Index: flang/CMakeLists.txt
===================================================================
--- flang/CMakeLists.txt
+++ flang/CMakeLists.txt
@@ -236,7 +236,11 @@
   ${FLANG_BINARY_DIR}/include/flang/Config/config.h)
 
 # Add global F18 flags.
-set(CMAKE_CXX_FLAGS "-fno-rtti -fno-exceptions -pedantic -Wall -Wextra -Werror -Wcast-qual -Wimplicit-fallthrough -Wdelete-non-virtual-dtor ${CMAKE_CXX_FLAGS}")
+if(MSVC)
+  set(CMAKE_CXX_FLAGS "/GR- /EHs-c- /Wall ${CMAKE_CXX_FLAGS}")
+else()
+  set(CMAKE_CXX_FLAGS "-fno-rtti -fno-exceptions -pedantic -Wall -Wextra -Werror -Wcast-qual -Wimplicit-fallthrough -Wdelete-non-virtual-dtor ${CMAKE_CXX_FLAGS}")
+endif()
 
 # Builtin check_cxx_compiler_flag doesn't seem to work correctly
 macro(check_compiler_flag flag resultVar)
Index: flang/lib/Optimizer/CMakeLists.txt
===================================================================
--- flang/lib/Optimizer/CMakeLists.txt
+++ flang/lib/Optimizer/CMakeLists.txt
@@ -1,5 +1,9 @@
 # Sources generated by tablegen have unused parameters.
-set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wno-unused-parameter")
+if(MSVC)
+  set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} /wd4100")
+else()
+  set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wno-unused-parameter")
+endif()
 
 add_subdirectory(Dialect)
 add_subdirectory(Support)