diff --git a/compiler-rt/cmake/Modules/CompilerRTUtils.cmake b/compiler-rt/cmake/Modules/CompilerRTUtils.cmake
--- a/compiler-rt/cmake/Modules/CompilerRTUtils.cmake
+++ b/compiler-rt/cmake/Modules/CompilerRTUtils.cmake
@@ -358,6 +358,10 @@
     if(DEFINED COMPILER_RT_DEFAULT_TARGET_TRIPLE)
       message(FATAL_ERROR "COMPILER_RT_DEFAULT_TARGET_TRIPLE isn't supported when building for default target only")
     endif()
+    if ("${CMAKE_C_COMPILER_TARGET}" STREQUAL "")
+      message(FATAL_ERROR "CMAKE_C_COMPILER_TARGET must also be set when COMPILER_RT_DEFAUT_TARGET_ONLY is ON")
+    endif()
+    message(STATUS "cmake c compiler target: ${CMAKE_C_COMPILER_TARGET}")
     set(COMPILER_RT_DEFAULT_TARGET_TRIPLE ${CMAKE_C_COMPILER_TARGET})
   else()
     set(COMPILER_RT_DEFAULT_TARGET_TRIPLE ${LLVM_TARGET_TRIPLE} CACHE STRING