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

4 files changed, 4 insertions, 4 deletions

diff --git a/llvm/lib/Target/NVPTX/MCTargetDesc/NVPTXMCTargetDesc.cpp b/llvm/lib/Target/NVPTX/MCTargetDesc/NVPTXMCTargetDesc.cpp
index c8b85b2718a..d758c2c8695 100644
--- a/llvm/lib/Target/NVPTX/MCTargetDesc/NVPTXMCTargetDesc.cpp
+++ b/llvm/lib/Target/NVPTX/MCTargetDesc/NVPTXMCTargetDesc.cpp
@@ -66,7 +66,7 @@ static MCTargetStreamer *createTargetAsmStreamer(MCStreamer &S,
 }
 
 // Force static initialization.
-extern "C" void LLVMInitializeNVPTXTargetMC() {
+extern "C" LLVM_EXTERNAL_VISIBILITY void LLVMInitializeNVPTXTargetMC() {
   for (Target *T : {&getTheNVPTXTarget32(), &getTheNVPTXTarget64()}) {
     // Register the MC asm info.
     RegisterMCAsmInfo<NVPTXMCAsmInfo> X(*T);
diff --git a/llvm/lib/Target/NVPTX/NVPTXAsmPrinter.cpp b/llvm/lib/Target/NVPTX/NVPTXAsmPrinter.cpp
index 307f4d58c3a..7117438dc50 100644
--- a/llvm/lib/Target/NVPTX/NVPTXAsmPrinter.cpp
+++ b/llvm/lib/Target/NVPTX/NVPTXAsmPrinter.cpp
@@ -2258,7 +2258,7 @@ void NVPTXAsmPrinter::printMemOperand(const MachineInstr *MI, int opNum,
 }
 
 // Force static initialization.
-extern "C" void LLVMInitializeNVPTXAsmPrinter() {
+extern "C" LLVM_EXTERNAL_VISIBILITY void LLVMInitializeNVPTXAsmPrinter() {
   RegisterAsmPrinter<NVPTXAsmPrinter> X(getTheNVPTXTarget32());
   RegisterAsmPrinter<NVPTXAsmPrinter> Y(getTheNVPTXTarget64());
 }
diff --git a/llvm/lib/Target/NVPTX/NVPTXTargetMachine.cpp b/llvm/lib/Target/NVPTX/NVPTXTargetMachine.cpp
index f58fb571777..0778706d936 100644
--- a/llvm/lib/Target/NVPTX/NVPTXTargetMachine.cpp
+++ b/llvm/lib/Target/NVPTX/NVPTXTargetMachine.cpp
@@ -72,7 +72,7 @@ void initializeNVPTXProxyRegErasurePass(PassRegistry &);
 
 } // end namespace llvm
 
-extern "C" void LLVMInitializeNVPTXTarget() {
+extern "C" LLVM_EXTERNAL_VISIBILITY void LLVMInitializeNVPTXTarget() {
   // Register the target.
   RegisterTargetMachine<NVPTXTargetMachine32> X(getTheNVPTXTarget32());
   RegisterTargetMachine<NVPTXTargetMachine64> Y(getTheNVPTXTarget64());
diff --git a/llvm/lib/Target/NVPTX/TargetInfo/NVPTXTargetInfo.cpp b/llvm/lib/Target/NVPTX/TargetInfo/NVPTXTargetInfo.cpp
index 2c71ec58ec4..235be9c0dbb 100644
--- a/llvm/lib/Target/NVPTX/TargetInfo/NVPTXTargetInfo.cpp
+++ b/llvm/lib/Target/NVPTX/TargetInfo/NVPTXTargetInfo.cpp
@@ -19,7 +19,7 @@ Target &llvm::getTheNVPTXTarget64() {
   return TheNVPTXTarget64;
 }
 
-extern "C" void LLVMInitializeNVPTXTargetInfo() {
+extern "C" LLVM_EXTERNAL_VISIBILITY void LLVMInitializeNVPTXTargetInfo() {
   RegisterTarget<Triple::nvptx> X(getTheNVPTXTarget32(), "nvptx",
                                   "NVIDIA PTX 32-bit", "NVPTX");
   RegisterTarget<Triple::nvptx64> Y(getTheNVPTXTarget64(), "nvptx64",