[CMake] Remove CLANG_DEFAULT_STD_C/CLANG_DEFAULT_STD_CXX

[MaskRay]

Summary

When Clang is used as a cross compiler, it should respect the target
platform default. Defaulting to a configure-time CLANG_DEFAULT_STD_C/CLANG_DEFAULT_STD_CXX
does not make much sense.

When Clang is used as a single-platform compiler, we should use Clang
configuration file (https://discourse.llvm.org/t/configuration-files/42529
https://clang.llvm.org/docs/UsersManual.html#configuration-files).

The Gentoo needs from D34365 has been satisfied by a configuration file.

[aaron.ballman]

So the basic idea here is that the default can be specified by a configuration file and thus we don't need any configure-time variable for it? But then why do we need other configuration macros like default stdlib or default linker? Mostly trying to understand what the rule of thumb is for when something should be a configure macro and when something should be left to a configuration file instead.

[thesamesam]

In D133375#3774293, @aaron.ballman wrote:

So the basic idea here is that the default can be specified by a configuration file and thus we don't need any configure-time variable for it? But then why do we need other configuration macros like default stdlib or default linker? Mostly trying to understand what the rule of thumb is for when something should be a configure macro and when something should be left to a configuration file instead.

Also, until https://reviews.llvm.org/D109621 or similar is resolved, I don't feel like the actual need mentioned in the original change above is addressed. Forcing distributions to use a prefixed Clang binary feels like a workaround for this deficiency?

[thesamesam]

My objection based on config support is no longer relevant, as it's been solved.

[mgorny]

clang/test/Preprocessor/lang-std.cpp

1

I suppose you want to remove the lit.cfg.py counterpart of this as well.

[MaskRay]

remove lit feature default_std_cxx

I think some other CLANG_DEFAULT_* probably should go away as well,
but we can start with STD_C STD_CXX (where there is little value for Clang to provide the configure-time customization) first.

[mgorny]

Thanks for doing this. For the record, Gentoo didn't need to use this for a long time now, so there's no backwards compatibility concern here.

[tambre]

A Clang 16 release note for this would be nice. I didn't immediately notice and got a bit broken by this change. 🙂

[aaron.ballman]

In D133375#4079251, @tambre wrote:

A Clang 16 release note for this would be nice. I didn't immediately notice and got a bit broken by this change. 🙂

+1 to the request, also adding clang vendors now for early warning despite this already being landed.

[tambre]

I've now run into a snag with this. Having clang++.cfg with -std=c++2b makes /usr/bin/clang++ -E -dM - fail with error: invalid argument '-std=c++2b' not allowed with 'C'.
This is invoked by Meson during its configuration process and causes it to fail.

Am I doing something wrong?

[thesamesam]

In D133375#4214214, @tambre wrote:

I've now run into a snag with this. Having clang++.cfg with -std=c++2b makes /usr/bin/clang++ -E -dM - fail with error: invalid argument '-std=c++2b' not allowed with 'C'.
This is invoked by Meson during its configuration process and causes it to fail.

Am I doing something wrong?

Filed https://github.com/llvm/llvm-project/issues/61641, thanks!

clang/CMakeLists.txt

 set(PPC_LINUX_DEFAULT_IEEELONGDOUBLE OFF CACHE BOOL
     "Enable IEEE binary128 as default long double format on PowerPC Linux.")
 
 set(CLANG_SPAWN_CC1 OFF CACHE BOOL
     "Whether clang should use a new process for the CC1 invocation")
 
 option(CLANG_DEFAULT_PIE_ON_LINUX "Default to -fPIE and -pie on linux-gnu" ON)
 
