[libcxx] By default, do not use internal_linkage to hide symbols from the ABI

Summary:
https://reviews.llvm.org/D49240 led to symbol size problems in Chromium, and
we expect this may be the case in other projects built in debug mode too.
Instead, unless users explicitly ask for internal_linkage, we use always_inline
like we used to.

In the future, when we have a solution that allows us to drop always_inline
without falling back on internal_linkage, we can replace always_inline by
that.

Note that this commit introduces a change in contract for existing libc++
users: by default, libc++ used to guarantee that TUs built with different
versions of libc++ could be linked together. With the introduction of the
_LIBCPP_HIDE_FROM_ABI_PER_TU macro, the default behavior is that TUs built
with different libc++ versions are not guaranteed to link. This is a change
in contract but not a change in behavior, since the current implementation
still allows linking TUs built with different libc++ versions together.

Reviewers: EricWF, mclow.lists, dexonsmith, hans, rnk

Subscribers: christof, cfe-commits

Differential Revision: https://reviews.llvm.org/D50652

2 files changed, 45 insertions, 3 deletions

diff --git a/libcxx/docs/BuildingLibcxx.rst b/libcxx/docs/BuildingLibcxx.rst
index d0b03c675c8..daf9229a0d0 100644
--- a/libcxx/docs/BuildingLibcxx.rst
+++ b/libcxx/docs/BuildingLibcxx.rst
@@ -332,6 +332,15 @@ libc++ Feature Options
   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/libcxx/docs/DesignDocs/VisibilityMacros.rst b/libcxx/docs/DesignDocs/VisibilityMacros.rst
index 493b445750e..9a757ff53f3 100644
--- a/libcxx/docs/DesignDocs/VisibilityMacros.rst
+++ b/libcxx/docs/DesignDocs/VisibilityMacros.rst
@@ -42,9 +42,7 @@ Visibility Macros
 
 **_LIBCPP_HIDE_FROM_ABI**
   Mark a function as not being part of the ABI of any final linked image that
-  uses it, and also as being internal to each TU that uses that function. In
-  other words, the address of a function marked with this attribute is not
-  guaranteed to be the same across translation units.
+  uses it.
 
 **_LIBCPP_HIDE_FROM_ABI_AFTER_V1**
   Mark a function as being hidden from the ABI (per `_LIBCPP_HIDE_FROM_ABI`)
@@ -61,6 +59,41 @@ Visibility Macros
   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 overriden by users.
+  Otherwise, per TU ABI insulation is disabled by default, and
+  `_LIBCPP_HIDE_FROM_ABI_PER_TU` is defined to 0 unless overriden 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 overriden 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.