The first example should have changed the heading to "C++ code" as opposed to "C code" due to the use of `std::unreachable()`. This matches the godbolt link.
Another alternative is to use `__builtin_unreachable()` (not standard, but available in GCC and Clang).
C23 also has the "no time travel" stipulation. I expect C23 should be immune to the example 1 program issue, because of that. A quick trial on godbolt (https://godbolt.org/z/8K81PYoYE) seems to agree, with the caveat that I didn't work out the right compiler options to show for sure.
C++26, IIRC, adds "no time travel" to C++. But note that your example still creates exploding chocolate, so what do you mean by "immune"? C compiler with -std=c23 -O1 produces literally the exact same assembly output.
This is sort of true, except that behavior can be undefined in the standard but defined in your implementation. One of the best examples is -fwrapv, which makes signed integer overflow not UB. And Fil-C has no undefined behavior that I'm aware of. But most implementations target the standard and "define" what the standard doesn't to be "whatever I feel like, in service of implementation simplicity and performance". What matters is what your implementation guarantees, which should be a superset of what the standard does.
Fil-C does still have UB. Signed-overflow, for instance, is still UB in Fil-C, and can still lead to things like the optimizer removing checks in the presence of UB.
"the definition of std::unreachable is: invokes undefined behaviour" does not mean that all undefined behaviour is equivalent to calling std::unreachable.
You saw it yourself: unreachable generates no code and suppresses the function epilog (duh - it's unreachable) but null deref generated a trap instruction in GCC. If you wanted a trap instruction, maybe this compiler reliably generates one from a hardcoded null deref.
> "the definition of std::unreachable is: invokes undefined behaviour" does not mean that all undefined behaviour is equivalent to calling std::unreachable.
Well, obviously not. The definition of UB is that anything can happen, so by transitive property it means that std::unreachable can trigger any behavior.
That's the whole point of UB, and the point of my post: It's UB! You cannot reason about what will happen! The whole point of the post is "No you CANNOT reason about what will happen". So of course I am making no claims about what will happen, just three interesting observations about what did in fact happen when I happened to try it, to prove this point.
> If you wanted a trap instruction, maybe this compiler reliably generates one from a hardcoded null deref.
No, even in the examples in my post I show that it's not reliable. It's UB!
The effect of UB is arbitrary and can change between compilers, between versions of the same compiler, between optimization levels of the same compiler version, between different platforms targeted by the same compiler, etc. It also can change depending on what other code is in the same compilation unit (which, with LTO, means any other code anywhere in the program can affect the behavior)
When you say "this UB does ... (like generates a trap instruction)" you are being hyper specific for a given compiler version and platform and optimization level and any other knob. It may not stay the same after you update the compiler. And what happens a lot is that newer, better optimizations (such as more opportunity to autovectorization), which attract people to newer compiler versions, may exploit UB in more creative ways.
For example, to exploit autovectorization, compilers may want to push some test out of a loop. If this test contains UB, this optimization will change where the UB is triggered. So that trap instruction may happen at a different place in the code, depending on optimizations.
The solution is to avoid UB. std::unreachable() is just an easy-to-invoke way of hitting UB, mentioned for the sake of the discussion. They did later take your advice “Let’s say we change std::unreachable() to int* ptr = nullptr; *ptr = 99; in the previous example.”. They still got improper codegen out of the compiler.
Though, it would be better if programmers read deliberate instances of UB as if they literally said "std::unreachable()" at that point in the code, as that would better convey how the optimizer passes will see things after dozens of frontend and middle-end passes have run before it.
> it would be better if programmers read deliberate instances of UB as if they literally said "std::unreachable()"
Except that isn't true.
There are basically two kinds of undefined behavior.
The first is undefinable behavior, the kind that easily goes disastrously wrong, like writing to random memory locations.
The second is incidentally undefined behavior, something where there just isn't enough implementation consensus for the standard to settle on it, like realloc(p, 0).
There's no standard terminology I'm aware of to distinguish the two. But the distinction exists.
And where programmers read deliberate instances of UB, it's usually the latter, e.g., writing directly to an address memory mapped to device I/O in that particular environment.
> There's no standard terminology I'm aware of to distinguish the two.
There is: implementation-defined behavior. Your example of writing to an MMIO address is such an example, as the standard says casting an int to a pointer is implementation-defined. (However, the C standard has plenty of cases of undefined behavior that probably should have been implementation-defined.)
unreachable is for when you are a certain that some code cannot be reached, so that it's okay to translate and optimize it under that assumption.
