[mytailorisrich]

> The C representation of memory (and all the pointer arithmetic) is not a real representation of your hardware, and this too is an abstraction.

By and large memory is a contiguous array and the C representation closely matches what is actually happening, so I am curious about which platforms you have worked on.

[kevin_thibedeau]

Tagged memory architectures don't match the C model of linear memory. They're essentially obsolete now but C is still designed to accommodate them.

A lot of the UB the people grouse about can generally be ignored because 99% of the platforms out there have the same behavior in areas where the standard is extra permissive for obsolete exotic hardware. Tagged memmory is dead, 1's complement is dead, big-endian is mostly dead. All the UB associated with them is not that relevant most of the time. The downside is that people write code that takes a lot of liberties assuming behavior that the standard doesn't guarantee. A common one is unaligned access because x86 has always been permissive about it and it took until C11 to have power tools needed to manage it in the language.

[marcosdumay]

The UB problems have no relation to the machine behavior. UB exists only on the compiler.

The fact that many C developers keep confusing it with implementation dependent behavior gives me no confidence on their other opinions about the language.

[Kamq]

> UB exists only on the compiler.

Sure, but UB exists so that compilers can generally do whatever's fastest on that particular architecture.

Your signed add instruction traps? Great, do that. Does one on a different architecture overflow? Fine. Just emit it and it's conformant.

[jstimpfle]

Does the C machine model in fact consist of a single linearly addressable memory space? I think the spec mostly talks about "objects" that are linearly addressable -- not about the whole "memory" (there might not even be such a thing). Technically you aren't even allowed to compare two pointers other than for equality (relational comparisons are possible only within the same array). Just making up pointers is probably already a stretch of the spec, although you'll see lots of that in e.g. embedded projects.

(Disclaimer: I really don't know all the details of the C standards, am not a language lawyer but know enough about the language to feel quite productive in it. Please fill me in or correct me where I'm wrong).

[kevin_thibedeau]

People will abuse [u]intptr_t to compare addresses from different objects. There is no guarantee that the integer value stored in such variables is representative of a linear memory space and you're supposed to treat them as opaque data but most platforms permit such comparisons. All you're permitted to do is cast a pointer into those types and cast it back to the original type.

[Voultapher]

A couple points:

- CPU memory subsystems are very complex these days and represent a lot of shared mutable micro-architectural state, which makes it hard to reason about. That's not linear and the C language does not offer concepts which represent that complexity. Short of some prefetching intrinsics.

- Pretty much all memory will be virtually addressed, pushing you even further from the concept of flat linear memory.

- Pointer provenance [0] binds memory to types and allocations which doesn't map onto the concept of linear memory and a pointer is just an offset.

[0] https://faultlore.com/blah/fix-rust-pointers/

[jstimpfle]

AFAIK C's machine model is not that linear (see my other comment). On the other hand, what most CPUs offer as an abstraction (through their instruction set) is very much so.

There are couple of arguments like that floating around and it just doesn't make a whole lot of sense. The C model is in fact a usable abstraction (and easy enough to peel off when required), otherwise it wouldn't have stuck around for so long. No amount of "network effects" and "free beer" arguments can discuss this away.

There is an argument that instruction sets might have developped a linear address space abstraction because of C, but I doubt it. Binding the IR closer to a specific physical layout would be very bad for portability and longevity of the code.

[adwn]

> the C representation closely matches what is actually happening

It really doesn't, though. Although your CPU might present system RAM as one contiguous array of bytes to your program, the C compiler follows different rules – see strict aliasing and other pointer dereference rules. For example, the following is Undefined Behavior and your C compiler may or may not generate the assembly you expect:

    int x = *(int *)0x1234568;

Your CPU would happily execute the equivalent machine instructions and load from address 0x12345678, while a C compiler is free to replace your entire program with return 0;

[binjooou]

Casting an integer to a pointer is implementation defined, not UB.

And every sane implementation does what everyone expects because its how memory mapped IO works (but you probably want a volatile in there and maybe a compiler or memory barrier as well depending on what the hardware guarantees about the access patterns for that particular range of addresses)

[adwn]

> Casting an integer to a pointer is implementation defined, not UB.

You're right, that was a bad example. Here's a better one:

    int x, y;
    ptrdiff_t diff = &x - &y;

This is Undefined Behavior, because &x and &y don't point to the same object.

[xscott]

The original author was talking about hardware not behaving like linear memory, and other than caches and maybe some thread local tricks, I'm not sure what he meant. However, it seems pretty clear that CPUs do try really hard to make:

    mov rax, qword ptr [0x12345678]

do what you think it would/should.

