[CMake] Clean up old code for handling MSVC runtime setting the old way

[mstorsjo]

Summary

This was left in place to reduce the risk of breaking anything,
and to keep the diff smaller, in D155233.

[smeenai]

LGTM, very nice.

llvm/cmake/modules/ChooseMSVCCRT.cmake

12–13

Does using generator expressions for this not work, as suggested in https://cmake.org/cmake/help/latest/variable/CMAKE_MSVC_RUNTIME_LIBRARY.html?

[mstorsjo]

llvm/cmake/modules/ChooseMSVCCRT.cmake

12–13

I haven't tested myself, but I would believe them to work.

What I'm trying to say here, is that I'm not trying to make a generic function that reads a number of LLVM_USE_CRT_* options and generates one single generator expression for CMAKE_MSVC_RUNTIME_LIBRARY that would express exactly the chosen combination.

I.e. for trivial cases with LLVM_USE_CRT_* this should help you along and you'd get a kind warning asking you to move onwards to CMAKE_MSVC_RUNTIME_LIBRARY at some point. If you have a nontrivial case, you'll to express it with a generator expression directly in CMAKE_MSVC_RUNTIME_LIBRARY right away.

I.e. "isn't supported" refers to the transitionary implementation here, not about what CMAKE_MSVC_RUNTIME_LIBRARY can do.

I guess I could reword it to be clearer.

[smeenai]

llvm/cmake/modules/ChooseMSVCCRT.cmake

12–13

Ah, got it, that makes sense.

[phosek]

LGTM

[yurybura]

This change breaks build on Windows for the Debug configuration with MSVC.

Due to /D_DEBUG and /MDd are still present on the command line, I see a build error and warning::

FAILED: projects/compiler-rt/lib/asan/CMakeFiles/RTAsan_dynamic.x86_64.dir/asan_win.cpp.obj 
C:\PROGRA~1\MICROS~1\2022\ENTERP~1\VC\Tools\MSVC\1437~1.328\bin\Hostx64\x64\cl.exe   /TP -DASAN_DYNAMIC=1 -DINTERCEPTION_DYNAMIC_CRT -DUNICODE -D_CRT_NONSTDC_NO_DEPRECATE -D_CRT_NONSTDC_NO_WARNINGS -D_CRT_SECURE_NO_DEPRECATE -D_CRT_SECURE_NO_WARNINGS -D_SCL_SECURE_NO_DEPRECATE -D_SCL_SECURE_NO_WARNINGS -D_UNICODE -D__STDC_CONSTANT_MACROS -D__STDC_FORMAT_MACROS -D__STDC_LIMIT_MACROS -ID:\buildtrees\llvm\x64-windows-dbg\projects\compiler-rt\lib\asan -ID:\buildtrees\llvm\src\org-17.0.1-4a67238d53.clean\compiler-rt\lib\asan -ID:\buildtrees\llvm\x64-windows-dbg\include -ID:\buildtrees\llvm\src\org-17.0.1-4a67238d53.clean\llvm\include -ID:\buildtrees\llvm\src\org-17.0.1-4a67238d53.clean\compiler-rt\lib\asan\.. /nologo /DWIN32 /D_WINDOWS /utf-8   /MP  /Zc:inline /Zc:preprocessor /Zc:__cplusplus /Oi /bigobj /permissive- -wd4141 -wd4146 -wd4244 -wd4267 -wd4291 -wd4351 -wd4456 -wd4457 -wd4458 -wd4459 -wd4503 -wd4624 -wd4722 -wd4100 -wd4127 -wd4512 -wd4505 -wd4610 -wd4510 -wd4702 -wd4245 -wd4706 -wd4310 -wd4701 -wd4703 -wd4389 -wd4611 -wd4805 -wd4204 -wd4577 -wd4091 -wd4592 -wd4319 -wd4709 -wd5105 -wd4324 -w14062 -we4238 /W4 /D_DEBUG /MDd /Z7 /Ob0 /Od /RTC1  -std:c++17 -MT /Oy- /GS- /Zc:threadSafeInit- /Z7 /wd4146 /wd4291 /wd4391 /wd4722 /wd4800 /GR- "/experimental:external /external:W0 /external:anglebrackets" /showIncludes /Foprojects\compiler-rt\lib\asan\CMakeFiles\RTAsan_dynamic.x86_64.dir\asan_win.cpp.obj /Fdprojects\compiler-rt\lib\asan\CMakeFiles\RTAsan_dynamic.x86_64.dir\ /FS -c D:\buildtrees\llvm\src\org-17.0.1-4a67238d53.clean\compiler-rt\lib\asan\asan_win.cpp
cl : Command line warning D9025 : overriding '/MDd' with '/MT'
cl : Command line warning D9002 : ignoring unknown option '/experimental:external /external:W0 /external:anglebrackets'
D:\buildtrees\llvm\src\org-17.0.1-4a67238d53.clean\compiler-rt\lib\asan\asan_win.cpp(257): fatal error C1189: #error:  Please build the runtime with a non-debug CRT: /MD or /MT