If you think a point in the program should not be reached unless there is a bug somewhere, you can write abort() rather than unreachable(), among other possibilities.
abort() doesn't return; any code after it is actually unreachable; we can confidently write
abort();
unreachable(); // redundant
The compiler can already assume that abort() doesn't return, and so anything after it is unreachable. But it may not assume that abort() itself is not reached.
So in essence we can get some of the same optimizations, somewhat blunted, so to speak.
For instance, if we put unreachable() at the end of a function, the compiler can omit generating the code for returning from the function; if that point is reached, the instruction pointer will wander off into whatever instructions occur after that.
If we put abort() at the end of a function, the compiler can likewise omit generating the code for returning from the function. But the instruction pointer won't wander off: there will be a call or jump to abort. So, not quite as optimized in size, but optimized still.
No. That's just one of the MANY ways to trigger UB. In the article I also trigger UB by dereferencing a null pointer instead.
In fact, here's an example where calling `unreachable()` and dereferencing a null pointer produce the EXACT same assembly: https://godbolt.org/z/538qfj9z9
im not an c++ expert, but it looks like the unreachable macro of rust.
In rust it allows optimizing the generated binary to not care about special paths.
If an integer can only be 1-4, then it doesn't make sense to dedicate an code path for 0 or 5+ that will never be called.
in an explicit for loop its easy to know for the compiler, but when this information crosses boundaries like function calls then it can make it easier and can reduce the binary size.
To elaborate on the other reply to your comment: In Rust, if you don't use `unsafe` and still manage to trigger UB, then that's a bug in the compiler or the library code you called (which in turn has to have used unsafe somewhere to trigger UB).
So the fact that `unreachable!()` does not require the use of the keyword `unsafe` means that it can't be like C++ `std::unreachable()`.
The first example should have changed the heading to "C++ code" as opposed to "C code" due to the use of `std::unreachable()`. This matches the godbolt link.
Another alternative is to use `__builtin_unreachable()` (not standard, but available in GCC and Clang).
Author here.
Yes, into the shame cube I go. It doesn't change anything, and it applies equally to C and C++, but you are correct. Thanks.
C23 has `unreachable()`. cf. https://en.cppreference.com/c/program/unreachable
C23 also has the "no time travel" stipulation. I expect C23 should be immune to the example 1 program issue, because of that. A quick trial on godbolt (https://godbolt.org/z/8K81PYoYE) seems to agree, with the caveat that I didn't work out the right compiler options to show for sure.
C++26, IIRC, adds "no time travel" to C++. But note that your example still creates exploding chocolate, so what do you mean by "immune"? C compiler with -std=c23 -O1 produces literally the exact same assembly output.
This is sort of true, except that behavior can be undefined in the standard but defined in your implementation. One of the best examples is -fwrapv, which makes signed integer overflow not UB. And Fil-C has no undefined behavior that I'm aware of. But most implementations target the standard and "define" what the standard doesn't to be "whatever I feel like, in service of implementation simplicity and performance". What matters is what your implementation guarantees, which should be a superset of what the standard does.
> This is sort of true, except that behavior can be undefined in the standard but defined in your implementation.
But then it's not UB, and not the topic at hand. So I wouldn't call it "sort of true" for a statement about UB, because it doesn't apply to not-UB.
It's also different in a consteval context. But that doesn't make it less true outside consteval context.
Fil-C does still have UB. Signed-overflow, for instance, is still UB in Fil-C, and can still lead to things like the optimizer removing checks in the presence of UB.
"the definition of std::unreachable is: invokes undefined behaviour" does not mean that all undefined behaviour is equivalent to calling std::unreachable.
You saw it yourself: unreachable generates no code and suppresses the function epilog (duh - it's unreachable) but null deref generated a trap instruction in GCC. If you wanted a trap instruction, maybe this compiler reliably generates one from a hardcoded null deref.
Author here.
> "the definition of std::unreachable is: invokes undefined behaviour" does not mean that all undefined behaviour is equivalent to calling std::unreachable.
Well, obviously not. The definition of UB is that anything can happen, so by transitive property it means that std::unreachable can trigger any behavior.
That's the whole point of UB, and the point of my post: It's UB! You cannot reason about what will happen! The whole point of the post is "No you CANNOT reason about what will happen". So of course I am making no claims about what will happen, just three interesting observations about what did in fact happen when I happened to try it, to prove this point.
> If you wanted a trap instruction, maybe this compiler reliably generates one from a hardcoded null deref.
No, even in the examples in my post I show that it's not reliable. It's UB!
