[libc++] Define new/delete in libc++abi only by default
Previously, we would define new/delete in both libc++ and libc++abi.
Not only does this cause code bloat, but also it's technically an ODR
violation since we don't know which operator will be selected. Furthermore,
since those are weak definitions, we should strive to have as few of them
as possible (to improve load times).
My preferred choice would have been to put the operators in libc++ only
by default, however that would create a circular dependency between
libc++ and libc++abi, which GNU linkers don't handle.
Folks who want to ship new/delete in libc++ instead of libc++abi are
free to do so by turning on LIBCXX_ENABLE_NEW_DELETE_DEFINITIONS at
CMake configure time.
On Apple platforms, this shouldn't be an ABI break because we re-export
the new/delete symbols from libc++abi. This change actually makes libc++
behave closer to the system libc++ shipped on Apple platforms.
On other platforms, this is an ABI break for people linking against libc++
but not libc++abi. However, vendors have been consulted in D68269 and no
objection was raised. Furthermore, the definitions can be controlled to
appear in libc++ instead with the CMake option.
Differential Revision: https://reviews.llvm.org/D68269
GitOrigin-RevId: 9b40ee8eb0c194f4b2787801ac6f9ef8fc1b8f46
diff --git a/docs/ReleaseNotes.rst b/docs/ReleaseNotes.rst
index 5157685..6079b6f 100644
--- a/docs/ReleaseNotes.rst
+++ b/docs/ReleaseNotes.rst
@@ -42,4 +42,10 @@
API Changes
-----------
-- ...
+- By default, libc++ will _not_ include the definition for new and delete,
+ since those are provided in libc++abi. Vendors wishing to provide new and
+ delete in libc++ can build the library with ``-DLIBCXX_ENABLE_NEW_DELETE_DEFINITIONS=ON``
+ to get back the old behavior. This was done to avoid providing new and delete
+ in both libc++ and libc++abi, which is technically an ODR violation. Also
+ note that we couldn't decide to put the operators in libc++ only, because
+ they are needed from libc++abi (which would create a circular dependency).