[builtins] Add COMPILER_RT_BUILTINS_HIDE_SYMBOLS

[rprichard]

Summary

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.

[rprichard]

I'm using this change in https://android-review.googlesource.com/c/toolchain/llvm_android/+/1532841.

The named seemed a bit long to me, but it was consistent with the *_HERMETIC_STATIC_LIBRARY option name in the other runtime libraries.

[MaskRay]

LG

compiler-rt/lib/builtins/CMakeLists.txt

51

Can this be clarified?

[rprichard]

compiler-rt/lib/builtins/CMakeLists.txt

51

I suppose -- do you have a suggestion?

Currently the option text is the same as the other *_HERMETIC_STATIC_LIBRARY options, and none of them have comments above them. The flag marks all defined global symbols as hidden. (I suppose on Mach-O, it marks them with .private_extern instead.) Even when the hermetic flag is turned off, some symbols are still hidden.

I think for the other projects, the hermetic switch also inhibits the use of dllexport on Windows, whereas the builtins library never uses dllexport.

I think there's no shared library configuration for the builtins library, and it's unusual to want the builtin symbols exported when they're linked into something.

[compnerd]

I really would prefer the alternate spelling, but the change itself is okay.

compiler-rt/lib/builtins/CMakeLists.txt

51

Im not sure that the name of the option is really that great - hermeticity in this case is not really the generally understood meaning as libraries on the system will not influence the build of the builtins, but rather the library will influence other libraries. I think that this really should be COMPILER_RT_BUILTINS_HIDE_SYMBOLS which really makes it much more obvious what this is doing.

[rprichard]

Rename flag to COMPILER_RT_BUILTINS_HIDE_SYMBOLS.

[compnerd]

Thanks!

compiler-rt/lib/builtins/CMakeLists.txt

if (COMPILER_RT_STANDALONE_BUILD)
   # LLVM configuration is not being utilized.
   option(COMPILER_RT_BUILTINS_ENABLE_PIC
     "Turns on or off -fPIC for the builtin library source"
     ON)
 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
   absvdi2.c
   absvsi2.c
   absvti2.c
   adddf3.c
   addsf3.c
else ()
 
   # These flags would normally be added to CMAKE_C_FLAGS by the llvm
   # cmake step. Add them manually if this is a standalone build.
   if(COMPILER_RT_STANDALONE_BUILD)
     if(COMPILER_RT_BUILTINS_ENABLE_PIC)
       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)
       append_list_if(COMPILER_RT_HAS_OMIT_FRAME_POINTER_FLAG -fomit-frame-pointer BUILTIN_CFLAGS)
     endif()
   endif()
 
   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()
 
   append_list_if(COMPILER_RT_HAS_ASM_LSE HAS_ASM_LSE BUILTIN_DEFS)
 
   foreach (arch ${BUILTIN_SUPPORTED_ARCH})
     if (CAN_TARGET_${arch})
       # For ARM archs, exclude any VFP builtins if VFP is not supported