-# TODO: verify the values against LangStandards.def?
-set(CLANG_DEFAULT_STD_C "" CACHE STRING
-  "Default standard to use for C/ObjC code (IDENT from LangStandards.def, empty for platform default)")
-set(CLANG_DEFAULT_STD_CXX "" CACHE STRING
-  "Default standard to use for C++/ObjC++ code (IDENT from LangStandards.def, empty for platform default)")
-
 set(CLANG_DEFAULT_LINKER "" CACHE STRING
   "Default linker to use (linker name or absolute path, empty for platform default)")
 
 set(CLANG_DEFAULT_CXX_STDLIB "" CACHE STRING
   "Default C++ stdlib to use (\"libstdc++\" or \"libc++\", empty for platform default")
 if (NOT(CLANG_DEFAULT_CXX_STDLIB STREQUAL "" OR
         CLANG_DEFAULT_CXX_STDLIB STREQUAL "libstdc++" OR
         CLANG_DEFAULT_CXX_STDLIB STREQUAL "libc++"))

clang/include/clang/Config/config.h.cmake

 /* This generated file is for internal use. Do not include it from headers. */
 
 #ifdef CLANG_CONFIG_H
 #error config.h can only be included once
 #else
 #define CLANG_CONFIG_H
 
 /* Bug report URL. */
 #define BUG_REPORT_URL "${BUG_REPORT_URL}"
 
 /* Default to -fPIE and -pie on Linux. */
 #cmakedefine01 CLANG_DEFAULT_PIE_ON_LINUX
 
 /* Default linker to use. */
 #define CLANG_DEFAULT_LINKER "${CLANG_DEFAULT_LINKER}"
 
-/* Default C/ObjC standard to use. */
-#cmakedefine CLANG_DEFAULT_STD_C LangStandard::lang_${CLANG_DEFAULT_STD_C}
-/* Always #define something so that missing the config.h #include at use sites
- * becomes a compile error.
- */
-#ifndef CLANG_DEFAULT_STD_C
-#define CLANG_DEFAULT_STD_C LangStandard::lang_unspecified
-#endif
-
-/* Default C++/ObjC++ standard to use. */
-#cmakedefine CLANG_DEFAULT_STD_CXX LangStandard::lang_${CLANG_DEFAULT_STD_CXX}
-/* Always #define something so that missing the config.h #include at use sites
- * becomes a compile error.
- */
-#ifndef CLANG_DEFAULT_STD_CXX
-#define CLANG_DEFAULT_STD_CXX LangStandard::lang_unspecified
-#endif
-
 /* Default C++ stdlib to use. */
 #define CLANG_DEFAULT_CXX_STDLIB "${CLANG_DEFAULT_CXX_STDLIB}"
 
 /* Default runtime library to use. */
 #define CLANG_DEFAULT_RTLIB "${CLANG_DEFAULT_RTLIB}"
 
 /* Default unwind library to use. */
 #define CLANG_DEFAULT_UNWINDLIB "${CLANG_DEFAULT_UNWINDLIB}"

clang/lib/Basic/LangStandards.cpp

LangStandard::Kind clang::getDefaultLanguageStandard(clang::Language Lang,
   case Language::OpenCL:
     return LangStandard::lang_opencl12;
   case Language::OpenCLCXX:
     return LangStandard::lang_openclcpp10;
   case Language::CUDA:
     return LangStandard::lang_cuda;
   case Language::Asm:
   case Language::C:
-    if (CLANG_DEFAULT_STD_C != LangStandard::lang_unspecified)
-      return CLANG_DEFAULT_STD_C;
-
     // The PS4 uses C99 as the default C standard.
     if (T.isPS4())
       return LangStandard::lang_gnu99;
     return LangStandard::lang_gnu17;
   case Language::ObjC:
-    if (CLANG_DEFAULT_STD_C != LangStandard::lang_unspecified)
-      return CLANG_DEFAULT_STD_C;
-
     return LangStandard::lang_gnu11;
   case Language::CXX:
   case Language::ObjCXX:
-    if (CLANG_DEFAULT_STD_CXX != LangStandard::lang_unspecified)
-      return CLANG_DEFAULT_STD_CXX;
-
     if (T.isPS())
       return LangStandard::lang_gnucxx14;
     return LangStandard::lang_gnucxx17;
   case Language::RenderScript:
     return LangStandard::lang_c99;
   case Language::HIP:
     return LangStandard::lang_hip;
   case Language::HLSL:
     return LangStandard::lang_hlsl2021;
   }
   llvm_unreachable("unhandled Language kind!");
 }

