diff --git a/library/core/src/sync/atomic.rs b/library/core/src/sync/atomic.rs index 71ee578f8b4..53a57412f82 100644 --- a/library/core/src/sync/atomic.rs +++ b/library/core/src/sync/atomic.rs @@ -87,33 +87,30 @@ //! atomic `load`s might be implemented using compare-exchange operations, even a `load` can fault //! on read-only memory. //! -//! (For the purpose of this section, "read-only memory" is defined as memory that is read-only in +//! For the purpose of this section, "read-only memory" is defined as memory that is read-only in //! the underlying target, i.e., the pages are mapped with a read-only flag and any attempt to write //! will cause a page fault. In particular, an `&u128` reference that points to memory that is //! read-write mapped is *not* considered to point to "read-only memory". In Rust, almost all memory //! is read-write; the only exceptions are memory created by `const` items or `static` items without //! interior mutability, and memory that was specifically marked as read-only by the operating -//! system via platform-specific APIs.) +//! system via platform-specific APIs. //! -//! However, as an exception from this general rule, "sufficiently small" atomic loads are -//! implemented in a way that works on read-only memory. The exact threshold for what makes a load -//! "sufficiently small" varies depending on the architecture and feature flags, but Rust guarantees -//! that atomic loads that do not exceed the size documented in the following table are guaranteed -//! to be read-only: +//! As an exception from the general rule stated above, "sufficiently small" atomic loads with +//! `Ordering::Relaxed` are implemented in a way that works on read-only memory, and are hence not +//! Undefined Behavior. The exact size limit for what makes a load "sufficiently small" varies +//! depending on the target: //! -//! | Target architecture | Atomic loads no larger than this are guaranteed read-only | +//! | Target triple prefix (regular expression) | Size limit | //! |---------------|---------| -//! | `x86` | 4 bytes | -//! | `x86_64` | 8 bytes | -//! | `arm` | 4 bytes | -//! | `aarch64` | 8 bytes | -//! | `riscv32` | 4 bytes | -//! | `riscv64` | 8 bytes | -//! | `powerpc64` | 8 bytes | +//! | `i(3|5|6)86-`, `arm`, `thumb`, `mips(|el)-`, `powerpc-`, `riscv32`, `sparc-` | 4 bytes | +//! | `x86_64-`, `aarch64-`, `loongarch64-`, `mips64(|el)-`, `powerpc64-`, `riscv64` | 8 bytes | +//! | `powerpc64le-` | 16 bytes | +//! | `s390x-` | 16 bytes | //! -//! Atomics loads that are larger than this threshold (and *all* atomic loads on targets not listed -//! in the table) might still be read-only under certain conditions, but that is not a stable -//! guarantee and should not be relied upon. +//! Atomics loads that are larger than this limit as well as atomic loads with ordering other +//! than `Relaxed`, as well as *all* atomic loads on targets not listed in the table, might still be +//! read-only under certain conditions, but that is not a stable guarantee and should not be relied +//! upon. //! //! # Examples //!