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Comment by notriddle | Hacker News Reader

Fifty bucks says it isn't even about performance, but is instead about passing pointers to C code. Zero-overhead FFI has killed a lot of radical performance improvements that Rust could have otherwise made.

I don't know, because nobody's actually posting a link to it.

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This strikes me as likely. Bitwise compatibility with machine ABI layout rules has powerful compatibility advantages even in places where it might make code slower. (And, for the large majority of code, slower doesn't matter anyway.)

Of course C and C++ themselves have to keep to machine ABI layout rules for backward compatibility to code built when those rules were (still) thought a good idea. Compilers offer annotations to dictate packing for specified types, and the Rust compiler certainly also offers such a choice. So, maybe such annotations should just be used a lot more in Rust, C, and C++.

This is not unlike the need to write "const" everywhere in C and C++ because the inherited default (from before it existed) was arguably wrong. We just need to get used to ignoring the annotation clutter.

But there is no doubt there are lots of people who think padding to alignment boundaries is faster. And, there can be other reasons to align even more strictly than the machine ABI says, even knowing what it costs.

Rust structs have non-C layouts. You can optionally specifiy that a struct should be laid out in the same way that C does it.

Structs aren’t the problem here. It’s the primitives.

You can take a pointer to an i32 and pass that pointer to C code as int32_t. This means it has to have the same alignment in Rust that it has in C.

The topic at hand is, specifically, that nobody makes cores that enforce alignment restrictions anymore. So, it doesn't matter where the pointer goes. All that matters is if your compiler lays out its structs the same way as whoever compiled the code a pointer to one ends up in.

There are embedded targets that still enforce alignment restrictions, but you are even less likely to pass pointers between code compiled with different compilers, there.

> The topic at hand is, specifically, that nobody makes cores that enforce alignment restrictions anymore. So, it doesn't matter where the pointer goes.

Compilers can rely on alignment even if the CPU doesn't. LLVM does, which is why older versions of rustc had segfaults when repr(packed) used to allow taking references. While it would be pretty easy to get rustc to stop emitting aligned loads, getting Clang and GCC to stop emitting aligned loads might be trickier. https://github.com/rust-lang/rust/issues/27060

People arguing against changing struct layout rules are probably mainly interested, then, in maintaining backward compatibility with older Rust, itself.

Anyway, same policy applies: annotate your structs "packed" anywhere performance matters and bitwise compatibility with other stuff doesn't.

Rust does not keep backward compatibility like that. In the absence of any statement forcing a specific ABI, the only guaranteed compatibility is with code that's part of the same build, or else part of the current Rust runtime and being linked into said build.

Even in Rust, mapping memory between processes, or via files between past and future processes, happens. Although structure-blasting is frowned upon in some circles, it is very fast where allowed.

But... when you are structure-blasting, you are probably also already paying generous attention to layout issues.

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