clang/test/Preprocessor/lang-std.cpp

-// UNSUPPORTED: default-std-cxx, ps4, ps5
-/// Test default standards when CLANG_DEFAULT_STD_CXX is unspecified.
+// UNSUPPORTED: ps4, ps5
+/// Test default standards.
 /// PS4/PS5 default to gnu++14.
 
 // RUN: %clang_cc1 -dM -E %s | FileCheck --check-prefix=CXX17 %s
 // RUN: %clang_cc1 -dM -E -x cuda %s | FileCheck --check-prefix=CXX14 %s
 // RUN: %clang_cc1 -dM -E -x hip %s | FileCheck --check-prefix=CXX14 %s
 
 // RUN: %clang_cc1 -dM -E -x cuda -std=c++14 %s | FileCheck --check-prefix=CXX14 %s
 // RUN: %clang_cc1 -dM -E -x hip -std=c++98 %s | FileCheck --check-prefix=CXX98 %s
 
 // CXX98: #define __cplusplus 199711L
 // CXX14: #define __cplusplus 201402L
 // CXX17: #define __cplusplus 201703L

clang/test/lit.cfg.py

 
 # Plugins (loadable modules)
 if config.has_plugins and config.llvm_plugin_ext:
     config.available_features.add('plugins')
 
 if config.clang_default_pie_on_linux:
     config.available_features.add('default-pie-on-linux')
 
-if config.clang_default_std_cxx != '':
-    config.available_features.add('default-std-cxx')
-
 # Set available features we allow tests to conditionalize on.
 #
 if config.clang_default_cxx_stdlib != '':
     config.available_features.add('default-cxx-stdlib={}'.format(config.clang_default_cxx_stdlib))
 
 # As of 2011.08, crash-recovery tests still do not pass on FreeBSD.
 if platform.system() not in ['FreeBSD']:
     config.available_features.add('crash-recovery')

clang/test/lit.site.cfg.py.in

 config.target_triple = "@LLVM_TARGET_TRIPLE@"
 config.host_cc = "@CMAKE_C_COMPILER@"
 config.host_cxx = "@CMAKE_CXX_COMPILER@"
 config.llvm_use_sanitizer = "@LLVM_USE_SANITIZER@"
 config.have_zlib = @LLVM_ENABLE_ZLIB@
 config.have_zstd = @LLVM_ENABLE_ZSTD@
 config.clang_arcmt = @CLANG_ENABLE_ARCMT@
 config.clang_default_pie_on_linux = @CLANG_DEFAULT_PIE_ON_LINUX@
-config.clang_default_std_cxx = "@CLANG_DEFAULT_STD_CXX@"
 config.clang_default_cxx_stdlib = "@CLANG_DEFAULT_CXX_STDLIB@"
 config.clang_staticanalyzer = @CLANG_ENABLE_STATIC_ANALYZER@
 config.clang_staticanalyzer_z3 = @LLVM_WITH_Z3@
 config.clang_examples = @CLANG_BUILD_EXAMPLES@
 config.enable_shared = @ENABLE_SHARED@
 config.enable_backtrace = @ENABLE_BACKTRACES@
 config.enable_threads = @LLVM_ENABLE_THREADS@
 config.host_arch = "@HOST_ARCH@"

llvm/utils/gn/secondary/clang/include/clang/Config/BUILD.gn

 import("//clang/lib/ARCMigrate/enable.gni")
 import("//clang/lib/StaticAnalyzer/Frontend/enable.gni")
 import("//llvm/utils/gn/build/libs/xml/enable.gni")
 import("//llvm/utils/gn/build/write_cmake_config.gni")
 import("//llvm/version.gni")
 
