Comment by lelanthran
10 hours ago
> They understand that all types "are" just bytes and that all pointers "are" just register-sized integer addresses, because that's how the hardware works and has worked for decades.
I'd clarify this with "They understand that all values are just bytes".
> Meanwhile, the actual computers we have been using for decades have no problems actually just loading 4 bytes through any arbitrary pointer with zero overhead.
It's partly the standards fault here - rather than saying "We don't know how vendors will implement this, so we shall leave it as implementation-defined", they say "We don't know how vendors will implement this, so we will leave it as undefined".
A clear majority of the UB problems with C could be fixed if the standards committee slowly moved all UB into IB. It's not that there isn't any progress (Signed twos-complement is coming, after all), it's that there is (I believe) much pushback from compiler authors (who dominate the standards) who don't want to make UB into IB.
> A clear majority of the UB problems with C could be fixed if the standards committee slowly moved all UB into IB
There is no such thing as getting rid of "all UB."
What behavior is the implementation supposed to prescribe for a write to an unpredictable garbage address you read from the network? It could overwrite your code. It could overwrite any value anywhere. It could overlap with anything. Prescribing defined behavior for absolutely everything would require defining a precise, unoptimizable 1-to-1 mapping to assembly code and disallowing any multithreading.
>It's partly the standards fault here - rather than saying "We don't know how vendors will implement this, so we shall leave it as implementation-defined", they say "We don't know how vendors will implement this, so we will leave it as undefined
I'd agree to a point. I still think it's unreasonable for compiler writers to get all lawyery about precise terminology. After all "implementation defined" could still be subject to the same lawyeriness (we implemented it, ergo we define it).
To me this is an issue of culture. We need to push back against the view that UB means anything can happen, therefore the compiler can do anything.
But it's genuinely useful. In all seriousness, are you sure you aren't perhaps just using the wrong language? At this point UB and leveraging it for optimization are core parts of the most performant C implementations.
That said, I think there are many cases where compilers could make a better effort to link UB they're optimizing against to UB that appears in the code as originally authored and emit a diagnostic or even error out. But at least we've got ubsan and friends so it seems like things are within reason if not optimal.
> At this point UB and leveraging it for optimization are core parts of the most performant C implementations.
I am skeptical that NULL-pointer checks being removed contribute anything more than a rounding error in performance gains in any non-trivial program.
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>are you sure you aren't perhaps just using the wrong language
Well I think there is a tension here. C is the language for microcontrollers and the language for high performance.
In ye olden days both groups interests were aligned because speed in C was about working with the machine. Now the UB has been highjacked for speed, that microcontroller that I'm working on, where I know and int will overflow and rely on that is UB so may be optimised out, so I then have to think about what the compiler may do.
I wouldn't say C is the wrong language. I would say there are wrong compilers though.
This series was a good explanation for me of why treating UB this way is genuinely useful: https://blog.llvm.org/2011/05/what-every-c-programmer-should...
Being able to assume certain things don't happen is powerful when you're writing optimisations, not doing that would have a real performance cost
> Being able to assume certain things don't happen is powerful when you're writing optimisations, not doing that would have a real performance cost
A few of those are significant performance gains, the majority are not.
Emitting the instruction for a NULL pointer dereference is effectively no more costly than not emitting that instruction.
It's the code removal that's killing me.
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Right. But to take the first example, the value of initialised memory.
It's undefined so it doesn't have to be zeroed therefore increasing efficiency.
But it's also UB so if you do know that memory contains something, you can't take advantage of that because it's UB. Having it UB is fine. It's the compilers assuming UB can't happen and optimising it away.
Turning undefined behavior into implementation defined behavior is rarely a fix, though.
It's a fix that removes the most pointy part of UB.
"Going past the end of the array results in addressing arbitrary values" I can live with. "Going past the end of an array results in anything happening" is a hard sell.
Is that really a meaningful distinction?
Once you are addressing arbitrary values you are firmly in the realm of "anything happening" in practice, but you've now given up optimization opportunities. As has been repeatedly demonstrated over the years, once memory safety breaks it is practically impossible to make any guarantees about program behavior.
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I think it’s a really easy sell, actually: if you go past the end of the array far enough you end up accessing the stack which includes parts of the program like “where does this function return to” or “what is the index used to perform this access” or “there is no page mapped there”. None of these are arbitrary values.
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Are you talking about creating a pointer (more than one item) past an array, or dereferencing that pointer? Both are currently UB.
For the former, I kinda get it. It may need to be there for cases like with segmented address space where p+10 could actually be a value less than p, for the eventually generated assembly. Maybe it should be fine to create such a pointer, but have it be "indeterminate value" or whatever, if you try to compare that pointer to anything? I don't know enough about compiler internals to say one way or the other.
Dereferencing, though, can only be UB. There may not be a "value" behind that address. There may be a motor that's been I/O mapped, or a self destruct button.
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