C, it is only undefined behavior to access an automatic object that has not been initialized.

Static objects are always initialized, so the situation cannot arise.

That leaves dynamic ones, like uninitialized struct members in a malloced structure.

Accessing uninitialized dynamic memory means isn't undefined behavior in C. It results in whatever value is implied by the uninitialized bits. If the type in question has no trap representations, then it cannot fail.