[CMake] Switch the CMP0091 policy (MSVC_RUNTIME_LIBRARY) to the new behaviour

With the new behaviour, the /MD or similar options aren't added to
e.g. CMAKE_CXX_FLAGS_RELEASE, but are added separately by CMake.
They can be changed by the cmake variable
CMAKE_MSVC_RUNTIME_LIBRARY or with the target property
MSVC_RUNTIME_LIBRARY.

LLVM has had its own custom CMake flags, e.g. LLVM_USE_CRT_RELEASE,
which affects which CRT is used for release mode builds. Deprecate
these and direct users to use CMAKE_MSVC_RUNTIME_LIBRARY directly
instead (and do a best effort attempt at setting CMAKE_MSVC_RUNTIME_LIBRARY
based on the existing LLVM_USE_CRT_ flags). This only handles the
simple cases, it doesn't handle multi-config generators with
different LLVM_USE_CRT_* variables for different configs though,
but that's probably fine - we should move over to the new upstream
CMake mechanism anyway, and push users towards that.

Change code in compiler-rt, that previously tried to override the
CRT choice to /MT, to set CMAKE_MSVC_RUNTIME_LIBRARY instead of
meddling in the old variables.

This resolves the policy issue in
https://github.com/llvm/llvm-project/issues/63286, and should
handle the issues that were observed originally when the
minimum CMake version was bumped, in
https://github.com/llvm/llvm-project/issues/62719 and
https://github.com/llvm/llvm-project/issues/62739.

Differential Revision: https://reviews.llvm.org/D155233

2 files changed, 13 insertions, 14 deletions

diff --git a/compiler-rt/CMakeLists.txt b/compiler-rt/CMakeLists.txt
index d004474..fd0430e 100644
--- a/compiler-rt/CMakeLists.txt
+++ b/compiler-rt/CMakeLists.txt
@@ -387,20 +387,19 @@ if("${COMPILER_RT_DEFAULT_TARGET_ARCH}" MATCHES "s390x")
 endif()
 
 if(MSVC)
-  # Replace the /M[DT][d] flags with /MT, and strip any definitions of _DEBUG,
-  # which cause definition mismatches at link time.
   # FIXME: In fact, sanitizers should support both /MT and /MD, see PR20214.
-  if(COMPILER_RT_HAS_MT_FLAG)
-    foreach(flag_var
-      CMAKE_C_FLAGS CMAKE_C_FLAGS_DEBUG CMAKE_C_FLAGS_RELEASE
-      CMAKE_C_FLAGS_MINSIZEREL CMAKE_C_FLAGS_RELWITHDEBINFO
-      CMAKE_CXX_FLAGS CMAKE_CXX_FLAGS_DEBUG CMAKE_CXX_FLAGS_RELEASE
-      CMAKE_CXX_FLAGS_MINSIZEREL CMAKE_CXX_FLAGS_RELWITHDEBINFO)
-      string(REGEX REPLACE "/M[DT]d" "/MT" ${flag_var} "${${flag_var}}")
-      string(REGEX REPLACE "/MD" "/MT" ${flag_var} "${${flag_var}}")
-      string(REGEX REPLACE "/D_DEBUG" "" ${flag_var} "${${flag_var}}")
-    endforeach()
-  endif()
+  set(CMAKE_MSVC_RUNTIME_LIBRARY MultiThreaded)
+  # Remove any /M[DT][d] flags, and strip any definitions of _DEBUG.
+  # TODO: We probably could remove this altogether.
+  foreach(flag_var
+    CMAKE_C_FLAGS CMAKE_C_FLAGS_DEBUG CMAKE_C_FLAGS_RELEASE
+    CMAKE_C_FLAGS_MINSIZEREL CMAKE_C_FLAGS_RELWITHDEBINFO
+    CMAKE_CXX_FLAGS CMAKE_CXX_FLAGS_DEBUG CMAKE_CXX_FLAGS_RELEASE
+    CMAKE_CXX_FLAGS_MINSIZEREL CMAKE_CXX_FLAGS_RELWITHDEBINFO)
+    string(REGEX REPLACE "[/-]M[DT]d" "" ${flag_var} "${${flag_var}}")
+    string(REGEX REPLACE "[/-]MD" "" ${flag_var} "${${flag_var}}")
+    string(REGEX REPLACE "[/-]D_DEBUG" "" ${flag_var} "${${flag_var}}")
+  endforeach()
   append_list_if(COMPILER_RT_HAS_Oy_FLAG /Oy- SANITIZER_COMMON_CFLAGS)
   append_list_if(COMPILER_RT_HAS_GS_FLAG /GS- SANITIZER_COMMON_CFLAGS)
 
diff --git a/compiler-rt/lib/orc/CMakeLists.txt b/compiler-rt/lib/orc/CMakeLists.txt
index 7943f3f..38dd233 100644
--- a/compiler-rt/lib/orc/CMakeLists.txt
+++ b/compiler-rt/lib/orc/CMakeLists.txt
@@ -131,7 +131,7 @@ else() # not Apple
       )
 
     if (MSVC)
-      set(ORC_CFLAGS "${ORC_CFLAGS} /MD")
+      set(CMAKE_MSVC_RUNTIME_LIBRARY "MultiThreadedDLL")
     endif()
   else()
     set(ORC_BUILD_TYPE STATIC)