[libcxx] Remove _LIBCPP_HAS_NO_BUILTIN_IS_CONSTANT_EVALUATED

[Mordante]

Summary

All supported compilers should support
_LIBCPP_HAS_NO_BUILTIN_IS_CONSTANT_EVALUATED so this can be removed.

[tschuett]

libcxx/include/__config

1295

Could you keep this check as a test to catch unsupported compilers, i.e. move it to the test folder?

[Mordante]

libcxx/include/__config

1295

I think it isn't useful to have this in the test folder. This will only catch issues if the user with an unsupported compiler runs the test suite.
If we want to validate the support status it's probably better to do something along the lines of

#if defined(_GNU_VER) && _GNU_VER < 1100
#errror "Only GCC 11 and 12 are supported."
#elif ... // all other supported platforms.

in __config.

Then the unsupported compiler is caught when a user uses it. (Obviously this also catches using it for unit tests.)
WDYT?

[tschuett]

Completely up to you. How about a separate header to catch unsupported compilers?

It would document in code the minimal supported compiler versions. At the moment it is some rst file I guess.

[Mordante]

In D107239#2920079, @tschuett wrote:

Completely up to you. How about a separate header to catch unsupported compilers?

Not too fond of that idea, __config seems like a good place, it also does other validations.

It would document in code the minimal supported compiler versions. At the moment it is some rst file I guess.

Yes at the moment it's https://libcxx.llvm.org/index.html# under Platform and Compiler Support. So it would be wise to add some comment in the .rst regarding updating the header.

@ldionne how do you feel about validating whether the compiler is supported in __config?

If we want this validation I'd rather do it in a separate patch.

[• Quuxplusone]

FWIW, this looks good to me. (I was initially prepared to disapprove of removing support for weird old compilers, but after seeing the diff, I agree: any compiler that expects to compile a C++20 standard library implementation needs to provide __builtin_is_constant_evaluated, full stop. And this patch simplifies the codebase nicely.)

[Mordante]

Thanks for the review. And indeed the intention is to remove some of the no longer supported compiler work-arounds from libc++.

[ldionne]

LGTM.

The issue of #erroring out on compilers that are not supported is a tricky one, and I'd tackle it as a separate effort. I see the following options:

1. Error-out on any compiler that isn't officially supported, even if parts of the library may still work.
2. Error-out only on compilers that we know will fail in crazy ways in order to provide a "fail fast and obviously" kind of experience to users.

The problem with (1) is that it could be perceived as raising the bar for people who want to create a setup that "kinda works". I can easily imagine people saying "my setup works just fine even though you don't officially support it - why are you making my life harder?".

The problem with (2) is that it's a moving target, and it's not easy to define "is obviously broken on compiler version X". For example, GCC 5 *clearly* doesn't work, but what about GCC 10? It's not supported, but parts of the library do work. How broken should compiler support be in order to explicitly #error out in __config?

In both cases, it's not an easy solution. Let's tackle it separately.

[Mordante]

Valid point. Probably GCC-10 will work perfectly when you compile in C++17 mode. Maybe we should not introduce the errors for now and wait and see how many bug reports we get of users using unsupported compilers. If that becomes a burden we can introduce the #errors.

[yaxunl]

This patch caused regression at https://lab.llvm.org/buildbot/#/builders/165/builds/5163

[yaxunl]

In D107239#2937895, @yaxunl wrote:

This patch caused regression at https://lab.llvm.org/buildbot/#/builders/165/builds/5163

Sorry this may be due to the build bot uses old compiler. I will see if I can upgrade it or use bootstrap.

[Mordante]

In D107239#2937923, @yaxunl wrote:

In D107239#2937895, @yaxunl wrote:

This patch caused regression at https://lab.llvm.org/buildbot/#/builders/165/builds/5163

Sorry this may be due to the build bot uses old compiler. I will see if I can upgrade it or use bootstrap.

This has been discussed off-line. I already had contact with the build-bot maintainer, before I saw this message.

libcxx/include/__config

 #else
 #  define _LIBCPP_SAFE_STATIC
 #endif
 
 #if !__has_builtin(__builtin_addressof) && _GNUC_VER < 700
 #define _LIBCPP_HAS_NO_BUILTIN_ADDRESSOF
 #endif
 