[mstorsjo]

In D155431#4651005, @yurybura wrote:

This change breaks build on Windows for the Debug configuration with MSVC.

Due to /D_DEBUG and /MDd are still present on the command line, I see a build error and warning::

FAILED: projects/compiler-rt/lib/asan/CMakeFiles/RTAsan_dynamic.x86_64.dir/asan_win.cpp.obj 
C:\PROGRA~1\MICROS~1\2022\ENTERP~1\VC\Tools\MSVC\1437~1.328\bin\Hostx64\x64\cl.exe   /TP -DASAN_DYNAMIC=1 -DINTERCEPTION_DYNAMIC_CRT -DUNICODE -D_CRT_NONSTDC_NO_DEPRECATE -D_CRT_NONSTDC_NO_WARNINGS -D_CRT_SECURE_NO_DEPRECATE -D_CRT_SECURE_NO_WARNINGS -D_SCL_SECURE_NO_DEPRECATE -D_SCL_SECURE_NO_WARNINGS -D_UNICODE -D__STDC_CONSTANT_MACROS -D__STDC_FORMAT_MACROS -D__STDC_LIMIT_MACROS -ID:\buildtrees\llvm\x64-windows-dbg\projects\compiler-rt\lib\asan -ID:\buildtrees\llvm\src\org-17.0.1-4a67238d53.clean\compiler-rt\lib\asan -ID:\buildtrees\llvm\x64-windows-dbg\include -ID:\buildtrees\llvm\src\org-17.0.1-4a67238d53.clean\llvm\include -ID:\buildtrees\llvm\src\org-17.0.1-4a67238d53.clean\compiler-rt\lib\asan\.. /nologo /DWIN32 /D_WINDOWS /utf-8   /MP  /Zc:inline /Zc:preprocessor /Zc:__cplusplus /Oi /bigobj /permissive- -wd4141 -wd4146 -wd4244 -wd4267 -wd4291 -wd4351 -wd4456 -wd4457 -wd4458 -wd4459 -wd4503 -wd4624 -wd4722 -wd4100 -wd4127 -wd4512 -wd4505 -wd4610 -wd4510 -wd4702 -wd4245 -wd4706 -wd4310 -wd4701 -wd4703 -wd4389 -wd4611 -wd4805 -wd4204 -wd4577 -wd4091 -wd4592 -wd4319 -wd4709 -wd5105 -wd4324 -w14062 -we4238 /W4 /D_DEBUG /MDd /Z7 /Ob0 /Od /RTC1  -std:c++17 -MT /Oy- /GS- /Zc:threadSafeInit- /Z7 /wd4146 /wd4291 /wd4391 /wd4722 /wd4800 /GR- "/experimental:external /external:W0 /external:anglebrackets" /showIncludes /Foprojects\compiler-rt\lib\asan\CMakeFiles\RTAsan_dynamic.x86_64.dir\asan_win.cpp.obj /Fdprojects\compiler-rt\lib\asan\CMakeFiles\RTAsan_dynamic.x86_64.dir\ /FS -c D:\buildtrees\llvm\src\org-17.0.1-4a67238d53.clean\compiler-rt\lib\asan\asan_win.cpp
cl : Command line warning D9025 : overriding '/MDd' with '/MT'
cl : Command line warning D9002 : ignoring unknown option '/experimental:external /external:W0 /external:anglebrackets'
D:\buildtrees\llvm\src\org-17.0.1-4a67238d53.clean\compiler-rt\lib\asan\asan_win.cpp(257): fatal error C1189: #error:  Please build the runtime with a non-debug CRT: /MD or /MT

I'm not able to reproduce this, when testing on latest git main and 17.0.1. Are you manually passing the /D_DEBUG /MDd options as cmake options somewhere? Have you done a properly clean reconfigure with cmake? Since the latest cmake policy setup, one is not supposed to pass those options manually anywhere, but just rely on setting CMAKE_MSVC_RUNTIME_LIBRARY, which does get hardcoded to MultiThreaded for the sanitizers.

