diff --git a/compiler-rt/cmake/Modules/CompilerRTDarwinUtils.cmake b/compiler-rt/cmake/Modules/CompilerRTDarwinUtils.cmake
--- a/compiler-rt/cmake/Modules/CompilerRTDarwinUtils.cmake
+++ b/compiler-rt/cmake/Modules/CompilerRTDarwinUtils.cmake
@@ -116,7 +116,7 @@
   if(NOT TEST_COMPILE_ONLY)
     message(STATUS "Finding valid architectures for ${os}...")
     set(SIMPLE_C ${CMAKE_BINARY_DIR}${CMAKE_FILES_DIRECTORY}/src.c)
-    file(WRITE ${SIMPLE_C} "#include <stdio.h>\nint main() { printf(__FILE__); return 0; }\n")
+    file(WRITE ${SIMPLE_C} "#include <stdio.h>\nint main(void) { printf(__FILE__); return 0; }\n")
 
     set(os_linker_flags)
     foreach(flag ${DARWIN_${os}_LINK_FLAGS})
diff --git a/compiler-rt/cmake/config-ix.cmake b/compiler-rt/cmake/config-ix.cmake
--- a/compiler-rt/cmake/config-ix.cmake
+++ b/compiler-rt/cmake/config-ix.cmake
@@ -224,7 +224,7 @@
 # runtime libraries supported by our current compilers cross-compiling
 # abilities.
 set(SIMPLE_SOURCE ${CMAKE_BINARY_DIR}${CMAKE_FILES_DIRECTORY}/simple.cc)
-file(WRITE ${SIMPLE_SOURCE} "#include <stdlib.h>\n#include <stdio.h>\nint main() { printf(\"hello, world\"); }\n")
+file(WRITE ${SIMPLE_SOURCE} "#include <stdlib.h>\n#include <stdio.h>\nint main(void) { printf(\"hello, world\"); }\n")
 
 # Detect whether the current target platform is 32-bit or 64-bit, and setup
 # the correct commandline flags needed to attempt to target 32-bit and 64-bit.
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
@@ -755,7 +755,7 @@
                            SOURCE "#if !(__ARM_FP & 0x8)
                                    #error No double-precision support!
                                    #endif
-                                   int main() { return 0; }")
+                                   int main(void) { return 0; }")
           if(NOT COMPILER_RT_HAS_${arch}_VFP_DP)
             list(REMOVE_ITEM ${arch}_SOURCES ${arm_Thumb1_VFPv2_DP_SOURCES})
           endif()
diff --git a/libcxx/cmake/config-ix.cmake b/libcxx/cmake/config-ix.cmake
--- a/libcxx/cmake/config-ix.cmake
+++ b/libcxx/cmake/config-ix.cmake
@@ -98,7 +98,7 @@
   set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} -Werror=unknown-pragmas")
   check_c_source_compiles("
 #pragma comment(lib, \"c\")
-int main() { return 0; }
+int main(void) { return 0; }
 " C_SUPPORTS_COMMENT_LIB_PRAGMA)
   cmake_pop_check_state()
 endif()
diff --git a/libcxxabi/cmake/config-ix.cmake b/libcxxabi/cmake/config-ix.cmake
--- a/libcxxabi/cmake/config-ix.cmake
+++ b/libcxxabi/cmake/config-ix.cmake
@@ -81,7 +81,7 @@
   set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} -Werror=unknown-pragmas")
   check_c_source_compiles("
 #pragma comment(lib, \"c\")
-int main() { return 0; }
+int main(void) { return 0; }
 " C_SUPPORTS_COMMENT_LIB_PRAGMA)
   cmake_pop_check_state()
 endif()
diff --git a/libunwind/cmake/config-ix.cmake b/libunwind/cmake/config-ix.cmake
--- a/libunwind/cmake/config-ix.cmake
+++ b/libunwind/cmake/config-ix.cmake
@@ -85,7 +85,7 @@
   set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} -Werror=unknown-pragmas")
   check_c_source_compiles("
 #pragma comment(lib, \"c\")
-int main() { return 0; }
+int main(void) { return 0; }
 " C_SUPPORTS_COMMENT_LIB_PRAGMA)
   cmake_pop_check_state()
 endif()
diff --git a/lldb/tools/debugserver/source/CMakeLists.txt b/lldb/tools/debugserver/source/CMakeLists.txt
--- a/lldb/tools/debugserver/source/CMakeLists.txt
+++ b/lldb/tools/debugserver/source/CMakeLists.txt
@@ -95,7 +95,7 @@
     #else
     #error Not building for ARM64
     #endif
-    int main() { return 0; }
+    int main(void) { return 0; }
     "
     BUILDING_FOR_ARM64_OSX
 )
diff --git a/llvm/cmake/config-ix.cmake b/llvm/cmake/config-ix.cmake
--- a/llvm/cmake/config-ix.cmake
+++ b/llvm/cmake/config-ix.cmake
@@ -71,7 +71,7 @@
   CHECK_C_SOURCE_COMPILES("
      static const char *__crashreporter_info__ = 0;
      asm(\".desc ___crashreporter_info__, 0x10\");
-     int main() { return 0; }"
+     int main(void) { return 0; }"
     HAVE_CRASHREPORTER_INFO)
 endif()
 
