[compiler-rt] suppress LNK4221 warning when creating static libraries

Summary:
We put empty object files in archives, which causes MSVC's linker to
complain about these objects not defining any previously undefined
symbols. Since we do it on purpose, this only creates noise during
the build process. This patch causes us to suppress the warnings.

Reviewers: rnk, samsonov

Subscribers: dberris, mgorny

Differential Revision: https://reviews.llvm.org/D28262

llvm-svn: 291011

Author: inglorion

compiler-rt/CMakeLists.txt

@@ -225,6 +225,17 @@ append_list_if(COMPILER_RT_HAS_WD4800_FLAG /wd4800 SANITIZER_COMMON_CFLAGS)
 # Warnings to turn off for all libraries, not just sanitizers.
 append_string_if(COMPILER_RT_HAS_WUNUSED_PARAMETER_FLAG -Wno-unused-parameter CMAKE_C_FLAGS CMAKE_CXX_FLAGS)
 
+if (CMAKE_LINKER MATCHES "link.exe$")
+  # Silence MSVC linker warnings caused by empty object files. The
+  # sanitizer libraries intentionally use ifdefs that result in empty
+  # files, rather than skipping these files in the build system.
+  # Ideally, we would pass this flag only for the libraries that need
+  # it, but CMake doesn't seem to have a way to set linker flags for
+  # individual static libraries, so we enable the suppression flag for
+  # the whole compiler-rt project.
+  append("/IGNORE:4221" CMAKE_STATIC_LINKER_FLAGS)
+endif()
+
 add_subdirectory(include)
 
 set(COMPILER_RT_LIBCXX_PATH ${LLVM_MAIN_SRC_DIR}/projects/libcxx)