-#if !__has_builtin(__builtin_is_constant_evaluated) && _GNUC_VER < 900
-#define _LIBCPP_HAS_NO_BUILTIN_IS_CONSTANT_EVALUATED
-#endif
-
 #if __has_attribute(diagnose_if) && !defined(_LIBCPP_DISABLE_ADDITIONAL_DIAGNOSTICS)
 #  define _LIBCPP_DIAGNOSE_WARNING(...) \
      __attribute__((diagnose_if(__VA_ARGS__, "warning")))
 #  define _LIBCPP_DIAGNOSE_ERROR(...) \
      __attribute__((diagnose_if(__VA_ARGS__, "error")))
 #else
 #  define _LIBCPP_DIAGNOSE_WARNING(...)
 #  define _LIBCPP_DIAGNOSE_ERROR(...)

libcxx/include/__format/format_parse_context.h

 _LIBCPP_BEGIN_NAMESPACE_STD
 
 #if _LIBCPP_STD_VER > 17
 
 // TODO FMT Remove this once we require compilers with proper C++20 support.
 // If the compiler has no concepts support, the format header will be disabled.
 // Without concepts support enable_if needs to be used and that too much effort
 // to support compilers with partial C++20 support.
-#if !defined(_LIBCPP_HAS_NO_CONCEPTS) &&                                       \
-    !defined(_LIBCPP_HAS_NO_BUILTIN_IS_CONSTANT_EVALUATED)
+#if !defined(_LIBCPP_HAS_NO_CONCEPTS)
 
 template <class _CharT>
 class _LIBCPP_TEMPLATE_VIS _LIBCPP_AVAILABILITY_FORMAT basic_format_parse_context {
 public:
   using char_type = _CharT;
   using const_iterator = typename basic_string_view<_CharT>::const_iterator;
   using iterator = const_iterator;
 
private:
   _Indexing __indexing_;
   size_t __next_arg_id_;
   size_t __num_args_;
 };
 
 using format_parse_context = basic_format_parse_context<char>;
 using wformat_parse_context = basic_format_parse_context<wchar_t>;
 
-#endif // !defined(_LIBCPP_HAS_NO_CONCEPTS) && !defined(_LIBCPP_HAS_NO_BUILTIN_IS_CONSTANT_EVALUATED)
+#endif // !defined(_LIBCPP_HAS_NO_CONCEPTS)
 
 #endif //_LIBCPP_STD_VER > 17
 
 _LIBCPP_END_NAMESPACE_STD
 
 _LIBCPP_POP_MACROS
 
 #endif // _LIBCPP___FORMAT_FORMAT_PARSE_CONTEXT_H

libcxx/include/type_traits

struct __can_extract_map_key
     : integral_constant<bool, _IsSame<_RawValTy, _Key>::value> {};
 
 // This specialization returns __extract_key_fail_tag for non-map containers
 // because _Key == _ContainerValueTy
 template <class _ValTy, class _Key, class _RawValTy>
 struct __can_extract_map_key<_ValTy, _Key, _Key, _RawValTy>
     : false_type {};
 
-#ifndef _LIBCPP_HAS_NO_BUILTIN_IS_CONSTANT_EVALUATED
 #if _LIBCPP_STD_VER > 17
 _LIBCPP_INLINE_VISIBILITY
 inline constexpr bool is_constant_evaluated() noexcept {
   return __builtin_is_constant_evaluated();
 }
 #endif
 
 inline _LIBCPP_CONSTEXPR
 bool __libcpp_is_constant_evaluated() _NOEXCEPT { return __builtin_is_constant_evaluated(); }
-#else
-inline _LIBCPP_CONSTEXPR
-bool __libcpp_is_constant_evaluated() _NOEXCEPT { return false; }
-#endif
 
 template <class _CharT>
 using _IsCharLikeType = _And<is_standard_layout<_CharT>, is_trivial<_CharT> >;
 
 template<class _Tp>
 using __make_const_lvalue_ref = const typename remove_reference<_Tp>::type&;
 
 #if _LIBCPP_STD_VER > 17

libcxx/include/version

 // #   define __cpp_lib_format                             201907L
 # endif
 # define __cpp_lib_generic_unordered_lookup             201811L
 # define __cpp_lib_int_pow2                             202002L
 # if !defined(_LIBCPP_HAS_NO_CONCEPTS)
 #   define __cpp_lib_integer_comparison_functions       202002L
 # endif
 # define __cpp_lib_interpolate                          201902L
-# if !defined(_LIBCPP_HAS_NO_BUILTIN_IS_CONSTANT_EVALUATED)
-#   define __cpp_lib_is_constant_evaluated              201811L
-# endif
+# define __cpp_lib_is_constant_evaluated                201811L
 // # define __cpp_lib_is_layout_compatible                 201907L
 # define __cpp_lib_is_nothrow_convertible               201806L
 // # define __cpp_lib_is_pointer_interconvertible          201907L
 # if !defined(_LIBCPP_HAS_NO_THREADS)
 // #   define __cpp_lib_jthread                            201911L
 # endif
 # if !defined(_LIBCPP_HAS_NO_THREADS) && !defined(_LIBCPP_AVAILABILITY_DISABLE_FTM___cpp_lib_latch)
 #   define __cpp_lib_latch                              201907L

