Pass PRIVATE to target_link_libraries if using shared libraries.

A shared library (unlike a .a), has its dependencies recorded in the library and
we can pass PRIVATE to target_link_libraries.

This patch then removes some bogus dependencies when using
BUILD_SHARED_LIBS=ON. For example, we go from

build lib/CodeGen/CMakeFiles/LLVMCodeGen.dir/AggressiveAntiDepBreaker.cpp.o:
CXX_COMPILER /home/espindola/llvm/llvm/lib/CodeGen/AggressiveAntiDepBreaker.cpp
|| include/llvm/IR/intrinsics_gen lib/libLLVMSupport.so
lib/libLLVMCore.so lib/libLLVMBitReader.so
lib/libLLVMTransformUtils.so lib/libLLVMInstCombine.so
lib/libLLVMScalarOpts.so lib/libLLVMipa.so lib/libLLVMAnalysis.so
lib/libLLVMMCParser.so lib/libLLVMMC.so lib/libLLVMObject.so
lib/libLLVMTarget.so lib/libLLVMProfileData.so

to

build lib/CodeGen/CMakeFiles/LLVMCodeGen.dir/AggressiveAntiDepBreaker.cpp.o:
CXX_COMPILER /home/espindola/llvm/llvm/lib/CodeGen/AggressiveAntiDepBreaker.cpp
|| include/llvm/IR/intrinsics_gen lib/libLLVMSupport.so
lib/libLLVMCore.so lib/libLLVMTransformUtils.so
lib/libLLVMScalarOpts.so lib/libLLVMAnalysis.so lib/libLLVMMC.so
lib/libLLVMTarget.so

In fact, build.ninja goes from 5231028 bytes to 4896759 bytes.

With this, old verisons of bfd ld (2.24 is OK, 2.23 warns) will print a bogus
warning when building with BUILD_SHARED_LIBS.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@221530 91177308-0d34-0410-b5e6-96231b3b80d8

CMakeLists.txt

@@ -469,6 +469,16 @@ else(UNIX)
   endif(NOT DEFINED CMAKE_INSTALL_RPATH)
 endif()
 
+# Work around a broken bfd ld behavior. When linking a binary with a
+# foo.so library, it will try to find any library that foo.so uses and
+# check its symbols. This is wasteful (the check was done when foo.so
+# was created) and can fail since it is not the dynamic linker and
+# doesn't know how to handle search paths correctly.
+if (UNIX AND NOT APPLE)
+  set(CMAKE_EXE_LINKER_FLAGS
+      "${CMAKE_EXE_LINKER_FLAGS} -Wl,-allow-shlib-undefined")
+endif()
+
 set(CMAKE_INCLUDE_CURRENT_DIR ON)
 
 include_directories( ${LLVM_INCLUDE_DIR} ${LLVM_MAIN_INCLUDE_DIR})

cmake/modules/AddLLVM.cmake

@@ -359,22 +359,8 @@ function(llvm_add_library name)
       ${lib_deps}
       ${llvm_libs}
       )
-  elseif((CYGWIN OR WIN32) AND ARG_SHARED)
-    # Win32's import library may be unaware of its dependent libs.
-    target_link_libraries(${name} PRIVATE
-      ${ARG_LINK_LIBS}
-      ${lib_deps}
-      ${llvm_libs}
-      )
-  elseif(ARG_SHARED AND BUILD_SHARED_LIBS)
-    # FIXME: It may be PRIVATE since SO knows its dependent libs.
-    target_link_libraries(${name} PUBLIC
-      ${ARG_LINK_LIBS}
-      ${lib_deps}
-      ${llvm_libs}
-      )
   else()
-    # MODULE|SHARED
+    # We can use PRIVATE since SO knows its dependent libs.
     target_link_libraries(${name} PRIVATE
       ${ARG_LINK_LIBS}
       ${lib_deps}