[libc++] Always query the compiler to find whether a type is always lockfree
In https://llvm.org/D56913, we added an emulation for the __atomic_always_lock_free
compiler builtin when compiling in Freestanding mode. However, the emulation
did (and could not) give exactly the same answer as the compiler builtin,
which led to a potential ABI break for e.g. enum classes.
After speaking to the original author of D56913, we agree that the correct
behavior is to instead always use the compiler builtin, since that provides
a more accurate answer, and __atomic_always_lock_free is a purely front-end
builtin which doesn't require any runtime support. Furthermore, it is
available regardless of the Standard mode (see https://godbolt.org/z/cazf3ssYY).
However, this patch does constitute an ABI break. As shown by https://godbolt.org/z/1eoex6zdK:
- In LLVM <= 11.0.1, an atomic<enum class with 1 byte> would not contain a lock byte.
- In LLVM >= 12.0.0, an atomic<enum class with 1 byte> would contain a lock byte.
This patch breaks the ABI again to bring it back to 1 byte, which seems
like the correct thing to do.
Fixes #57440
Differential Revision: https://reviews.llvm.org/D133377
NOKEYCHECK=True
GitOrigin-RevId: f1a601fe88f99d52ca80617266897b217bcd4d64
diff --git a/docs/ReleaseNotes.rst b/docs/ReleaseNotes.rst
index 2f0fb9d..c013326 100644
--- a/docs/ReleaseNotes.rst
+++ b/docs/ReleaseNotes.rst
@@ -85,6 +85,14 @@
ABI Affecting Changes
---------------------
+- In freestanding mode, ``atomic<small enum class>`` does not contain a lock byte anymore if the platform
+ can implement lockfree atomics for that size. More specifically, in LLVM <= 11.0.1, an ``atomic<small enum class>``
+ would not contain a lock byte. This was broken in LLVM >= 12.0.0, where it started including a lock byte despite
+ the platform supporting lockfree atomics for that size. Starting in LLVM 15.0.1, the ABI for these types has been
+ restored to what it used to be (no lock byte), which is the most efficient implementation.
+
+ This ABI break only affects users that compile with ``-ffreestanding``, and only for ``atomic<T>`` where ``T``
+ is a non-builtin type that could be lockfree on the platform. See https://llvm.org/D133377 for more details.
Build System Changes
--------------------