[yurybura]

I'm not able to reproduce this, when testing on latest git main and 17.0.1. Are you manually passing the /D_DEBUG /MDd options as cmake options somewhere? Have you done a properly clean reconfigure with cmake? Since the latest cmake policy setup, one is not supposed to pass those options manually anywhere, but just rely on setting CMAKE_MSVC_RUNTIME_LIBRARY, which does get hardcoded to MultiThreaded for the sanitizers.

Yes, the /D_DEBUG /MDd flags are defined manually. More precisely, I use VCPKG to build LLVM, and it declares them implicitly for debug configuration. LLVM and Clang themselves build with these flags, but the compiler-rt project fails (it is added as an external CMake project to the Clang build, and without patching the code I don't know how to fix this failure). I think it's better to leave the CMake code to remove unsupported compiler flags. You can find the same code in other project. For example please refer libcxx\CMakeLists.txt :

# FIXME: Remove all debug flags and flags that change which Windows
# default libraries are linked. Currently we only support linking the
# non-debug DLLs
remove_flags("/D_DEBUG" "/MTd" "/MDd" "/MT" "/Md")

[mstorsjo]

In D155431#4651633, @yurybura wrote:

I'm not able to reproduce this, when testing on latest git main and 17.0.1. Are you manually passing the /D_DEBUG /MDd options as cmake options somewhere? Have you done a properly clean reconfigure with cmake? Since the latest cmake policy setup, one is not supposed to pass those options manually anywhere, but just rely on setting CMAKE_MSVC_RUNTIME_LIBRARY, which does get hardcoded to MultiThreaded for the sanitizers.

Yes, the /D_DEBUG /MDd flags are defined manually. More precisely, I use VCPKG to build LLVM, and it declares them implicitly for debug configuration.

Right - I'm not sure if we'd like to consider that supported - the right way of choosing CRT for the LLVM build is by using CMAKE_MSVC_RUNTIME_LIBRARY. I guess we could consider readding a couple lines in compiler-rt to strip that out though. But we don't really in general want to support users manually injecting conflicting CRT flags via the cmake flags. What do others think here, @hans @phosek?

and without patching the code I don't know how to fix this failure

Fixing the sanitizers to build correctly in that mode is most probably out of scope here; the fix you can do is stop passing such flags manually. That's the general migration the vcpkg build script should do, in order to handle CRT choice correctly in a world after https://cmake.org/cmake/help/latest/policy/CMP0091.html is in effect.

You can find the same code in other project. For example please refer libcxx\CMakeLists.txt :

# FIXME: Remove all debug flags and flags that change which Windows
# default libraries are linked. Currently we only support linking the
# non-debug DLLs
remove_flags("/D_DEBUG" "/MTd" "/MDd" "/MT" "/Md")

That code is removed now as well; both since we expect to be using CMAKE_MSVC_RUNTIME_LIBRARY as the only mechanism for selecting the CRT, and also because libcxx now does support building with the other CRT configurations. See https://github.com/llvm/llvm-project/commit/e777e44546f903146e7cfcd241a9dd9d7f217865.

[yurybura]

I guess we could consider readding a couple lines in compiler-rt to strip that out though. But we don't really in general want to support users manually injecting conflicting CRT flags via the cmake flags. What do others think here, @hans @phosek?

If we talk about CRT flags, then I can partially agree. By default CMake defines MultiThreaded$<$<CONFIG:Debug>:Debug>DLL (/MDd for Debug configuration). The Clang project can build with default flags without special handling. The compiler-rt project overwrites specified flags with MultiThreaded (/MT for Debug configuration). CMake doesn't provide any support for remove obsolete flags or warnings or other way to inform somehow about using conflicted/unsupported flags. At least for backward compatibility it is better to leave code to remove unsupported flags.

The main reason of the failure is /D_DEBUG. CMake doesn't provide custom variables to handle this... In either case, this definition is checked in the source code and causes the build to fail.

