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> Undefined behavior is critical for performance

Not only is this not true, it's trivially easy to prove it's not true. Both rustc and clang generate LLVM IR and use LLVM to optimise and generate machine code. The code that's generated is equally performant, as you'd expect since most of the optimisation is being done by LLVM, not the front end.

The difference between the two frontends is that rustc is stricter, rejecting programs where UB may arise.

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I do agree that if you remove `unsafe` blocks from Rust, then you have a performant language without UB. However: (1) We are talking about C here, not Rust (2) Rust has UB due to `unsafe` (3) LLVM IR has UB.

To play devil's advocate, that doesn't negate the claim that undefined behavior is critical for performance. Rust (as well as LLVM IR itself) also have a concept of undefined behavior.

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