clang/llvm: add initial configuration file for clang

Add an LLVM backend and a clang toolchain variant to support building
with llvm coming with popular Linux distributions.

This has been tested with X86 boards:
- quark_d2000_crb
- quark_se_c1000_devboard/Arduino 101

Use:

export ZEPHYR_TOOLCHAIN_VARIANT=clang

Signed-off-by: Anas Nashif <anas.nashif@intel.com>

CMakeLists.txt

@@ -230,9 +230,17 @@ zephyr_compile_options(${CONFIG_COMPILER_OPT_AS_LIST})
 
 # TODO: Include arch compiler options at this point.
 
-# TODO: This Clang check is broken
 if(CMAKE_C_COMPILER_ID STREQUAL "Clang")
   zephyr_cc_option(
+    #FIXME: need to fix all of those
+    -Wno-sometimes-uninitialized
+    -Wno-shift-overflow
+    -Wno-missing-braces
+    -Wno-self-assign
+    -Wno-address-of-packed-member
+    -Wno-unused-function
+    -Wno-initializer-overrides
+    -Wno-section
     -Wno-unknown-warning-option
     -Wno-unused-variable
     -Wno-format-invalid-specifier

arch/x86/core/CMakeLists.txt

@@ -1,12 +1,12 @@
 zephyr_library()
 
-if (COMPILER STREQUAL "clang")
+if (CMAKE_LANG_COMPILER_ID STREQUAL "Clang")
   # We rely on GAS for assembling, so don't use the integrated assembler
   zephyr_compile_options_ifndef(CONFIG_X86_IAMCU $<$<COMPILE_LANGUAGE:ASM>:-no-integrated-as>)
+elseif(CMAKE_LANG_COMPILER_ID STREQUAL "GNU")
+  zephyr_compile_options($<$<COMPILE_LANGUAGE:ASM>:-Wa,--divide>)
 endif()
 
-zephyr_compile_options($<$<COMPILE_LANGUAGE:ASM>:-Wa,--divide>)
-
 zephyr_library_sources(
   cache.c
   cache_s.S

cmake/compiler/llvm.cmake

@@ -0,0 +1,22 @@
+# Configuration for host installed llvm
+#
+
+set(NOSTDINC "")
+
+foreach(file_name include include-fixed)
+  execute_process(
+    COMMAND ${CMAKE_C_COMPILER} --print-file-name=${file_name}
+    OUTPUT_VARIABLE _OUTPUT
+    )
+  string(REGEX REPLACE "\n" "" _OUTPUT ${_OUTPUT})
+
+  list(APPEND NOSTDINC ${_OUTPUT})
+endforeach()
+
+foreach(isystem_include_dir ${NOSTDINC})
+  list(APPEND isystem_include_flags -isystem ${isystem_include_dir})
+endforeach()
+
+set(CMAKE_REQUIRED_FLAGS -nostartfiles -nostdlib ${isystem_include_flags} -Wl,--unresolved-symbols=ignore-in-object-files)
+string(REPLACE ";" " " CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS}")
+

cmake/toolchain/clang.cmake

@@ -0,0 +1,24 @@
+set(CLANG_ROOT $ENV{CLANG_ROOT_DIR})
+set_ifndef(CLANG_ROOT /usr)
+set(COMPILER clang)
+
+if(CONFIG_ARM)
+set(triple arm-none-eabi)
+set(CMAKE_EXE_LINKER_FLAGS_INIT "--specs=nosys.specs")
+elseif(CONFIG_X86)
+set(triple i686-pc-none-elf)
+endif()
+
+set(CMAKE_C_COMPILER          ${CLANG_ROOT}/bin/clang)
+set(CMAKE_C_COMPILER_TARGET   ${triple})
+set(CMAKE_ASM_COMPILER_TARGET ${triple})
+set(CMAKE_CXX_COMPILER        ${CLANG_ROOT}/bin/clang++)
+set(CMAKE_CXX_COMPILER_TARGET ${triple})
+set(CMAKE_AR                  "${CLANG_ROOT}/bin/llvm-ar"  CACHE INTERNAL " " FORCE)
+set(CMAKE_LINKER              "${CLANG_ROOT}/bin/llvm-link" CACHE INTERNAL " " FORCE)
+SET(CMAKE_NM                  "${CLANG_ROOT}/bin/llvm-nm"       CACHE INTERNAL " " FORCE)
+SET(CMAKE_OBJDUMP             "${CLANG_ROOT}/bin/llvm-objdump" CACHE INTERNAL " " FORCE)
+SET(CMAKE_RANLIB              "${CLANG_ROOT}/bin/llvm-ranlib" CACHE INTERNAL " " FORCE)
+set(CMAKE_OBJCOPY             objcopy CACHE INTERNAL " " FORCE)
+set(CMAKE_READELF             readelf CACHE INTERNAL " " FORCE)
+