Support atomic CAS on no-std pre-v6 ARM using SWP instruction

CHANGELOG.md

@@ -10,6 +10,8 @@ Note: In this file, do not use the hard wrap in the middle of a sentence for com
 
 ## [Unreleased]
 
+- Support atomic CAS on no-std pre-v6 ARM targets (e.g., thumbv4t-none-eabi) under `--cfg portable_atomic_arm_swp_lock`.
+
 ## [0.3.10] - 2022-08-03
 
 - Optimize AArch64 128-bit atomic load when the `lse` target feature is enabled at compile-time. ([#20](https://github.com/taiki-e/portable-atomic/pull/20))

README.md

@@ -14,7 +14,7 @@ Portable atomic types including support for 128-bit atomics, atomic float, etc.
 - Provide `AtomicF32` and `AtomicF64`. (optional)
 <!-- - Provide generic `Atomic<T>` type. (optional) -->
 - Provide atomic load/store for targets where atomic is not available at all in the standard library. (RISC-V without A-extension, MSP430, AVR)
-- Provide atomic CAS for targets where atomic CAS is not available in the standard library. (thumbv6m, RISC-V without A-extension, MSP430, AVR) (optional, [single-core only](#optional-cfg))
+- Provide atomic CAS for targets where atomic CAS is not available in the standard library. (thumbv6m, thumbv4t, RISC-V without A-extension, MSP430, AVR) ([optional](#optional-cfg))
 - Provide stable equivalents of the standard library atomic types' unstable APIs, such as [`AtomicPtr::fetch_*`](https://github.com/rust-lang/rust/issues/99108), [`AtomicBool::fetch_not`](https://github.com/rust-lang/rust/issues/98485).
 - Make features that require newer compilers, such as [fetch_max](https://doc.rust-lang.org/std/sync/atomic/struct.AtomicUsize.html#method.fetch_max), [fetch_min](https://doc.rust-lang.org/std/sync/atomic/struct.AtomicUsize.html#method.fetch_min), [fetch_update](https://doc.rust-lang.org/std/sync/atomic/struct.AtomicPtr.html#method.fetch_update), and [stronger CAS failure ordering](https://github.com/rust-lang/rust/pull/98383) available on Rust 1.34+.
 
@@ -75,6 +75,15 @@ See [this list](https://github.com/taiki-e/portable-atomic/issues/10#issuecommen
 
   Enabling this cfg for targets that have atomic CAS will result in a compile error.
 
+  Supported platforms: ARMv6-M (thumbv6m), RISC-V without A-extension, MSP430, AVR
+
+- **`--cfg portable_atomic_arm_swp_lock`**<br>
+  Use global lock using `SWP` instruction on pre-v6 ARM targets where atomic CAS is not available in the standard library.
+
+  Enabling this cfg for targets that have atomic CAS will result in a compile error.
+
+  Supported platforms: pre-v6 ARM (thumbv4t, armv4t, etc.)
+
 ## Related Projects
 
 - [atomic-maybe-uninit]: Atomic operations on potentially uninitialized integers.