CMake: Make most target symbols hidden by default

Summary:
For builds with LLVM_BUILD_LLVM_DYLIB=ON and BUILD_SHARED_LIBS=OFF
this change makes all symbols in the target specific libraries hidden
by default.

A new macro called LLVM_EXTERNAL_VISIBILITY has been added to mark symbols in these
libraries public, which is mainly needed for the definitions of the
LLVMInitialize* functions.

This patch reduces the number of public symbols in libLLVM.so by about
25%.  This should improve load times for the dynamic library and also
make abi checker tools, like abidiff require less memory when analyzing
libLLVM.so

One side-effect of this change is that for builds with
LLVM_BUILD_LLVM_DYLIB=ON and LLVM_LINK_LLVM_DYLIB=ON some unittests that
access symbols that are no longer public will need to be statically linked.

Before and after public symbol counts (using gcc 8.2.1, ld.bfd 2.31.1):
nm before/libLLVM-9svn.so | grep ' [A-Zuvw] ' | wc -l
36221
nm after/libLLVM-9svn.so | grep ' [A-Zuvw] ' | wc -l
26278

Reviewers: chandlerc, beanz, mgorny, rnk, hans

Reviewed By: rnk, hans

Subscribers: merge_guards_bot, luismarques, smeenai, ldionne, lenary, s.egerton, pzheng, sameer.abuasal, MaskRay, wuzish, echristo, Jim, hiraditya, michaelplatings, chapuni, jholewinski, arsenm, dschuff, jyknight, dylanmckay, sdardis, nemanjai, jvesely, javed.absar, sbc100, jgravelle-google, aheejin, kbarton, fedor.sergeev, asb, rbar, johnrusso, simoncook, apazos, sabuasal, niosHD, jrtc27, zzheng, edward-jones, mgrang, atanasyan, rogfer01, MartinMosbeck, brucehoult, the_o, PkmX, jocewei, kristina, jsji, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D54439

1 files changed, 20 insertions, 0 deletions

diff --git a/llvm/utils/unittest/CMakeLists.txt b/llvm/utils/unittest/CMakeLists.txt
index 0850d49e3a3..5aad048f2c3 100644
--- a/llvm/utils/unittest/CMakeLists.txt
+++ b/llvm/utils/unittest/CMakeLists.txt
@@ -71,3 +71,23 @@ add_llvm_library(gtest
 )
 
 add_subdirectory(UnitTestMain)
+
+# When LLVM_LINK_LLVM_DYLIB is enabled, libLLVM.so is added to the interface
+# link libraries for gtest and gtest_main.  This means that any target, like
+# unittests for example, that links against gtest will be forced to link
+# against libLLVM.so.  In some cases we may want to statically unittests if they
+# need access to symbols that are marked private in libLLVM.so.  The only
+# way we can make this work is to remove libLLVM.so from the list of interface
+# link libraries for gtest and then make gtest users responsible for explicitly
+# adding libLLVM.so to their targets link libraries if they need it.
+
+function (gtest_remove_dylib_from_link_interface target)
+  get_target_property(interface_libs ${target} INTERFACE_LINK_LIBRARIES)
+  if (interface_libs)
+    list(REMOVE_ITEM interface_libs LLVM)
+    set_target_properties(${target} PROPERTIES INTERFACE_LINK_LIBRARIES "${interface_libs}")
+  endif()
+endfunction()
+
+gtest_remove_dylib_from_link_interface(gtest)
+gtest_remove_dylib_from_link_interface(gtest_main)