Add singlethreaded (compiler-only) fences.

[tari]

Rendered.

[pythonesque]

This seems pretty useful, +1.

[reem]

Agreed.

[oli-obk]

alternatives: using black_box instead. parameters to black_box are not moved after results of black_box. allows fine-grain fences. drawback: not as easy to use

[brson]

Why does the llvm reference say 'this is useful for interacting with signal handlers'? Reading the docs for atomic_signal_fence does not enlighten.

[tari]

My understanding is that signals preempt the thread or process receiving them, meaning they will execute on the same physical thread. Because there's no hardware context switch, memory reordering is not a concern since operations always appear to happen in program order within a single thread's context.

If code interacts with signal handlers via memory, compiler reordering of memory operations may change the order of operations as seen by the signal handler (which the compiler is not aware of), potentially violating program-maintained invariants.

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Neglected to mention it elsewhere; this RFC comes from Rust PR #22358, which includes an implementation.

[brson]

Accepted. Tracking bug rust-lang/rust#24118. Sorry for the long delays.