And as for the C memory model, aliasing, and optimizations, I'm firmly in the camp that thinks the standards originally gave the compiler writers an inch to work on weird platforms and they've taken a mile when they work on reasonable ones. The intent of your integer to pointer cast is very clear, but it's been undefined to insanity. So now there is some variant of the following, which doesn't have UB but does the exact same thing less clearly:

    uintptr_t i = 0x12345678;
    int* p = 0;
    memcpy(&p, &i, sizeof(int*));
    int x = *p;

I'm sure some language lawyer will correct me on some obscure detail of the standard, but it could be fixed with some modification. The point to me is that using memcpy instead of pointer casts is NOT an improvement. The good compilers will generate the same code as the assembly above, so all they've done is made the C source less readable.

[haberman]

> The point to me is that using memcpy instead of pointer casts is NOT an improvement.

The improvement comes when there are multiple accesses that could potentially point to the same memory. Consider a silly function:

    void f(int16_t* a, int32_t* b) {
      for (int32_t i = 0; i < 100; i++) {
        b[i] = a[0] + i;
      }
    }

If type-based alias analysis is enabled, then the compiler can assume that a[0] does not alias b[i] because they are different pointer types. So it can hoist the load of a[0] outside the loop, improving efficiency. If strict aliasing is disabled, it cannot assume this, so it must reload a[0] each time: https://godbolt.org/z/E7jxfYsbx

The memcpy() makes it clear that the memory could alias anything, so it will generate the less efficient code even if strict aliasing is enabled: https://godbolt.org/z/KoPxK9fPj

Memory aliasing is a huge thorn in the side of the optimizer, because the compiler frequently has to allow for the possibility that different pointers will alias each other, even if they never will in practice. The code might end up being slower than necessary for no real reason. Strict aliasing is one of the few tools we have to tell the compiler that aliasing will not occur.

I don't think that C actually forbids this code:

     *(int*)0x12345678

The rule is just: if you access it as an int, you have to consistently access as an int. You can't mix types from one access to the next, eg:

    *(long*)0x12345678
    *(int*)0x12345678

[xscott]

> Strict aliasing is one of the few tools we have to tell the compiler that aliasing will not occur.

I can see the argument, but there's a much better way to indicate what you want with your example:

    void f(int16_t* a, int32_t* b) {
      const int16_t a0 = a[0];
      for (int32_t i = 0; i < 100; i++) {
        b[i] = a0 + i;
      }
    }

Now a clean (well defined) compiler could do what you asked.

I've seen other people suggest that UB is a mechanism to have these magical backdoor conversations with the compiler to express optimization opportunities. I think that's absurd and reckless. Propose adding assertions or "declare" statements instead, and quit thinking of interpretive dance through a minefield as a method of communication.

[haberman]

You are entitled to your opinion. C isn't perfect, but as someone who spends my life trying to optimize the efficiency and code size of critical loops to the max, I like the direction C has gone with UB and optimizations. It's not the right tool for every problem, but for the most size/speed critical code it's hard to beat IMO.

[xscott]

> I don't think that C actually forbids this code:

     *(int*)0x12345678

If not, give it time. It was only a few years ago when you were allowed to use a union for that kind of thing. I really believe they'll eventually make everything except unsigned integers be UB.

"Oh, the code was never correct. You just got lucky before."

[cvnamx]

If you want to load from address 0x1234568, assign it to a char pointer first. Then the cast is legal and defined.

Your point that C is stricter than asm of course still stands.

[zabzonk]

> and load from address 0x12345678

and most likely seg fault, or similar

[adwn]

1. If the CPU lacks an MMU and the address falls into an accessible address space, it won't segfault.

2. If the CPU has an MMU, it won't segfault if the address is mapped to an accessible region of memory.

3. This is besides the point, because the CPU will execute the instruction and attempt to load from that address. A C compiler might emit the load instruction, or it might assume that this code branch will never be executed and can therefore be replaced with code that sends an angry email to your mother.

[DragonStrength]

Only if you ignore memory layout and understand the UB on your platform. The language does not make as many guarantees as "C is simple" folks seem to think. Throw a sanitizer at any of their code, and you'll see unaligned memory accesses all over the place.

[SkiFire13]

Most modern hardware make use of registers and multiple layers of caches and you need ton of UB to justify the compiler making use of them.

[mytailorisrich]

Registers are orthogonal with memory layout. Cache does not change the general model.