[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
llvm/llvm-project#63286, and should
handle the issues that were observed originally when the
minimum CMake version was bumped, in
llvm/llvm-project#62719 and
llvm/llvm-project#62739.

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

Author: mstorsjo

Diff for: cmake/Modules/CMakePolicy.cmake

@@ -1,10 +1,5 @@
 # CMake policy settings shared between LLVM projects
 
-# CMP0091: MSVC runtime library flags are selected by an abstraction.
-# New in CMake 3.15. https://cmake.org/cmake/help/latest/policy/CMP0091.html
-if(POLICY CMP0091)
-  cmake_policy(SET CMP0091 OLD)
-endif()
 # CMP0114: ExternalProject step targets fully adopt their steps.
 # New in CMake 3.19: https://cmake.org/cmake/help/latest/policy/CMP0114.html
 if(POLICY CMP0114)

Diff for: 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 for: 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)

Diff for: llvm/CMakeLists.txt

@@ -684,7 +684,7 @@ if( WIN32 AND NOT CYGWIN )
 endif()
 set(LLVM_NATIVE_TOOL_DIR "" CACHE PATH "Path to a directory containing prebuilt matching native tools (such as llvm-tblgen)")
 
-set(LLVM_INTEGRATED_CRT_ALLOC "" CACHE PATH "Replace the Windows CRT allocator with any of {rpmalloc|mimalloc|snmalloc}. Only works with /MT enabled.")
+set(LLVM_INTEGRATED_CRT_ALLOC "" CACHE PATH "Replace the Windows CRT allocator with any of {rpmalloc|mimalloc|snmalloc}. Only works with CMAKE_MSVC_RUNTIME_LIBRARY=MultiThreaded.")
 if(LLVM_INTEGRATED_CRT_ALLOC)
   if(NOT WIN32)
     message(FATAL_ERROR "LLVM_INTEGRATED_CRT_ALLOC is only supported on Windows.")

Diff for: llvm/cmake/modules/ChooseMSVCCRT.cmake

@@ -62,33 +62,32 @@ variables (LLVM_USE_CRT_DEBUG, etc) instead.")
 
   foreach(build_type ${CMAKE_CONFIGURATION_TYPES} ${CMAKE_BUILD_TYPE})
     string(TOUPPER "${build_type}" build)
-    if (NOT LLVM_USE_CRT_${build})
-      get_current_crt(LLVM_USE_CRT_${build}
-        MSVC_CRT_REGEX
-        CMAKE_CXX_FLAGS_${build})
-      set(LLVM_USE_CRT_${build}
-        "${LLVM_USE_CRT_${build}}"
-        CACHE STRING "Specify VC++ CRT to use for ${build_type} configurations."
-        FORCE)
-      set_property(CACHE LLVM_USE_CRT_${build}
-        PROPERTY STRINGS ;${${MSVC_CRT}})
-    endif(NOT LLVM_USE_CRT_${build})
-  endforeach(build_type)
-
-  foreach(build_type ${CMAKE_CONFIGURATION_TYPES} ${CMAKE_BUILD_TYPE})
-    string(TOUPPER "${build_type}" build)
-    if ("${LLVM_USE_CRT_${build}}" STREQUAL "")
-      set(flag_string " ")
-    else()
-      set(flag_string " /${LLVM_USE_CRT_${build}} ")
+    if (NOT "${LLVM_USE_CRT_${build}}" STREQUAL "")
       if (NOT ${LLVM_USE_CRT_${build}} IN_LIST ${MSVC_CRT})
         message(FATAL_ERROR
           "Invalid value for LLVM_USE_CRT_${build}: ${LLVM_USE_CRT_${build}}. Valid options are one of: ${${MSVC_CRT}}")
       endif()
-      message(STATUS "Using ${build_type} VC++ CRT: ${LLVM_USE_CRT_${build}}")
+      set(library "MultiThreaded")
+      if ("${LLVM_USE_CRT_${build}}" MATCHES "d$")
+        set(library "${library}Debug")
+      endif()
+      if ("${LLVM_USE_CRT_${build}}" MATCHES "^MD")
+        set(library "${library}DLL")
+      endif()
+      if(${runtime_library_set})
+        message(WARNING "Conflicting LLVM_USE_CRT_* options")
+      else()
+        message(WARNING "The LLVM_USE_CRT_* options are deprecated, use the CMake provided CMAKE_MSVC_RUNTIME_LIBRARY setting instead")
+      endif()
+      set(CMAKE_MSVC_RUNTIME_LIBRARY "${library}" CACHE STRING "" FORCE)
+      message(STATUS "Using VC++ CRT: ${CMAKE_MSVC_RUNTIME_LIBRARY}")
+      set(runtime_library_set 1)
     endif()
     foreach(lang C CXX)
-      set_flag_in_var(CMAKE_${lang}_FLAGS_${build} MSVC_CRT_REGEX flag_string)
+      # Clear any potentially manually set options from these variables.
+      # Kept as temporary backwards compat (unsure if necessary).
+      # TODO: We probably should remove it.
+      set_flag_in_var(CMAKE_${lang}_FLAGS_${build} MSVC_CRT_REGEX "")
     endforeach(lang)
   endforeach(build_type)
 endmacro(choose_msvc_crt MSVC_CRT)