[mstorsjo]

In D155431#4651646, @yurybura wrote:

The main reason of the failure is /D_DEBUG. CMake doesn't provide custom variables to handle this... In either case, this definition is checked in the source code and causes the build to fail.

That define should be set implicitly if CMAKE_MSVC_RUNTIME_LIBRARY is set to one of the debug choices, no?

[yurybura]

That define should be set implicitly if CMAKE_MSVC_RUNTIME_LIBRARY is set to one of the debug choices, no?

I think, no. At least I cannot find information about this in CMake documentation.

[mstorsjo]

In D155431#4651666, @yurybura wrote:

That define should be set implicitly if CMAKE_MSVC_RUNTIME_LIBRARY is set to one of the debug choices, no?

I think, no. At least I cannot find information about this in CMake documentation.

It's not set explicitly by CMake, but if you set CMAKE_MSVC_RUNTIME_LIBRARY to MultiThreadedDebug or MultiThreadedDebugDLL, it passes -MTd or -MDd to the compiler. When the compiler gets either of those, it implicitly sets -D_DEBUG. Example:

$ cat crtdefs.c 
#ifdef _DEBUG
#error _DEBUG
#endif
$ cl -c crtdefs.c -MDd
Microsoft (R) C/C++ Optimizing Compiler Version 19.30.30705 for x64
Copyright (C) Microsoft Corporation.  All rights reserved.

crtdefs.c
crtdefs.c(2): fatal error C1189: #error:  _DEBUG

[yurybura]

In D155431#4651667, @mstorsjo wrote:

In D155431#4651666, @yurybura wrote:

That define should be set implicitly if CMAKE_MSVC_RUNTIME_LIBRARY is set to one of the debug choices, no?

I think, no. At least I cannot find information about this in CMake documentation.

It's not set explicitly by CMake, but if you set CMAKE_MSVC_RUNTIME_LIBRARY to MultiThreadedDebug or MultiThreadedDebugDLL, it passes -MTd or -MDd to the compiler. When the compiler gets either of those, it implicitly sets -D_DEBUG. Example:

$ cat crtdefs.c 
#ifdef _DEBUG
#error _DEBUG
#endif
$ cl -c crtdefs.c -MDd
Microsoft (R) C/C++ Optimizing Compiler Version 19.30.30705 for x64
Copyright (C) Microsoft Corporation.  All rights reserved.

crtdefs.c
crtdefs.c(2): fatal error C1189: #error:  _DEBUG

Once again, the default value for CMAKE_MSVC_RUNTIME_LIBRARY is MultiThreaded$<$<CONFIG:Debug>:Debug>DLL for the entire LLVM build, /D_DEBUG is added to CMAKE_CXX_FLAGS_DEBUG. The compiler-rt project overwrites CMAKE_MSVC_RUNTIME_LIBRARY, but does not change other compiler flags that conflict with the modified CRT version.

[mstorsjo]

In D155431#4651673, @yurybura wrote:

Once again, the default value for CMAKE_MSVC_RUNTIME_LIBRARY is MultiThreaded$<$<CONFIG:Debug>:Debug>DLL for the entire LLVM build, /D_DEBUG is added to CMAKE_CXX_FLAGS_DEBUG.

Are you saying that this gets added automatically by CMake in this variable, or that it is set manually by your build configuration? I don't think it's added automatically by CMake.

The compiler-rt project overwrites CMAKE_MSVC_RUNTIME_LIBRARY, but does not change other compiler flags that conflict with the modified CRT version.

The point is that by switching CMAKE_MSVC_RUNTIME_LIBRARY to the right choice, debug/non-debug, the define _DEBUG gets added automatically by the compiler if building in a debug mode. You shouldn't be adding that manually.

[mstorsjo]

In D155431#4651674, @mstorsjo wrote:

In D155431#4651673, @yurybura wrote:

Once again, the default value for CMAKE_MSVC_RUNTIME_LIBRARY is MultiThreaded$<$<CONFIG:Debug>:Debug>DLL for the entire LLVM build, /D_DEBUG is added to CMAKE_CXX_FLAGS_DEBUG.

Are you saying that this gets added automatically by CMake in this variable, or that it is set manually by your build configuration? I don't think it's added automatically by CMake.

I just tested building a CMake project, setting CMAKE_BUILD_TYPE=Debug, not touching CMAKE_MSVC_RUNTIME_LIBRARY. CMake invokes cl.exe with /MDd, /D_DEBUG is never passed explicit to the compiler - this is implied by /MDd.

