[builtins] Add COMPILER_RT_BUILTINS_HIDE_SYMBOLS

On Android, when the builtins are linked into a binary, they are
typically linked using -Wl,--exclude-libs so that the symbols aren't
reexported. For the NDK, compiler-rt's default behavior (build the
builtins archive with -fvisibility=hidden) is better so that builtins
are hidden even without -Wl,--exclude-libs.

Android needs the builtins with non-hidden symbols only for a special
case: for backwards compatibility with old binaries, the libc.so and
libm.so DSOs in the platform need to export some builtins for arm32 and
32-bit x86. See D56977.

Control the behavior with a new flag,
`COMPILER_RT_BUILTINS_HIDE_SYMBOLS`, that behaves similarly to the
`*_HERMETIC_STATIC_LIBRARY` in libunwind/libcxx/libcxxabi, so that
Android can build a special builtins variant for libc.so/libm.so.

Unlike the hermetic flags for other projects, this new flag is enabled
by default.

Reviewed By: compnerd, MaskRay

Differential Revision: https://reviews.llvm.org/D93431

diff --git a/compiler-rt/lib/builtins/CMakeLists.txt b/compiler-rt/lib/builtins/CMakeLists.txt
index 5259e95..b511a9a 100644 (file)
--- a/compiler-rt/lib/builtins/CMakeLists.txt
+++ b/compiler-rt/lib/builtins/CMakeLists.txt
@@ -47,6 +47,9 @@ endif()
 
 include(builtin-config-ix)
 
+option(COMPILER_RT_BUILTINS_HIDE_SYMBOLS
+  "Do not export any symbols from the static library." ON)
+
 # TODO: Need to add a mechanism for logging errors when builtin source files are
 # added to a sub-directory and not this CMakeLists file.
 set(GENERIC_SOURCES
@@ -666,7 +669,7 @@ else ()
       append_list_if(COMPILER_RT_HAS_FPIC_FLAG -fPIC BUILTIN_CFLAGS)
     endif()
     append_list_if(COMPILER_RT_HAS_FNO_BUILTIN_FLAG -fno-builtin BUILTIN_CFLAGS)
-    if(NOT ANDROID)
+    if(COMPILER_RT_BUILTINS_HIDE_SYMBOLS)
       append_list_if(COMPILER_RT_HAS_VISIBILITY_HIDDEN_FLAG -fvisibility=hidden BUILTIN_CFLAGS)
     endif()
     if(NOT COMPILER_RT_DEBUG)
@@ -676,7 +679,7 @@ else ()
 
   set(BUILTIN_DEFS "")
 
-  if(NOT ANDROID)
+  if(COMPILER_RT_BUILTINS_HIDE_SYMBOLS)
     append_list_if(COMPILER_RT_HAS_VISIBILITY_HIDDEN_FLAG VISIBILITY_HIDDEN BUILTIN_DEFS)
   endif()