4 ms·

I think that we are subtly talking past each other, because what you're saying doesn't line up against what I thought I was saying. I'm not looking for addition

by Twisol 7y ago

I think that we are subtly talking past each other, because what you're saying doesn't line up against what I thought I was saying. I'm not looking for additional type safety or security protections from this model. See my sibling comment to your earlier comment for a little more context [1]. As someone who works with Rust often, your assertions don't match my experience. I want to engage honestly and understand where Rust itself falters, versus where we need to invest in educating newcomers about what kinds of models are better represented in Rust than others. I have found the model I'm describing to be valuable in many ways -- more than a "hack" to get around Rust. > And of course the extra indirection is costly, especially for a systems language. To my knowledge, base-offset addressing is natively present on at least x86. With a vector-based index space like I was describing, a[b] == * (a + b) is no more costly than * b. If you would need a more complex index space, you were probably doing more than bare pointers already. > And you're also doing much more memory management. > because such hacks turn a simple logical operation into a series of multiple memory allocations No, no more than you would have needed with pointer-based naming. The top-level arena (a vector in the graph example) owns everything and can be deallocated all at once when you're done with it. Arenas were already a well-known pattern before Rust, and they can reduce the amount of manual memory management over tracking a bunch of small, separate heap allocations. Relatedly, Catherine West gave an excellent talk at RustConf 2018 on how the Entity-Component-System architecture maps nicely into Rust [2]. The address/name distinction comes from ECS, but it becomes even more useful in Rust. > The analog in the land of pointers is simply to NULL node member pointers and fail if a member is non-NULL at destruction or insertion I don't see how this is hindered by Rust. We use an Option type -- with Some(T) and None variants -- to model something that might not be present, so the logic can be exactly the same. Rust can even optimize away the "extra" space you would need for the Some/None variant flag in common situations [3]. [1] https://news.ycombinator.com/item?id=20822003 https://news.ycombinator.com/item?id=20822003 [2] https://kyren.github.io/2018/09/14/rustconf-talk.html https://kyren.github.io/2018/09/14/rustconf-talk.html [3] https://github.com/rust-lang/rust/pull/45225 https://github.com/rust-lang/rust/pull/45225

wahern 7y ago
My original point is that real-world programs are complex data structures where any particular object may need to be referenced from multiple containers. At which point you're right back to square one--which container is the true owner responsible for maintaining all invariants and releasing the object when it's dead? Rust memory management does not and cannot automatically "free" resources with such references; you're still doing this manually. Indeed, this is irreducible complexity; it's why programming is hard, even in GC'd languages. The indexing scheme is a hack to subvert Rust's typing system. It's even admitted in the talk you mentioned, > One more criticism of this style is that you might say that using indexes instead of pointers is “safe” in the strict sense, but possibly only technically, you’re trading UB and potential crashes with pointers for “random but unspecified” behavior if you access the wrong or outdated index, and potentially panics. You’d be right, btw! The generational indexing discussed therein is just a runtime invariant assertion, just as-if you asserted on internal pointer members being NULL at destruction (which implies but doesn't guarantee that the node has been properly unlinked). This is all just more memory games.[1] In some respects they may be marginally safer, but they're also marginally more complicated. And that's friction. And even if on the whole it makes programs safer, the cost exists. ATS is an even safer language than Rust. If we're gonna pretend that the friction doesn't matter why wouldn't we just pretend we should all use ATS? I'm not arguing that C or C++ is better than Rust. It just bugs me the way people rationalize some of the difficulty with arguments about why it makes things easier. Yeah, single-ownership does make things easier. Good C programs and good programmers generally learn to strive for single ownership semantics. Without an argument, Rust's enforcement of this invariant makes programs safer. But arguing that the enforcement of the invariant makes it easier to program? Come on! I know single-ownership is safer and simpler, which means when I choose to have a more complex ownership pattern it's for a darned good reason. Having to jump through hoops to accomplish it, even if it's "good" for me, is still jumping through hoops. [1] And game makers love memory hacks; it's why their apps crash all the time, and why the thought of handling allocation failure would never cross their minds. To understand why people stopped using arenas and ad hoc invariant assertions for systems programming, see Heartbleed.
wahern 7y ago
> With a vector-based index space like I was describing, a[b] == * (a + b) is no more costly than * b. If you would need a more complex index space, you were probably doing more than bare pointers already. Of course you're doing more than bare pointers. Which is why your example isn't applicable. (a + b) is fast when you're accessing record fields, because you already have a in a register and b is usually a constant. In the vector scheme you're doing (v + a + b), where v needs to be fetched (often through another level of indirection), and unless you're doing a long series of operation to the same structure, you're constantly reloading v and recalculating a.[1] It doesn't follow that technique A is equivalent to B simply because when you reduce B to a microbenchmark the difference to A is de minimis. Microbenchmarks obscure and often elide the very reasons B would be slower than A. That said, I'm usually dubious of such performance arguments. This level of performance rarely matters, though when you do it ubiquitously it's more likely to matter, which is why it crossed my mind. [1] It's often pointed out that loops like `for (size_t i = 0; i < n; i++) {}` are as fast as `for (; p < pe; p++) { ... }`, and that the former is preferable for various dubious reasons related to the ease of implementing boundary checks. But the performance is often the same because (and to the extent that) the compiler can reduce the former to the latter. Even today I regularly encounter situations where using pointers is cleaner and faster than indexing. Though that fact that it's faster is more a novelty; what matters most to me is clarity and correctness, which is highly context dependent.
- Twisol 7y ago
  > Though that fact that it's faster is more a novelty; what matters most to me is clarity and correctness, which is highly context dependent. We agree.