 write_cmake_config("Config") {
   input = "config.h.cmake"
   output = "$target_gen_dir/config.h"
   values = [
     "BUG_REPORT_URL=https://github.com/llvm/llvm-project/issues/",
     "CLANG_DEFAULT_PIE_ON_LINUX=1",
     "CLANG_DEFAULT_LINKER=",
-    "CLANG_DEFAULT_STD_C=",
-    "CLANG_DEFAULT_STD_CXX=",
     "CLANG_DEFAULT_CXX_STDLIB=",
     "CLANG_DEFAULT_RTLIB=",
     "CLANG_DEFAULT_OBJCOPY=objcopy",
     "CLANG_DEFAULT_OPENMP_RUNTIME=libomp",
     "CLANG_DEFAULT_UNWINDLIB=",
     "CLANG_INSTALL_LIBDIR_BASENAME=lib",
     "CLANG_OPENMP_NVPTX_DEFAULT_ARCH=sm_35",
     "CLANG_RESOURCE_DIR=",

llvm/utils/gn/secondary/clang/test/BUILD.gn

write_lit_config("lit_site_cfg") {
   # Fully-qualified instead of relative for LIT_SITE_CFG_IN_HEADER.
   input = "//clang/test/lit.site.cfg.py.in"
   output = clang_lit_site_cfg_file
   dir = get_path_info(output, "dir")
   extra_values = [
     "LLVM_WITH_Z3=0",
     "CLANG_BUILD_EXAMPLES=0",
     "CLANG_BUILT_STANDALONE=0",
-    "CLANG_DEFAULT_STD_CXX=",
     "CLANG_DEFAULT_CXX_STDLIB=",  # Empty string means "default value" here.
     "CLANG_DEFAULT_PIE_ON_LINUX=0",
     "CLANG_PLUGIN_SUPPORT=0",  # FIXME: Analysis/plugins need global -fPIC
     "CURRENT_TOOLS_DIR=" + rebase_path("$root_out_dir/bin", dir),
     "CLANG_VENDOR_UTI=org.llvm.clang",
 
     # This is only used if LLVM_USE_SANITIZER includes lsan and the host
     # OS is macOS. Since the GN build currently never uses LLVM_USE_SANITIZER,

utils/bazel/llvm-project-overlay/clang/include/clang/Config/config.h

 #define BUG_REPORT_URL "https://github.com/llvm/llvm-project/issues/"
 
 /* Default to -fPIE and -pie on Linux. */
 #define CLANG_DEFAULT_PIE_ON_LINUX 1
 
 /* Default linker to use. */
 #define CLANG_DEFAULT_LINKER ""
 
-/* Default C/ObjC standard to use. */
-/* #undef CLANG_DEFAULT_STD_C */
-/* Always #define something so that missing the config.h #include at use sites
- * becomes a compile error.
- */
-#ifndef CLANG_DEFAULT_STD_C
-#define CLANG_DEFAULT_STD_C LangStandard::lang_unspecified
-#endif
-
-/* Default C++/ObjC++ standard to use. */
-/* #undef CLANG_DEFAULT_STD_CXX */
-/* Always #define something so that missing the config.h #include at use sites
- * becomes a compile error.
- */
-#ifndef CLANG_DEFAULT_STD_CXX
-#define CLANG_DEFAULT_STD_CXX LangStandard::lang_unspecified
-#endif
-
 /* Default C++ stdlib to use. */
 #define CLANG_DEFAULT_CXX_STDLIB ""
 
 /* Default runtime library to use. */
 #define CLANG_DEFAULT_RTLIB ""
 
 /* Default unwind library to use. */
 #define CLANG_DEFAULT_UNWINDLIB ""