toolchain: xcc: use Clang if exists

The XCC toolchain may come with Clang front-end depending on
how it's built. Currently, the only SoC/board using XCC is
the intel_s1000_crb and its XCC toolchain comes with Clang
3.9.0 which has a lot better support for C99 and C++11 than
the portion based on GCC 4.2 (which does not even support
C++11). So this change attempts to use the Clang portion
instead of GCC if the Clang executable exists.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>

cmake/compiler/xcc/target.cmake

@@ -73,6 +73,11 @@ include(${ZEPHYR_BASE}/cmake/compiler/gcc/target_optimizations.cmake)
 include(${ZEPHYR_BASE}/cmake/compiler/gcc/target_cpp.cmake)
 include(${ZEPHYR_BASE}/cmake/compiler/gcc/target_asm.cmake)
 include(${ZEPHYR_BASE}/cmake/compiler/gcc/target_baremetal.cmake)
-include(${ZEPHYR_BASE}/cmake/compiler/gcc/target_warnings.cmake)
 include(${ZEPHYR_BASE}/cmake/compiler/gcc/target_imacros.cmake)
 include(${ZEPHYR_BASE}/cmake/compiler/gcc/target_base.cmake)
+
+if(CC STREQUAL "clang")
+  include(${ZEPHYR_BASE}/cmake/compiler/clang/target_warnings.cmake)
+else()
+  include(${ZEPHYR_BASE}/cmake/compiler/gcc/target_warnings.cmake)
+endif()

cmake/toolchain/xcc/generic.cmake

@@ -23,15 +23,20 @@ set(SYSROOT_DIR    ${TOOLCHAIN_HOME}/${SYSROOT_TARGET})
 # xt-xcc does not support -Og, so make it -O0
 set(OPTIMIZE_FOR_DEBUG_FLAG "-O0")
 
-set(CC xcc)
-set(C++ xc++)
+if(EXISTS ${CROSS_COMPILE}clang)
+  set(CC clang)
+  set(C++ clang++)
+else()
+  set(CC xcc)
+  set(C++ xc++)
+
+  list(APPEND TOOLCHAIN_C_FLAGS
+    -imacros${ZEPHYR_BASE}/include/toolchain/xcc_missing_defs.h
+    )
+endif()
 
 set(NOSYSDEF_CFLAG "")
 
 list(APPEND TOOLCHAIN_C_FLAGS -fms-extensions)
 
-list(APPEND TOOLCHAIN_C_FLAGS
-  -imacros${ZEPHYR_BASE}/include/toolchain/xcc_missing_defs.h
-  )
-
 set(TOOLCHAIN_HAS_NEWLIB OFF CACHE BOOL "True if toolchain supports newlib")

include/toolchain/xcc.h

@@ -11,10 +11,18 @@
  * there. However, __BYTE_ORDER__ is actually being defined later in
  * this file. So define __BYTE_ORDER__ to skip the check in gcc.h
  * and undefine after including gcc.h.
+ *
+ * Clang has it defined so there is no need to work around.
  */
+#ifndef __clang__
 #define __BYTE_ORDER__
+#endif
+
 #include <toolchain/gcc.h>
+
+#ifndef __clang__
 #undef __BYTE_ORDER__
+#endif
 
 #include <stdbool.h>
 
@@ -22,8 +30,12 @@
 #define UINT32_C(x)	x ## U
 #endif
 
-/* XCC doesn't support __COUNTER__ but this should be good enough */
+#ifndef __COUNTER__
+/* XCC (GCC-based compiler) doesn't support __COUNTER__
+ * but this should be good enough
+ */
 #define __COUNTER__ __LINE__
+#endif
 
 #undef __in_section_unique
 #define __in_section_unique(seg) \

soc/xtensa/intel_s1000/xcc/CMakeLists.txt

@@ -2,5 +2,10 @@
 
 zephyr_library_sources_ifdef(CONFIG_CPLUSPLUS cpp_fixes.c)
 zephyr_library_sources_ifdef(CONFIG_NEWLIB_LIBC newlib_fixes.c)
+
+if(NOT (CC STREQUAL "clang"))
+# These two are due to IS_ENABLED() not being parsed correctly
+# in old GCC 4.2 based XCC. Clang is not affected.
 zephyr_library_sources_ifdef(CONFIG_LOG_MINIMAL log_minimal_fixes.c)
 zephyr_library_sources_ifdef(CONFIG_TEST ztest_fixes.c)
+endif()