[libc++] Re-apply the use of ABI tags to provide per-TU insulation
This commit re-applies 9ee97ce3b830, which was reverted by 61d417ce
because it broke the LLDB data formatter tests. It also re-applies
6148c79a (the manual GN change associated to it).
Differential Revision: https://reviews.llvm.org/D127444
NOKEYCHECK=True
GitOrigin-RevId: d2e86866be0f17295c5f677333c2a9952e259dd7
diff --git a/docs/BuildingLibcxx.rst b/docs/BuildingLibcxx.rst
index 3f4e314..5069c7f 100644
--- a/docs/BuildingLibcxx.rst
+++ b/docs/BuildingLibcxx.rst
@@ -412,15 +412,6 @@
Use the specified GCC toolchain and standard library when building the native
stdlib benchmark tests.
-.. option:: LIBCXX_HIDE_FROM_ABI_PER_TU_BY_DEFAULT:BOOL
-
- **Default**: ``OFF``
-
- Pick the default for whether to constrain ABI-unstable symbols to
- each individual translation unit. This setting controls whether
- `_LIBCPP_HIDE_FROM_ABI_PER_TU_BY_DEFAULT` is defined by default --
- see the documentation of that macro for details.
-
libc++ ABI Feature Options
--------------------------
diff --git a/docs/DesignDocs/VisibilityMacros.rst b/docs/DesignDocs/VisibilityMacros.rst
index a165fc4..d1c1626 100644
--- a/docs/DesignDocs/VisibilityMacros.rst
+++ b/docs/DesignDocs/VisibilityMacros.rst
@@ -65,41 +65,6 @@
ABI, we should create a new _LIBCPP_HIDE_FROM_ABI_AFTER_XXX macro, and we can
use it to start removing symbols from the ABI after that stable version.
-**_LIBCPP_HIDE_FROM_ABI_PER_TU**
- This macro controls whether symbols hidden from the ABI with `_LIBCPP_HIDE_FROM_ABI`
- are local to each translation unit in addition to being local to each final
- linked image. This macro is defined to either 0 or 1. When it is defined to
- 1, translation units compiled with different versions of libc++ can be linked
- together, since all non ABI-facing functions are local to each translation unit.
- This allows static archives built with different versions of libc++ to be linked
- together. This also means that functions marked with `_LIBCPP_HIDE_FROM_ABI`
- are not guaranteed to have the same address across translation unit boundaries.
-
- When the macro is defined to 0, there is no guarantee that translation units
- compiled with different versions of libc++ can interoperate. However, this
- leads to code size improvements, since non ABI-facing functions can be
- deduplicated across translation unit boundaries.
-
- This macro can be defined by users to control the behavior they want from
- libc++. The default value of this macro (0 or 1) is controlled by whether
- `_LIBCPP_HIDE_FROM_ABI_PER_TU_BY_DEFAULT` is defined, which is intended to
- be used by vendors only (see below).
-
-**_LIBCPP_HIDE_FROM_ABI_PER_TU_BY_DEFAULT**
- This macro controls the default value for `_LIBCPP_HIDE_FROM_ABI_PER_TU`.
- When the macro is defined, per TU ABI insulation is enabled by default, and
- `_LIBCPP_HIDE_FROM_ABI_PER_TU` is defined to 1 unless overridden by users.
- Otherwise, per TU ABI insulation is disabled by default, and
- `_LIBCPP_HIDE_FROM_ABI_PER_TU` is defined to 0 unless overridden by users.
-
- This macro is intended for vendors to control whether they want to ship
- libc++ with per TU ABI insulation enabled by default. Users can always
- control the behavior they want by defining `_LIBCPP_HIDE_FROM_ABI_PER_TU`
- appropriately.
-
- By default, this macro is not defined, which means that per TU ABI insulation
- is not provided unless explicitly overridden by users.
-
**_LIBCPP_TYPE_VIS**
Mark a type's typeinfo, vtable and members as having default visibility.
This attribute cannot be used on class templates.
@@ -194,22 +159,6 @@
versioning namespace. This allows throwing and catching some exception types
between libc++ and libstdc++.
-**_LIBCPP_INTERNAL_LINKAGE**
- Mark the affected entity as having internal linkage (i.e. the `static`
- keyword in C). This is only a best effort: when the `internal_linkage`
- attribute is not available, we fall back to forcing the function to be
- inlined, which approximates internal linkage since an externally visible
- symbol is never generated for that function. This is an internal macro
- used as an implementation detail by other visibility macros. Never mark
- a function or a class with this macro directly.
-
-**_LIBCPP_ALWAYS_INLINE**
- Forces inlining of the function it is applied to. For visibility purposes,
- this macro is used to make sure that an externally visible symbol is never
- generated in an object file when the `internal_linkage` attribute is not
- available. This is an internal macro used by other visibility macros, and
- it should not be used directly.
-
Links
=====
diff --git a/docs/ReleaseNotes.rst b/docs/ReleaseNotes.rst
index c2917d4..c6eb835 100644
--- a/docs/ReleaseNotes.rst
+++ b/docs/ReleaseNotes.rst
@@ -224,3 +224,8 @@
means that the same set of installed headers works for both DLL and static
linkage. This means that distributors finally can build both library
versions with a single CMake invocation.
+
+- The ``LIBCXX_HIDE_FROM_ABI_PER_TU_BY_DEFAULT`` configuration option has been removed. Indeed,
+ the risk of ODR violations from mixing different versions of libc++ in the same program has
+ been mitigated with a different technique that is simpler and does not have the drawbacks of
+ using internal linkage.