libcxx/test/libcxx/type_traits/is_constant_evaluated.pass.cpp

 #include <cassert>
 
 #include "test_macros.h"
 
 int main (int, char**) {
     ASSERT_SAME_TYPE(decltype(std::__libcpp_is_constant_evaluated()), bool);
     ASSERT_NOEXCEPT(std::__libcpp_is_constant_evaluated());
 
-#if !defined(_LIBCPP_HAS_NO_BUILTIN_IS_CONSTANT_EVALUATED) && !defined(_LIBCPP_CXX03_LANG)
+#if !defined(_LIBCPP_CXX03_LANG)
     static_assert(std::__libcpp_is_constant_evaluated(), "");
 #endif
 
     bool p = std::__libcpp_is_constant_evaluated();
     assert(!p);
 
     return 0;
     }

libcxx/test/std/language.support/support.limits/support.limits.general/type_traits.version.pass.cpp

 #     error "__cpp_lib_is_aggregate should have the value 201703L in c++20"
 #   endif
 # else
 #   ifdef __cpp_lib_is_aggregate
 #     error "__cpp_lib_is_aggregate should not be defined when TEST_HAS_BUILTIN_IDENTIFIER(__is_aggregate) || TEST_GCC_VER_NEW >= 7001 is not defined!"
 #   endif
 # endif
 
-# if TEST_HAS_BUILTIN(__builtin_is_constant_evaluated) || TEST_GCC_VER >= 900
-#   ifndef __cpp_lib_is_constant_evaluated
-#     error "__cpp_lib_is_constant_evaluated should be defined in c++20"
-#   endif
-#   if __cpp_lib_is_constant_evaluated != 201811L
-#     error "__cpp_lib_is_constant_evaluated should have the value 201811L in c++20"
-#   endif
-# else
-#   ifdef __cpp_lib_is_constant_evaluated
-#     error "__cpp_lib_is_constant_evaluated should not be defined when TEST_HAS_BUILTIN(__builtin_is_constant_evaluated) || TEST_GCC_VER >= 900 is not defined!"
-#   endif
-# endif
+# ifndef __cpp_lib_is_constant_evaluated
+#   error "__cpp_lib_is_constant_evaluated should be defined in c++20"
+# endif
+# if __cpp_lib_is_constant_evaluated != 201811L
+#   error "__cpp_lib_is_constant_evaluated should have the value 201811L in c++20"
+# endif
 
 # ifndef __cpp_lib_is_final
 #   error "__cpp_lib_is_final should be defined in c++20"
 # endif
 # if __cpp_lib_is_final != 201402L
 #   error "__cpp_lib_is_final should have the value 201402L in c++20"
 # endif
 

 #     error "__cpp_lib_is_aggregate should have the value 201703L in c++2b"
 #   endif
 # else
 #   ifdef __cpp_lib_is_aggregate
 #     error "__cpp_lib_is_aggregate should not be defined when TEST_HAS_BUILTIN_IDENTIFIER(__is_aggregate) || TEST_GCC_VER_NEW >= 7001 is not defined!"
 #   endif
 # endif
 
-# if TEST_HAS_BUILTIN(__builtin_is_constant_evaluated) || TEST_GCC_VER >= 900
-#   ifndef __cpp_lib_is_constant_evaluated
-#     error "__cpp_lib_is_constant_evaluated should be defined in c++2b"
-#   endif
-#   if __cpp_lib_is_constant_evaluated != 201811L
-#     error "__cpp_lib_is_constant_evaluated should have the value 201811L in c++2b"
-#   endif
-# else
-#   ifdef __cpp_lib_is_constant_evaluated
-#     error "__cpp_lib_is_constant_evaluated should not be defined when TEST_HAS_BUILTIN(__builtin_is_constant_evaluated) || TEST_GCC_VER >= 900 is not defined!"
-#   endif
-# endif
+# ifndef __cpp_lib_is_constant_evaluated
+#   error "__cpp_lib_is_constant_evaluated should be defined in c++2b"
+# endif
+# if __cpp_lib_is_constant_evaluated != 201811L
+#   error "__cpp_lib_is_constant_evaluated should have the value 201811L in c++2b"
+# endif
 