[yurybura]

Are you saying that this gets added automatically by CMake in this variable, or that it is set manually by your build configuration? I don't think it's added automatically by CMake.

I'm saying that VCPKG adds these compiler flags. I use this package manager to build LLVM projects, including compiler-rt. Starting from LLVM 17.0.1 I see the build error.

[mstorsjo]

In D155431#4651678, @yurybura wrote:

Are you saying that this gets added automatically by CMake in this variable, or that it is set manually by your build configuration? I don't think it's added automatically by CMake.

I'm saying that VCPKG adds these compiler flags.

And I'm saying that it's redundant.

The point is that by switching CMAKE_MSVC_RUNTIME_LIBRARY to the right choice, debug/non-debug, the define _DEBUG gets added automatically by the compiler if building in a debug mode. You shouldn't be adding that manually.

As I understand _DEBUG is not added by default. How to add it for Debug config for the whole LLVM/Clang build but exclude adding it to compiler-rt or libcxx?

Why do you feel that you need to add _DEBUG at all?

If you set CMAKE_MSVC_RUNTIME_LIBRARY to a debug CRT, this define gets added automatically by the compiler. (And conversely, if something further down the build chain chooses to override CMAKE_MSVC_RUNTIME_LIBRARY, there's no conflicting _DEBUG define either.)

[yurybura]

Are you saying that this gets added automatically by CMake in this variable, or that it is set manually by your build configuration? I don't think it's added automatically by CMake.

I'm saying that VCPKG adds these compiler flags.

And I'm saying that it's redundant.

Yes, it is. I'm just asking to save removing these flags for backward compatibility.

[mstorsjo]

In D155431#4651689, @yurybura wrote:

Are you saying that this gets added automatically by CMake in this variable, or that it is set manually by your build configuration? I don't think it's added automatically by CMake.

I'm saying that VCPKG adds these compiler flags.

And I'm saying that it's redundant.

Yes, it is. I'm just asking to save removing these flags for backward compatibility.

Right.

I'd be ok with restoring the removing of these flags temporarily (for a little while in git main, and backporting that to the 17.x release branch), but going forward, in an environment set up according to https://cmake.org/cmake/help/latest/policy/CMP0091.html, I think the user shouldn't be adding these flags manually.

Unfortunately, while we can keep this cleanup in 17.x, users of such setups won't necessarily get visible deprecation warnings telling them to move forward with their setups though. (Or I guess we could add similar logic scanning the variables and giving a warning if they are set there?)

[mstorsjo]

In D155431#4651692, @mstorsjo wrote:

In D155431#4651689, @yurybura wrote:

And I'm saying that it's redundant.

Yes, it is. I'm just asking to save removing these flags for backward compatibility.

Right.

I'd be ok with restoring the removing of these flags temporarily (for a little while in git main, and backporting that to the 17.x release branch), but going forward, in an environment set up according to https://cmake.org/cmake/help/latest/policy/CMP0091.html, I think the user shouldn't be adding these flags manually.

I posted a PR for reinstating this flag removal in https://github.com/llvm/llvm-project/pull/67935.

[mstorsjo]

In D155431#4652239, @mstorsjo wrote:

I posted a PR for reinstating this flag removal in https://github.com/llvm/llvm-project/pull/67935.

This was landed now.

compiler-rt/CMakeLists.txt

 # testsuite failures.
 if("${COMPILER_RT_DEFAULT_TARGET_ARCH}" MATCHES "s390x")
   append_list_if(COMPILER_RT_HAS_FNO_PARTIAL_INLINING_FLAG -fno-partial-inlining SANITIZER_COMMON_CFLAGS)
 endif()
 
 if(MSVC)
   # FIXME: In fact, sanitizers should support both /MT and /MD, see PR20214.
   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)
 
