How to disable loop unrolling in Rust?

Question

I am looking at Rust to rewrite a C++ codebase and I would like to know how to disable loop unrolling/vectorizing and control unroll count as well. In Clang we would use:

#pragma clang loop unroll(disable)
#pragma clang loop unroll(disable) vectorize(disable)
#pragma clang loop unroll_count(2)

It is highly performance sensitive code and not having control over loop codegen could be a showstopper.

Answer 1

You cannot control how a given loop is/is not unrolled. It has been proposed, but no real progress has been made on considering such an RFC, much less accepting and implementing it.

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If you are looking for very broad capabilities, you can pass -C llvm-args=... to the compiler to influence LLVM. If there's a way to disable loop unrolling completely, that might do what you want.

Pragmatically, I'd encourage you to get a walking skeleton of your code in Rust and see if the performance is acceptable. My personal experience has shown that "general" Rust code is more performant because of

• The borrow checker making it safe to be aggressive about using references instead of copies
• Comparatively easy usage of multithreading
• A strong type system that can help avoid performing work multiple times