Module Multicore_magic.Transparent_atomic

A replacement for Stdlib.Atomic with fixes and performance improvements

Stdlib.Atomic.get is incorrectly subject to CSE optimization in OCaml 5.0.0 and 5.1.0. This can result in code being generated that can produce results that cannot be explained with the OCaml memory model. It can also sometimes result in code being generated where a manual optimization to avoid writing to memory is defeated by the compiler as the compiler eliminates a (repeated) read access. This module implements get such that argument to Stdlib.Atomic.get is passed through Sys.opaque_identity, which prevents the compiler from applying the CSE optimization.

OCaml 5 generates inefficient accesses of 'a Stdlib.Atomic.t arrays assuming that the array might be an array of floating point numbers. That is because the Stdlib.Atomic.t type constructor is opaque, which means that the compiler cannot assume that _ Stdlib.Atomic.t is not the same as float. This module defines the type as private 'a ref, which allows the compiler to know that it cannot be the same as float, which allows the compiler to generate more efficient array accesses. This can both improve performance and reduce size of generated code when using arrays of atomics.

type !'a t = private 'a ref
val make : 'a -> 'a t
val make_contended : 'a -> 'a t
val get : 'a t -> 'a
val fenceless_get : 'a t -> 'a
val set : 'a t -> 'a -> unit
val fenceless_set : 'a t -> 'a -> unit
val exchange : 'a t -> 'a -> 'a
val compare_and_set : 'a t -> 'a -> 'a -> bool
val fetch_and_add : int t -> int -> int
val incr : int t -> unit
val decr : int t -> unit