Use a sufficiently current MSVCRT when using MinGW

[jsroemer]

Summary

The patch finds all installed MSVCRT versions from 11.0 (highest currently supported by MinGW) down to 9.0 and sets the appropriate compiler and linker flags for the newest version found. An installed MSVCRT is required due to the call to llvm-tblgen at build time.

[Bigcheese]

lgtm

[jsroemer]

Thanks for the review. As I do not have commit access someone else will have to land the patch.

[Bigcheese]

I've committed a different fix that removes the need for this. Thanks for the patch though.

[jsroemer]

I assume you are referring to http://reviews.llvm.org/D7230?

Is it really desirable to disable the functionality for MinGW? Any recent MinGW is perfectly capable of compiling it with the right flags. My issue was only that running the generated executables requires the corresponding runtimes to be installed (also true for MSVC, but if that is installed the runtime is automatically installed as well)

CMakeLists.txt

   # People report that -O3 is unreliable on MinGW. The traditional
   # build also uses -O2 for that reason:
   llvm_replace_compiler_option(CMAKE_CXX_FLAGS_RELEASE "-O3" "-O2")
+
+  # _set_abort_behavior requires an MSVCRT version >= 9.0
+  # as llvm-tblgen is called during the build process only installed
+  # MSVCRT versions should be used
+  function(get_newest_msvcrt OUTVAR)
+    file(TO_CMAKE_PATH $ENV{WINDIR} WINDIR)
+    file(GLOB INSTALLED_MSVCRT_RUNTIMES "${WINDIR}/System32/msvcr*.dll")
+    set(INSTALLED_MSVCRT_NAMES)
+    foreach(MSVCRT ${INSTALLED_MSVCRT_RUNTIMES})
+      get_filename_component(MSVCRT ${MSVCRT} NAME_WE)
+      list(APPEND INSTALLED_MSVCRT_NAMES ${MSVCRT})
+    endforeach()
+    foreach(VERSION 110 100 90) # try newest first
+      list(FIND INSTALLED_MSVCRT_NAMES "msvcr${VERSION}" INDEX)
+      if(INDEX GREATER -1)
+        set(${OUTVAR} ${VERSION} PARENT_SCOPE)
+        return()
+        endif()
+    endforeach()
+    message(FATAL_ERROR "No compatible MSVCRT runtime installed")
+  endfunction()
+  get_newest_msvcrt(NEWEST_MSVCRT)
+  foreach(LANG C CXX)
+    set(CMAKE_${LANG}_FLAGS "${CMAKE_${LANG}_FLAGS} -D__MSVCRT_VERSION__=0x${NEWEST_MSVCRT}0")
+    set(CMAKE_${LANG}_STANDARD_LIBRARIES "${CMAKE_${LANG}_STANDARD_LIBRARIES} -lmsvcr${NEWEST_MSVCRT}")
+  endforeach()
 endif()
 
 # Put this before tblgen. Else we have a circular dependence.