   # Disable thread safe initialization for static locals. ASan shouldn't need
   # it. Thread safe initialization assumes that the CRT has already been
   # initialized, but ASan initializes before the CRT.
   list(APPEND SANITIZER_COMMON_CFLAGS /Zc:threadSafeInit-)
 endif()

llvm/cmake/modules/ChooseMSVCCRT.cmake

 # The macro choose_msvc_crt() takes a list of possible
 # C runtimes to choose from, in the form of compiler flags,
 # to present to the user. (MTd for /MTd, etc)
 #
 # The macro is invoked at the end of the file.
 #
-# CMake already sets CRT flags in the CMAKE_CXX_FLAGS_* and
-# CMAKE_C_FLAGS_* variables by default. To let the user
-# override that for each build type:
-# 1. Detect which CRT is already selected, and reflect this in
-# LLVM_USE_CRT_* so the user can have a better idea of what
-# changes they're making.
-# 2. Replace the flags in both variables with the new flag via a regex.
-# 3. set() the variables back into the cache so the changes
-# are user-visible.
-
-### Helper macros: ###
-macro(make_crt_regex regex crts)
-  set(${regex} "")
-  foreach(crt ${${crts}})
-    # Trying to match the beginning or end of the string with stuff
-    # like [ ^]+ didn't work, so use a bunch of parentheses instead.
-    set(${regex} "${${regex}}|(^| +)/${crt}($| +)")
-  endforeach(crt)
-  string(REGEX REPLACE "^\\|" "" ${regex} "${${regex}}")
-endmacro(make_crt_regex)
-
-macro(get_current_crt crt_current regex flagsvar)
-  # Find the selected-by-CMake CRT for each build type, if any.
-  # Strip off the leading slash and any whitespace.
-  string(REGEX MATCH "${${regex}}" ${crt_current} "${${flagsvar}}")
-  string(REPLACE "/" " " ${crt_current} "${${crt_current}}")
-  string(STRIP "${${crt_current}}" ${crt_current})
-endmacro(get_current_crt)
-
-# Replaces or adds a flag to a variable.
-# Expects 'flag' to be padded with spaces.
-macro(set_flag_in_var flagsvar regex flag)
-  string(REGEX MATCH "${${regex}}" current_flag "${${flagsvar}}")
-  if("${current_flag}" STREQUAL "")
-    set(${flagsvar} "${${flagsvar}}${${flag}}")
-  else()
-    string(REGEX REPLACE "${${regex}}" "${${flag}}" ${flagsvar} "${${flagsvar}}")
-  endif()
-  string(STRIP "${${flagsvar}}" ${flagsvar})
-  # Make sure this change gets reflected in the cache/gui.
-  # CMake requires the docstring parameter whenever set() touches the cache,
-  # so get the existing docstring and re-use that.
-  get_property(flagsvar_docs CACHE ${flagsvar} PROPERTY HELPSTRING)
-  set(${flagsvar} "${${flagsvar}}" CACHE STRING "${flagsvar_docs}" FORCE)
-endmacro(set_flag_in_var)
-
+# This mechanism is deprecated, but kept for transitioning users.
+#
+# This reads the LLVM_USE_CRT_<CONFIG> options and sets
+# CMAKE_MSVC_RUNTIME_LIBRARY accordingly. The previous mechanism allowed
+# setting different choices for different build configurations (for
+# multi-config generators), but translating multiple differing choices to
+# the corresponding CMAKE_MSVC_RUNTIME_LIBRARY generator expression isn't
+# supported by this transitional helper.
 
 macro(choose_msvc_crt MSVC_CRT)
   if(LLVM_USE_CRT)
     message(FATAL_ERROR
       "LLVM_USE_CRT is deprecated. Use the CMAKE_BUILD_TYPE-specific
 variables (LLVM_USE_CRT_DEBUG, etc) instead.")
   endif()
 
-  make_crt_regex(MSVC_CRT_REGEX ${MSVC_CRT})
-
   foreach(build_type ${CMAKE_CONFIGURATION_TYPES} ${CMAKE_BUILD_TYPE})
     string(TOUPPER "${build_type}" 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()
       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: ${CMAKE_MSVC_RUNTIME_LIBRARY}")
       set(runtime_library_set 1)
     endif()
-    foreach(lang C CXX)
-      # Clear any potentially manually set options from these variables.
-      # Kept as temporary backwards compat (unsure if necessary).
-      # TODO: We probably should remove it.
-      set_flag_in_var(CMAKE_${lang}_FLAGS_${build} MSVC_CRT_REGEX "")
-    endforeach(lang)
   endforeach(build_type)
 endmacro(choose_msvc_crt MSVC_CRT)
 
 
 # List of valid CRTs for MSVC
 set(MSVC_CRT
   MD
   MDd
   MT
   MTd)
 
 choose_msvc_crt(MSVC_CRT)