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Revise the notes about atomics on Arm.

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Jonathan 'theJPster' Pallant 2025-11-23 21:52:00 +00:00
parent 99a393444c
commit 120a464d26
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20
library/core/src/sync/atomic.rs
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@ -130,16 +130,18 @@
//!
//! * PowerPC and MIPS platforms with 32-bit pointers do not have `AtomicU64` or
//! `AtomicI64` types.
//! * ARM platforms like `armv5te` that aren't for Linux only provide `load`
//! and `store` operations, and do not support Compare and Swap (CAS)
//! operations, such as `swap`, `fetch_add`, etc. Additionally on Linux,
//! these CAS operations are implemented via [operating system support], which
//! may come with a performance penalty.
//! * ARM targets with `thumbv6m` only provide `load` and `store` operations,
//! and do not support Compare and Swap (CAS) operations, such as `swap`,
//! `fetch_add`, etc.
//! * Legacy ARM platforms like ARMv4T and ARMv5TE have very limited hardware
//! support for atomics. The bare-metal targets disable this module
//! entirely, but the Linux targets [use the kernel] to assist (which comes
//! with a performance penalty). It's not until ARMv6K onwards that ARM CPUs
//! have support for load/store and Compare and Swap (CAS) atomics in hardware.
//! * ARMv6-M and ARMv8-M baseline targets (`thumbv6m-*` and
//! `thumbv8m.base-*`) only provide `load` and `store` operations, and do
//! not support Compare and Swap (CAS) operations, such as `swap`,
//! `fetch_add`, etc. Full CAS support is available on ARMv7-M and ARMv8-M
//! Mainline (`thumbv7m-*`, `thumbv7em*` and `thumbv8m.main-*`).
//!
//! [operating system support]: https://www.kernel.org/doc/Documentation/arm/kernel_user_helpers.txt
//! [use the kernel]: https://www.kernel.org/doc/Documentation/arm/kernel_user_helpers.txt
//!
//! Note that future platforms may be added that also do not have support for
//! some atomic operations. Maximally portable code will want to be careful