Diff for: llvm/docs/CMake.rst

@@ -680,7 +680,7 @@ enabled sub-projects. Nearly all of these variable names begin with
     $ D:\llvm-project> cmake ... -DLLVM_INTEGRATED_CRT_ALLOC=D:\git\rpmalloc
 
   This flag needs to be used along with the static CRT, ie. if building the
-  Release target, add -DLLVM_USE_CRT_RELEASE=MT.
+  Release target, add -DCMAKE_MSVC_RUNTIME_LIBRARY=MultiThreaded.
 
 **LLVM_INSTALL_DOXYGEN_HTML_DIR**:STRING
   The path to install Doxygen-generated HTML documentation to. This path can
@@ -768,7 +768,8 @@ enabled sub-projects. Nearly all of these variable names begin with
 **LLVM_USE_CRT_{target}**:STRING
   On Windows, tells which version of the C runtime library (CRT) should be used.
   For example, -DLLVM_USE_CRT_RELEASE=MT would statically link the CRT into the
-  LLVM tools and library.
+  LLVM tools and library. This is deprecated; use ``CMAKE_MSVC_RUNTIME_LIBRARY``
+  instead.
 
 **LLVM_USE_INTEL_JITEVENTS**:BOOL
   Enable building support for Intel JIT Events API. Defaults to OFF.

Diff for: llvm/lib/Support/CMakeLists.txt

@@ -89,8 +89,8 @@ endif()
 
 # Override the C runtime allocator on Windows and embed it into LLVM tools & libraries
 if(LLVM_INTEGRATED_CRT_ALLOC)
-  if (CMAKE_BUILD_TYPE AND NOT ${LLVM_USE_CRT_${uppercase_CMAKE_BUILD_TYPE}} MATCHES "^(MT|MTd)$")
-    message(FATAL_ERROR "LLVM_INTEGRATED_CRT_ALLOC only works with /MT or /MTd. Use LLVM_USE_CRT_${uppercase_CMAKE_BUILD_TYPE} to set the appropriate option.")
+  if (NOT CMAKE_MSVC_RUNTIME_LIBRARY OR CMAKE_MSVC_RUNTIME_LIBRARY MATCHES "DLL$")
+    message(FATAL_ERROR "LLVM_INTEGRATED_CRT_ALLOC only works with CMAKE_MSVC_RUNTIME_LIBRARY set to MultiThreaded or MultiThreadedDebug.")
   endif()
 
   string(REGEX REPLACE "(/|\\\\)$" "" LLVM_INTEGRATED_CRT_ALLOC "${LLVM_INTEGRATED_CRT_ALLOC}")