Make clang-cl able to self host on Windows ✿ヾ╲(｡◕‿◕｡)╱✿

[zturner]

Summary

CMake 3.x adds the notion of "simulated compilers" [1, 2]. CMAKE_CXX_COMPILER_ID will still report the actual compiler you're using, but it will set additional variables CMAKE_CXX_SIMULATE_ID and CMAKE_CXX_SIMULATE_VERSION. So, for example, if you set CC and CXX to clang-cl and run CMake while MSVC in your path (for example you've run vcvarsall), then CMAKE_CXX_SIMULATE_ID will get set to "MSVC", and CMAKE_CXX_SIMULATE_VERSION will get set to the value of -fms-compatibility-version.

With this patch and one more patch on the clang side to get -fms-compatibility-version to default to 18.00 instead of 17.00, I was able to compile with clang-cl with the following CMake invocation:

set CC=clang-cl.exe
set CXX=clang-cl.exe
cmake -G Ninja ..\..\llvm

I need to test this more exhaustively, and with CMake 2.8. I know CMake 2.8 doesn't support this, but I want to make sure it doesn't completely break the build. I also haven't completed a full build and run the test suite. But I was able to get ninja to [100/3000] with no major blowups before I stopped due to clang being so slow (I had installed a debug build). So this is promising, and I want to throw it up for discussion.

[1] http://cmake.org/gitweb?p=cmake.git;a=commitdiff;h=51ab85c3
[2] http://cmake.org/gitweb?p=cmake.git;a=commitdiff;h=3d8356d4

[hans]

Thanks for looking into this!

With this patch and one more patch on the clang side to get -fms-compatibility-version to default to 18.00 instead of 17.00,

Yes, I think we should just bump the compatibility version now.

[rnk]

Nice! Looks good. Can we tweak the libstdc++ check instead, though? I think it makes more sense to come into the Clang branch in case we add more stuff there in the future.

-    if(NOT LLVM_ENABLE_LIBCXX)
+    if (CMAKE_CXX_SIMULATE_ID MATCHES "MSVC")
+      ...
+    elseif(NOT LLVM_ENABLE_LIBCXX)

llvm/trunk/cmake/modules/HandleLLVMOptions.cmake

if(CMAKE_CXX_COMPILER_ID STREQUAL "GNU")
     if(CMAKE_CXX_COMPILER_VERSION VERSION_LESS 4.7)
       message(FATAL_ERROR "Host GCC version must be at least 4.7!")
     endif()
   elseif(CMAKE_CXX_COMPILER_ID STREQUAL "Clang")
     if(CMAKE_CXX_COMPILER_VERSION VERSION_LESS 3.1)
       message(FATAL_ERROR "Host Clang version must be at least 3.1!")
     endif()
 
-    # Also test that we aren't using too old of a version of libstdc++ with the
-    # Clang compiler. This is tricky as there is no real way to check the
-    # version of libstdc++ directly. Instead we test for a known bug in
-    # libstdc++4.6 that is fixed in libstdc++4.7.
-    if(NOT LLVM_ENABLE_LIBCXX)
+    if (CMAKE_CXX_SIMULATE_ID MATCHES "MSVC")
+      if (CMAKE_CXX_SIMULATE_VERSION VERSION_LESS 18.0)
+        message(FATAL_ERROR "Host Clang must have at least -fms-compatibility-version=18.0")
+      endif()
+    elseif(NOT LLVM_ENABLE_LIBCXX)
+      # Otherwise, test that we aren't using too old of a version of libstdc++
+      # with the Clang compiler. This is tricky as there is no real way to
+      # check the version of libstdc++ directly. Instead we test for a known
+      # bug in libstdc++4.6 that is fixed in libstdc++4.7.
       set(OLD_CMAKE_REQUIRED_FLAGS ${CMAKE_REQUIRED_FLAGS})
       set(OLD_CMAKE_REQUIRED_LIBRARIES ${CMAKE_REQUIRED_LIBRARIES})
       set(CMAKE_REQUIRED_FLAGS "-std=c++0x")
       check_cxx_source_compiles("
 #include <atomic>
 std::atomic<float> x(0.0f);
 int main() { return (float)x; }"
         LLVM_NO_OLD_LIBSTDCXX)