diff --git a/compiler-rt/cmake/builtin-config-ix.cmake b/compiler-rt/cmake/builtin-config-ix.cmake
--- a/compiler-rt/cmake/builtin-config-ix.cmake
+++ b/compiler-rt/cmake/builtin-config-ix.cmake
@@ -13,6 +13,7 @@
 builtin_check_c_compiler_flag(-fomit-frame-pointer  COMPILER_RT_HAS_OMIT_FRAME_POINTER_FLAG)
 builtin_check_c_compiler_flag(-ffreestanding        COMPILER_RT_HAS_FFREESTANDING_FLAG)
 builtin_check_c_compiler_flag(-fxray-instrument     COMPILER_RT_HAS_XRAY_COMPILER_FLAG)
+builtin_check_c_compiler_flag(-Wbuiltin-declaration-mismatch COMPILER_RT_HAS_WBUILTIN_DECLARATION_MISMATCH_FLAG)
 
 builtin_check_c_compiler_source(COMPILER_RT_HAS_ATOMIC_KEYWORD
 "
diff --git a/compiler-rt/lib/builtins/CMakeLists.txt b/compiler-rt/lib/builtins/CMakeLists.txt
--- a/compiler-rt/lib/builtins/CMakeLists.txt
+++ b/compiler-rt/lib/builtins/CMakeLists.txt
@@ -754,6 +754,7 @@
   endif()
 
   append_list_if(COMPILER_RT_HAS_STD_C11_FLAG -std=c11 BUILTIN_CFLAGS)
+  append_list_if(COMPILER_RT_HAS_WBUILTIN_DECLARATION_MISMATCH_FLAG -Werror=builtin-declaration-mismatch BUILTIN_CFLAGS)
 
   # These flags would normally be added to CMAKE_C_FLAGS by the llvm
   # cmake step. Add them manually if this is a standalone build.
diff --git a/compiler-rt/test/builtins/lit.cfg.py b/compiler-rt/test/builtins/lit.cfg.py
--- a/compiler-rt/test/builtins/lit.cfg.py
+++ b/compiler-rt/test/builtins/lit.cfg.py
@@ -10,9 +10,14 @@
 
 # Test suffixes.
 config.suffixes = [".c", ".cpp", ".m", ".mm"]
+extra_flags = ['-Wall']
+if config.compiler_id == "GNU":
+    # detect incorrect declarations of libgcc functions
+    extra_flags.append("-Werror=builtin-declaration-mismatch")
+
 
 # Define %clang and %clangxx substitutions to use in test RUN lines.
-config.substitutions.append(("%clang ", " " + config.clang + " "))
+config.substitutions.append(("%clang ", " " + config.clang + " " + " ".join(extra_flags)))
 
 if config.host_os == "Darwin":
     config.substitutions.append(