Why Not Rust?

[guevara]

Why Not Rust?



https://ift.tt/3iPgp7A

“Using LLVM” is not a universal solution to all performance problems. While I am not aware of benchmarks comparing performance of C++ and Rust at scale, it’s not to hard to come up with a list of cases where Rust leaves some performance on the table relative to C++.

The biggest one is probably the fact that Rust’s move semantics is based on values (memcpy at the machine code level). In contrast, C++ semantics uses special references you can steal data from (pointers at the machine code level). In theory, compiler should be able to see through chain of copies; in practice it often doesn’t: #57077. A related problem is the absence of placement new — Rust sometimes need to copy bytes to/from the stack, while C++ can construct the thing in place.

Somewhat amusingly, Rust’s default ABI (which is not stable, to make it as efficient as possible) is sometimes worse than that of C: #26494.

Finally, while in theory Rust code should be more efficient due to the significantly richer aliasing information, enabling aliasing-related optimizations triggers LLVM bugs and miscompilations: #54878.

But, to reiterate, these are cherry-picked examples, sometimes the field is tilted the other way. For example, std::unique_ptr has a performance problem which Rust’s Box lacks.

A potentially bigger issue is that Rust, with its definition time checked generics, is less expressive than C++. So, some C++ template tricks for high performance are not expressible in Rust using a nice syntax.

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January 27, 2022 at 09:17PM