Hell, we get very different output just between -O1 (https://godbolt.org/z/33e1nGE38) and -O2 (https://godbolt.org/z/3coTqP31W).
The effect of UB is arbitrary and can change between compilers, between versions of the same compiler, between optimization levels of the same compiler version, between different platforms targeted by the same compiler, etc. It also can change depending on what other code is in the same compilation unit (which, with LTO, means any other code anywhere in the program can affect the behavior)
When you say "this UB does ... (like generates a trap instruction)" you are being hyper specific for a given compiler version and platform and optimization level and any other knob. It may not stay the same after you update the compiler. And what happens a lot is that newer, better optimizations (such as more opportunity to autovectorization), which attract people to newer compiler versions, may exploit UB in more creative ways.
For example, to exploit autovectorization, compilers may want to push some test out of a loop. If this test contains UB, this optimization will change where the UB is triggered. So that trap instruction may happen at a different place in the code, depending on optimizations.
UB - undefined behaviour
Isn't a solution to simply not use unreachable()?
The solution is to avoid UB. std::unreachable() is just an easy-to-invoke way of hitting UB, mentioned for the sake of the discussion. They did later take your advice “Let’s say we change std::unreachable() to int* ptr = nullptr; *ptr = 99; in the previous example.”. They still got improper codegen out of the compiler.
Though, it would be better if programmers read deliberate instances of UB as if they literally said "std::unreachable()" at that point in the code, as that would better convey how the optimizer passes will see things after dozens of frontend and middle-end passes have run before it.
> it would be better if programmers read deliberate instances of UB as if they literally said "std::unreachable()"
Except that isn't true.
There are basically two kinds of undefined behavior.
The first is undefinable behavior, the kind that easily goes disastrously wrong, like writing to random memory locations.
The second is incidentally undefined behavior, something where there just isn't enough implementation consensus for the standard to settle on it, like realloc(p, 0).
There's no standard terminology I'm aware of to distinguish the two. But the distinction exists.
And where programmers read deliberate instances of UB, it's usually the latter, e.g., writing directly to an address memory mapped to device I/O in that particular environment.
> There's no standard terminology I'm aware of to distinguish the two.
There is: implementation-defined behavior. Your example of writing to an MMIO address is such an example, as the standard says casting an int to a pointer is implementation-defined. (However, the C standard has plenty of cases of undefined behavior that probably should have been implementation-defined.)
unreachable is for when you are a certain that some code cannot be reached, so that it's okay to translate and optimize it under that assumption.
If you think a point in the program should not be reached unless there is a bug somewhere, you can write abort() rather than unreachable(), among other possibilities.
abort() doesn't return; any code after it is actually unreachable; we can confidently write
The compiler can already assume that abort() doesn't return, and so anything after it is unreachable. But it may not assume that abort() itself is not reached.So in essence we can get some of the same optimizations, somewhat blunted, so to speak.
For instance, if we put unreachable() at the end of a function, the compiler can omit generating the code for returning from the function; if that point is reached, the instruction pointer will wander off into whatever instructions occur after that.
If we put abort() at the end of a function, the compiler can likewise omit generating the code for returning from the function. But the instruction pointer won't wander off: there will be a call or jump to abort. So, not quite as optimized in size, but optimized still.
Author here.
No. That's just one of the MANY ways to trigger UB. In the article I also trigger UB by dereferencing a null pointer instead.
In fact, here's an example where calling `unreachable()` and dereferencing a null pointer produce the EXACT same assembly: https://godbolt.org/z/538qfj9z9
im not an c++ expert, but it looks like the unreachable macro of rust. In rust it allows optimizing the generated binary to not care about special paths. If an integer can only be 1-4, then it doesn't make sense to dedicate an code path for 0 or 5+ that will never be called. in an explicit for loop its easy to know for the compiler, but when this information crosses boundaries like function calls then it can make it easier and can reduce the binary size.
To elaborate on the other reply to your comment: In Rust, if you don't use `unsafe` and still manage to trigger UB, then that's a bug in the compiler or the library code you called (which in turn has to have used unsafe somewhere to trigger UB).
So the fact that `unreachable!()` does not require the use of the keyword `unsafe` means that it can't be like C++ `std::unreachable()`.
unreachable! in Rust is panic! with a different name [1]. The unsafe unreachable_unchecked [2] is C++ std::unreachable.
[1] https://doc.rust-lang.org/std/macro.unreachable.html [2] https://doc.rust-lang.org/std/hint/fn.unreachable_unchecked....