diff --git a/llvm/cmake/modules/FindFFI.cmake b/llvm/cmake/modules/FindFFI.cmake
--- a/llvm/cmake/modules/FindFFI.cmake
+++ b/llvm/cmake/modules/FindFFI.cmake
@@ -45,7 +45,7 @@
     struct ffi_cif;
     typedef struct ffi_cif ffi_cif;
     void ffi_call(ffi_cif *cif, void (*fn)(void), void *rvalue, void **avalue);
-    int main() { ffi_call(0, 0, 0, 0); }"
+    int main(void) { ffi_call(0, 0, 0, 0); }"
     HAVE_FFI_CALL)
   cmake_pop_check_state()
 endif()
diff --git a/llvm/cmake/modules/FindTerminfo.cmake b/llvm/cmake/modules/FindTerminfo.cmake
--- a/llvm/cmake/modules/FindTerminfo.cmake
+++ b/llvm/cmake/modules/FindTerminfo.cmake
@@ -20,7 +20,7 @@
   list(APPEND CMAKE_REQUIRED_LIBRARIES ${Terminfo_LIBRARIES})
   check_c_source_compiles("
     int setupterm(char *term, int filedes, int *errret);
-    int main() { return setupterm(0, 0, 0); }"
+    int main(void) { return setupterm(0, 0, 0); }"
     Terminfo_LINKABLE)
   cmake_pop_check_state()
 endif()
diff --git a/llvm/cmake/modules/FindZ3.cmake b/llvm/cmake/modules/FindZ3.cmake
--- a/llvm/cmake/modules/FindZ3.cmake
+++ b/llvm/cmake/modules/FindZ3.cmake
@@ -18,8 +18,9 @@
   # The program that will be executed to print Z3's version.
   file(WRITE ${CMAKE_BINARY_DIR}${CMAKE_FILES_DIRECTORY}/CMakeTmp/testz3.cpp
        "#include <assert.h>
+        #include <stdio.h> 
         #include <z3.h>
-        int main() {
+        int main(void) {
           unsigned int major, minor, build, rev;
           Z3_get_version(&major, &minor, &build, &rev);
           printf(\"%u.%u.%u\", major, minor, build);
diff --git a/llvm/cmake/modules/HandleLLVMOptions.cmake b/llvm/cmake/modules/HandleLLVMOptions.cmake
--- a/llvm/cmake/modules/HandleLLVMOptions.cmake
+++ b/llvm/cmake/modules/HandleLLVMOptions.cmake
@@ -779,7 +779,7 @@
   # line is also a // comment.
   set(OLD_CMAKE_REQUIRED_FLAGS ${CMAKE_REQUIRED_FLAGS})
   set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} -Werror -Wcomment")
-  CHECK_C_SOURCE_COMPILES("// \\\\\\n//\\nint main() {return 0;}"
+  CHECK_C_SOURCE_COMPILES("// \\\\\\n//\\nint main(void) {return 0;}"
                           C_WCOMMENT_ALLOWS_LINE_WRAP)
   set(CMAKE_REQUIRED_FLAGS ${OLD_CMAKE_REQUIRED_FLAGS})
   if (NOT C_WCOMMENT_ALLOWS_LINE_WRAP)
diff --git a/openmp/runtime/cmake/config-ix.cmake b/openmp/runtime/cmake/config-ix.cmake
--- a/openmp/runtime/cmake/config-ix.cmake
+++ b/openmp/runtime/cmake/config-ix.cmake
@@ -27,7 +27,7 @@
     void func2() { printf(\"World\"); }
     __asm__(\".symver func1, func@VER1\");
     __asm__(\".symver func2, func@VER2\");
-    int main() {
+    int main(void) {
       func1();
       func2();
       return 0;
diff --git a/polly/lib/External/CMakeLists.txt b/polly/lib/External/CMakeLists.txt
--- a/polly/lib/External/CMakeLists.txt
+++ b/polly/lib/External/CMakeLists.txt
@@ -64,7 +64,7 @@
     check_c_source_compiles("
     ${_includes}
     ${_type} typeVar;
-    int main() {
+    int main(void) {
     return 0;
     }
     " ${_variable})
@@ -73,7 +73,7 @@
 