by Twisol 7y ago

wahern 7y ago

My original point is that real-world programs are complex data structures where any particular object may need to be referenced from multiple containers. At which point you're right back to square one--which container is the true owner responsible for maintaining all invariants and releasing the object when it's dead? Rust memory management does not and cannot automatically "free" resources with such references; you're still doing this manually. Indeed, this is irreducible complexity; it's why programming is hard, even in GC'd languages. The indexing scheme is a hack to subvert Rust's typing system. It's even admitted in the talk you mentioned, > One more criticism of this style is that you might say that using indexes instead of pointers is “safe” in the strict sense, but possibly only technically, you’re trading UB and potential crashes with pointers for “random but unspecified” behavior if you access the wrong or outdated index, and potentially panics. You’d be right, btw! The generational indexing discussed therein is just a runtime invariant assertion, just as-if you asserted on internal pointer members being NULL at destruction (which implies but doesn't guarantee that the node has been properly unlinked). This is all just more memory games.[1] In some respects they may be marginally safer, but they're also marginally more complicated. And that's friction. And even if on the whole it makes programs safer, the cost exists. ATS is an even safer language than Rust. If we're gonna pretend that the friction doesn't matter why wouldn't we just pretend we should all use ATS? I'm not arguing that C or C++ is better than Rust. It just bugs me the way people rationalize some of the difficulty with arguments about why it makes things easier. Yeah, single-ownership does make things easier. Good C programs and good programmers generally learn to strive for single ownership semantics. Without an argument, Rust's enforcement of this invariant makes programs safer. But arguing that the enforcement of the invariant makes it easier to program? Come on! I know single-ownership is safer and simpler, which means when I choose to have a more complex ownership pattern it's for a darned good reason. Having to jump through hoops to accomplish it, even if it's "good" for me, is still jumping through hoops. [1] And game makers love memory hacks; it's why their apps crash all the time, and why the thought of handling allocation failure would never cross their minds. To understand why people stopped using arenas and ad hoc invariant assertions for systems programming, see Heartbleed.

wahern 7y ago

> With a vector-based index space like I was describing, a[b] == * (a + b) is no more costly than * b. If you would need a more complex index space, you were probably doing more than bare pointers already. Of course you're doing more than bare pointers. Which is why your example isn't applicable. (a + b) is fast when you're accessing record fields, because you already have a in a register and b is usually a constant. In the vector scheme you're doing (v + a + b), where v needs to be fetched (often through another level of indirection), and unless you're doing a long series of operation to the same structure, you're constantly reloading v and recalculating a.[1] It doesn't follow that technique A is equivalent to B simply because when you reduce B to a microbenchmark the difference to A is de minimis. Microbenchmarks obscure and often elide the very reasons B would be slower than A. That said, I'm usually dubious of such performance arguments. This level of performance rarely matters, though when you do it ubiquitously it's more likely to matter, which is why it crossed my mind. [1] It's often pointed out that loops like `for (size_t i = 0; i < n; i++) {}` are as fast as `for (; p < pe; p++) { ... }`, and that the former is preferable for various dubious reasons related to the ease of implementing boundary checks. But the performance is often the same because (and to the extent that) the compiler can reduce the former to the latter. Even today I regularly encounter situations where using pointers is cleaner and faster than indexing. Though that fact that it's faster is more a novelty; what matters most to me is clarity and correctness, which is highly context dependent.

Twisol 7y ago
> Though that fact that it's faster is more a novelty; what matters most to me is clarity and correctness, which is highly context dependent. We agree.