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0888-compiler-fence-intrinsics.md

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Summary

Add intrinsics for single-threaded memory fences.

Motivation

Rust currently supports memory barriers through a set of intrinsics, atomic_fence and its variants, which generate machine instructions and are suitable as cross-processor fences. However, there is currently no compiler support for single-threaded fences which do not emit machine instructions.

Certain use cases require that the compiler not reorder loads or stores across a given barrier but do not require a corresponding hardware guarantee, such as when a thread interacts with a signal handler which will run on the same thread. By omitting a fence instruction, relatively costly machine operations can be avoided.

The C++ equivalent of this feature is std::atomic_signal_fence.

Detailed design

Add four language intrinsics for single-threaded fences:

  • atomic_compilerfence
  • atomic_compilerfence_acq
  • atomic_compilerfence_rel
  • atomic_compilerfence_acqrel

These have the same semantics as the existing atomic_fence intrinsics but only constrain memory reordering by the compiler, not by hardware.

The existing fence intrinsics are exported in libstd with safe wrappers, but this design does not export safe wrappers for the new intrinsics. The existing fence functions will still perform correctly if used where a single-threaded fence is called for, but with a slight reduction in efficiency. Not exposing these new intrinsics through a safe wrapper reduces the possibility for confusion on which fences are appropriate in a given situation, while still providing the capability for users to opt in to a single-threaded fence when appropriate.

Alternatives

  • Do nothing. The existing fence intrinsics support all use cases, but with a negative impact on performance in some situations where a compiler-only fence is appropriate.

  • Recommend inline assembly to get a similar effect, such as asm!("" ::: "memory" : "volatile"). LLVM provides an IR item specifically for this case (fence singlethread), so I believe taking advantage of that feature in LLVM is most appropriate, since its semantics are more rigorously defined and less likely to yield unexpected (but not necessarily wrong) behavior.

Unresolved questions

These intrinsics may be better represented with a different name, such as atomic_signal_fence or atomic_singlethread_fence. The existing implementation of atomic intrinsics in the compiler precludes the use of underscores in their names and I believe it is clearer to refer to this construct as a "compiler fence" rather than a "signal fence" because not all use cases necessarily involve signal handlers, hence the current choice of name.