   check_c_source_compiles("
   int func(void) __attribute__((__warn_unused_result__));
-  int main() { return 0; }
+  int main(void) { return 0; }
   " HAS_ATTRIBUTE_WARN_UNUSED_RESULT)
   set(GCC_WARN_UNUSED_RESULT)
   if (HAS_ATTRIBUTE_WARN_UNUSED_RESULT)
@@ -82,22 +82,22 @@
 
   check_c_source_compiles("
   __attribute__ ((unused)) static void foo(void);
-  int main() { return 0; }
+  int main(void) { return 0; }
   " HAVE___ATTRIBUTE__)
 
 
   check_c_source_compiles_numeric("
   #include <strings.h>
-  int main() { (void)ffs(0); return 0; }
+  int main(void) { (void)ffs(0); return 0; }
   " HAVE_DECL_FFS)
 
   check_c_source_compiles_numeric("
-  int main() { (void)__builtin_ffs(0); return 0; }
+  int main(void) { (void)__builtin_ffs(0); return 0; }
   " HAVE_DECL___BUILTIN_FFS)
 
   check_c_source_compiles_numeric("
   #include <intrin.h>
-  int main() { (void)_BitScanForward(NULL, 0); return 0; }
+  int main(void) { (void)_BitScanForward(NULL, 0); return 0; }
   " HAVE_DECL__BITSCANFORWARD)
 
   if (NOT HAVE_DECL_FFS AND
@@ -109,12 +109,12 @@
 
   check_c_source_compiles_numeric("
   #include <strings.h>
-  int main() { (void)strcasecmp(\"\", \"\"); return 0; }
+  int main(void) { (void)strcasecmp(\"\", \"\"); return 0; }
   " HAVE_DECL_STRCASECMP)
 
   check_c_source_compiles_numeric("
   #include <string.h>
-  int main() { (void)_stricmp(\"\", \"\"); return 0; }
+  int main(void) { (void)_stricmp(\"\", \"\"); return 0; }
   " HAVE_DECL__STRICMP)
 
   if (NOT HAVE_DECL_STRCASECMP AND NOT HAVE_DECL__STRICMP)
@@ -124,12 +124,12 @@
 
   check_c_source_compiles_numeric("
   #include <strings.h>
-  int main() { (void)strncasecmp(\"\", \"\", 0); return 0; }
+  int main(void) { (void)strncasecmp(\"\", \"\", 0); return 0; }
   " HAVE_DECL_STRNCASECMP)
 
   check_c_source_compiles_numeric("
   #include <string.h>
-  int main() { (void)_strnicmp(\"\", \"\", 0); return 0; }
+  int main(void) { (void)_strnicmp(\"\", \"\", 0); return 0; }
   " HAVE_DECL__STRNICMP)
 
   if (NOT HAVE_DECL_STRNCASECMP AND NOT HAVE_DECL__STRNICMP)
@@ -139,12 +139,12 @@
 
   check_c_source_compiles_numeric("
   #include <stdio.h>
-  int main() { snprintf((void*)0, 0, \" \"); return 0; }
+  int main(void) { snprintf((void*)0, 0, \" \"); return 0; }
   " HAVE_DECL_SNPRINTF)
 
   check_c_source_compiles_numeric("
   #include <stdio.h>
-  int main() { _snprintf((void*)0, 0, \" \"); return 0; }
+  int main(void) { _snprintf((void*)0, 0, \" \"); return 0; }
   " HAVE_DECL__SNPRINTF)
 
   if (NOT HAVE_DECL_SNPRINTF AND NOT HAVE_DECL__SNPRINTF)