Resubmit "[CMake] Change lldbAPI to be a CMake OBJECT library."

This resubmits r230380.  The primary cause of the failure was
actually just a warning, which we can disable at the CMake level
in a followup patch on the LLVM side.  The other thing which was
actually an error on the bot should be able to be fixed with
a clean.

llvm-svn: 230389

1 files changed, 17 insertions, 2 deletions

diff --git a/lldb/source/API/CMakeLists.txt b/lldb/source/API/CMakeLists.txt
index 65ce88e4b97..a6f4349b7b7 100644
--- a/lldb/source/API/CMakeLists.txt
+++ b/lldb/source/API/CMakeLists.txt
@@ -1,10 +1,25 @@
 set(LLVM_NO_RTTI 1)
 
 if ( CMAKE_SYSTEM_NAME MATCHES "Windows" )
-add_definitions( -DEXPORT_LIBLLDB )
+  add_definitions( -DEXPORT_LIBLLDB )
 endif()
 
-add_lldb_library(lldbAPI
+# An OBJECT library is a special type of CMake library that produces
+# no archive, has no link interface, and no link inputs.  It is like
+# a regular archive, just without the physical output.  To link against
+# an OBJECT library, you reference it in the *source* file list of a
+# library using the special syntax $<TARGET_OBJECTS:lldbAPI>.  This will
+# cause every object file to be passed to the linker independently, as
+# opposed to a single archive being passed to the linker.
+#
+# This is *extremely* important on Windows.  lldbAPI exports all of the
+# SB classes using __declspec(dllexport).  Unfortunately for technical
+# reasons it is not possible (well, extremely difficult) to get the linker
+# to propagate a __declspec(dllexport) attribute from a symbol in an
+# object file in an archive to a DLL that links against that archive.
+# The solution to this is for the DLL to link the object file directly.
+# So lldbAPI must be an OBJECT library.
+add_lldb_library(lldbAPI OBJECT
   SBAddress.cpp
   SBAttachInfo.cpp
   SBBlock.cpp