[builtins] Build with -Wbuiltin-declaration-mismatch if supported

[arichardson]

Summary

GCC is able to check that the signatures of the builtins are as expected
and this shows some incorrect signatures on ld80 platforms (i.e. x86).
The *tf* functions should take 128-bit arguments but until the latest fixes
they used 80-bit long double.

[MaskRay]

Do you have an example how this is useful for lib/builtins functions? I think GCC -Wbuiltin-declaration-mismatch is enabled by default and applies to C standard functions (e.g., printf), but I cannot get it to warn on libgcc_s.so.1 functions, e.g.

% cat a.c
float __extendhfdf2(float a) { return a; }
% gcc -c a.c -Wbuiltin-declaration-mismatch  # no warning

[arichardson]

In D153814#4485924, @MaskRay wrote:

Do you have an example how this is useful for lib/builtins functions? I think GCC -Wbuiltin-declaration-mismatch is enabled by default and applies to C standard functions (e.g., printf), but I cannot get it to warn on libgcc_s.so.1 functions, e.g.

% cat a.c
float __extendhfdf2(float a) { return a; }
% gcc -c a.c -Wbuiltin-declaration-mismatch  # no warning

I only see one error right now:

compiler-rt/lib/builtins/divtc3.c:20:26: error: conflicting types for built-in function ‘__divtc3’; expected ‘_Complex _Float128(_Float128,  _Float128,  _Float128,  _Float128)’ [-Werror=builtin-declaration-mismatch]
   20 | COMPILER_RT_ABI Lcomplex __divtc3(long double __a, long double __b,
      |                          ^~~~~~~~

[arichardson]

So, yes this is of limited usefulness, but it did catch one incorrect declaration.

[compnerd]

I don't see any harm in adding the additional flag, the overall complexity is pretty simple and is more a patterned extension.

compiler-rt/cmake/builtin-config-ix.cmake

 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(-fno-lto              COMPILER_RT_HAS_FNO_LTO_FLAG)
 builtin_check_c_compiler_flag(-fno-profile-generate COMPILER_RT_HAS_FNO_PROFILE_GENERATE_FLAG)
 builtin_check_c_compiler_flag(-fno-profile-instr-generate COMPILER_RT_HAS_FNO_PROFILE_INSTR_GENERATE_FLAG)
 builtin_check_c_compiler_flag(-fno-profile-instr-use COMPILER_RT_HAS_FNO_PROFILE_INSTR_USE_FLAG)
 builtin_check_c_compiler_flag(-Wno-pedantic         COMPILER_RT_HAS_WNO_PEDANTIC)
+builtin_check_c_compiler_flag(-Wbuiltin-declaration-mismatch COMPILER_RT_HAS_WBUILTIN_DECLARATION_MISMATCH_FLAG)
 builtin_check_c_compiler_flag(/Zl COMPILER_RT_HAS_ZL_FLAG)
 
 builtin_check_c_compiler_source(COMPILER_RT_HAS_ATOMIC_KEYWORD
 "
 int foo(int x, int y) {
  _Atomic int result = x * y;
  return result;
 }

compiler-rt/lib/builtins/CMakeLists.txt

else ()
   set(BUILTIN_CFLAGS "")
   add_security_warnings(BUILTIN_CFLAGS 0)
 
   if (COMPILER_RT_HAS_FCF_PROTECTION_FLAG)
     append_list_if(COMPILER_RT_ENABLE_CET -fcf-protection=full BUILTIN_CFLAGS)
   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)
 
   # Don't embed directives for picking any specific CRT
   if (MSVC)
     set(CMAKE_MSVC_RUNTIME_LIBRARY "")
     append_list_if(COMPILER_RT_HAS_ZL_FLAG /Zl BUILTIN_CFLAGS)
   endif()
 
   # These flags would normally be added to CMAKE_C_FLAGS by the llvm

compiler-rt/test/builtins/lit.cfg.py

 # -*- Python -*-
 
 import os
 
 # Setup config name.
 config.name = "Builtins"
 
 # Setup source root.
 config.test_source_root = os.path.dirname(__file__)
 
 # 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(
         ("%macos_version_major", str(config.darwin_osx_version[0]))
     )
     config.substitutions.append(
         ("%macos_version_minor", str(config.darwin_osx_version[1]))
     )
     config.substitutions.append(
         ("%macos_version_subminor", str(config.darwin_osx_version[2]))
     )