[libc++] Introduce _LIBCPP_HIDE_FROM_ABI to replace _LIBCPP_INLINE_VISIBILITY

Summary:
This commit introduces a new macro, _LIBCPP_HIDE_FROM_ABI, whose goal is to
mark functions that shouldn't be part of libc++'s ABI. It marks the functions
as being hidden for dylib visibility purposes, and as having internal linkage
using Clang's __attribute__((internal_linkage)) when available, and
__always_inline__ otherwise.

It replaces _LIBCPP_INLINE_VISIBILITY, which was always using __always_inline__
to achieve similar goals, but suffered from debuggability and code size problems.
The full proposal, along with more background information, can be found here:

    http://lists.llvm.org/pipermail/cfe-dev/2018-July/058419.html

This commit does not rename uses of _LIBCPP_INLINE_VISIBILITY to
_LIBCPP_HIDE_FROM_ABI: this wide reaching but mechanical change can
be done later when we've confirmed we're happy with the new macro.

In the future, it would be nice if we could optionally allow dropping
any internal_linkage or __always_inline__ attribute, which could result
in code size improvements. However, this is currently impossible for
reasons explained here: http://lists.llvm.org/pipermail/cfe-dev/2018-July/058450.html

Reviewers: EricWF, dexonsmith, mclow.lists

Subscribers: christof, dexonsmith, llvm-commits, mclow.lists

Differential Revision: https://reviews.llvm.org/D49240

llvm-svn: 338122

libcxx/docs/DesignDocs/VisibilityMacros.rst

@@ -40,7 +40,7 @@ Visibility Macros
   this macro therefore expands to `__declspec(dllexport)` when building the
   library and has an empty definition otherwise.
 
-**_LIBCPP_INLINE_VISIBILITY**
+**_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
@@ -155,6 +155,22 @@ Visibility Macros
   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
 =====
 

libcxx/include/__config

@@ -491,6 +491,8 @@ namespace std {
 #define _LIBCPP_HAS_UNIQUE_OBJECT_REPRESENTATIONS
 #endif
 
+#define _LIBCPP_ALWAYS_INLINE __attribute__ ((__always_inline__))
+
 #elif defined(_LIBCPP_COMPILER_GCC)
 
 #define _ALIGNAS(x) __attribute__((__aligned__(x)))
@@ -582,6 +584,8 @@ namespace std {
 #define _LIBCPP_HAS_UNIQUE_OBJECT_REPRESENTATIONS
 #endif
 
+#define _LIBCPP_ALWAYS_INLINE __attribute__ ((__always_inline__))
+
 #elif defined(_LIBCPP_COMPILER_MSVC)
 
 #define _LIBCPP_TOSTRING2(x) #x
@@ -614,6 +618,8 @@ namespace std {
 
 #define _LIBCPP_HAS_NO_ASAN
 
+#define _LIBCPP_ALWAYS_INLINE __forceinline
+
 #elif defined(_LIBCPP_COMPILER_IBM)
 
 #define _ALIGNAS(x) __attribute__((__aligned__(x)))
@@ -644,6 +650,8 @@ namespace std {
 
 #define _LIBCPP_HAS_NO_ASAN
 
+#define _LIBCPP_ALWAYS_INLINE __attribute__ ((__always_inline__))
+
 #endif // _LIBCPP_COMPILER_[CLANG|GCC|MSVC|IBM]
 
 #if _LIBCPP_STD_VER >= 17
@@ -695,10 +703,8 @@ namespace std {
 #define _LIBCPP_ENUM_VIS
 
 #if defined(_LIBCPP_COMPILER_MSVC)
-#  define _LIBCPP_INLINE_VISIBILITY __forceinline
 #  define _LIBCPP_EXTERN_TEMPLATE_INLINE_VISIBILITY __forceinline
 #else
-#  define _LIBCPP_INLINE_VISIBILITY __attribute__ ((__always_inline__))
 #  define _LIBCPP_EXTERN_TEMPLATE_INLINE_VISIBILITY __attribute__ ((__always_inline__))
 #endif
 
@@ -785,14 +791,19 @@ namespace std {
 #define _LIBCPP_CLASS_TEMPLATE_INSTANTIATION_VIS
 #endif
 
-#ifndef _LIBCPP_INLINE_VISIBILITY
-#  if !defined(_LIBCPP_DISABLE_VISIBILITY_ANNOTATIONS)
-#    define _LIBCPP_INLINE_VISIBILITY __attribute__ ((__visibility__("hidden"), __always_inline__))
-#  else
-#    define _LIBCPP_INLINE_VISIBILITY __attribute__ ((__always_inline__))
-#  endif
+#if __has_attribute(internal_linkage)
+#  define _LIBCPP_INTERNAL_LINKAGE __attribute__ ((internal_linkage))
+#else
+#  define _LIBCPP_INTERNAL_LINKAGE _LIBCPP_ALWAYS_INLINE
 #endif
 
+#ifndef _LIBCPP_HIDE_FROM_ABI
+#  define _LIBCPP_HIDE_FROM_ABI _LIBCPP_HIDDEN _LIBCPP_INTERNAL_LINKAGE
+#endif
+
+// Just so we can migrate to _LIBCPP_HIDE_FROM_ABI gradually.
+#define _LIBCPP_INLINE_VISIBILITY _LIBCPP_HIDE_FROM_ABI
+
 #ifndef _LIBCPP_EXTERN_TEMPLATE_INLINE_VISIBILITY
 #  if !defined(_LIBCPP_DISABLE_VISIBILITY_ANNOTATIONS)
 #    define _LIBCPP_EXTERN_TEMPLATE_INLINE_VISIBILITY __attribute__((__visibility__("default"), __always_inline__))