 # ifndef __cpp_lib_is_final
 #   error "__cpp_lib_is_final should be defined in c++2b"
 # endif
 # if __cpp_lib_is_final != 201402L
 #   error "__cpp_lib_is_final should have the value 201402L in c++2b"
 # endif
 

libcxx/test/std/language.support/support.limits/support.limits.general/version.version.pass.cpp

 #     error "__cpp_lib_is_aggregate should have the value 201703L in c++20"
 #   endif
 # else
 #   ifdef __cpp_lib_is_aggregate
 #     error "__cpp_lib_is_aggregate should not be defined when TEST_HAS_BUILTIN_IDENTIFIER(__is_aggregate) || TEST_GCC_VER_NEW >= 7001 is not defined!"
 #   endif
 # endif
 
-# if TEST_HAS_BUILTIN(__builtin_is_constant_evaluated) || TEST_GCC_VER >= 900
-#   ifndef __cpp_lib_is_constant_evaluated
-#     error "__cpp_lib_is_constant_evaluated should be defined in c++20"
-#   endif
-#   if __cpp_lib_is_constant_evaluated != 201811L
-#     error "__cpp_lib_is_constant_evaluated should have the value 201811L in c++20"
-#   endif
-# else
-#   ifdef __cpp_lib_is_constant_evaluated
-#     error "__cpp_lib_is_constant_evaluated should not be defined when TEST_HAS_BUILTIN(__builtin_is_constant_evaluated) || TEST_GCC_VER >= 900 is not defined!"
-#   endif
-# endif
+# ifndef __cpp_lib_is_constant_evaluated
+#   error "__cpp_lib_is_constant_evaluated should be defined in c++20"
+# endif
+# if __cpp_lib_is_constant_evaluated != 201811L
+#   error "__cpp_lib_is_constant_evaluated should have the value 201811L in c++20"
+# endif
 
 # ifndef __cpp_lib_is_final
 #   error "__cpp_lib_is_final should be defined in c++20"
 # endif
 # if __cpp_lib_is_final != 201402L
 #   error "__cpp_lib_is_final should have the value 201402L in c++20"
 # endif
 

 #     error "__cpp_lib_is_aggregate should have the value 201703L in c++2b"
 #   endif
 # else
 #   ifdef __cpp_lib_is_aggregate
 #     error "__cpp_lib_is_aggregate should not be defined when TEST_HAS_BUILTIN_IDENTIFIER(__is_aggregate) || TEST_GCC_VER_NEW >= 7001 is not defined!"
 #   endif
 # endif
 
-# if TEST_HAS_BUILTIN(__builtin_is_constant_evaluated) || TEST_GCC_VER >= 900
-#   ifndef __cpp_lib_is_constant_evaluated
-#     error "__cpp_lib_is_constant_evaluated should be defined in c++2b"
-#   endif
-#   if __cpp_lib_is_constant_evaluated != 201811L
-#     error "__cpp_lib_is_constant_evaluated should have the value 201811L in c++2b"
-#   endif
-# else
-#   ifdef __cpp_lib_is_constant_evaluated
-#     error "__cpp_lib_is_constant_evaluated should not be defined when TEST_HAS_BUILTIN(__builtin_is_constant_evaluated) || TEST_GCC_VER >= 900 is not defined!"
-#   endif
-# endif
+# ifndef __cpp_lib_is_constant_evaluated
+#   error "__cpp_lib_is_constant_evaluated should be defined in c++2b"
+# endif
+# if __cpp_lib_is_constant_evaluated != 201811L
+#   error "__cpp_lib_is_constant_evaluated should have the value 201811L in c++2b"
+# endif
 
 # ifndef __cpp_lib_is_final
 #   error "__cpp_lib_is_final should be defined in c++2b"
 # endif
 # if __cpp_lib_is_final != 201402L
 #   error "__cpp_lib_is_final should have the value 201402L in c++2b"
 # endif
 

libcxx/utils/generate_feature_test_macro_components.py

feature_test_macros = [ add_version_header(x) for x in [
     "values": { "c++17": 201703 },
     "headers": ["type_traits"],
     "test_suite_guard": "TEST_HAS_BUILTIN_IDENTIFIER(__is_aggregate) || TEST_GCC_VER_NEW >= 7001",
     "libcxx_guard": "!defined(_LIBCPP_HAS_NO_IS_AGGREGATE)",
   }, {
     "name": "__cpp_lib_is_constant_evaluated",
     "values": { "c++20": 201811 },
     "headers": ["type_traits"],
-    "test_suite_guard": "TEST_HAS_BUILTIN(__builtin_is_constant_evaluated) || TEST_GCC_VER >= 900",
-    "libcxx_guard": "!defined(_LIBCPP_HAS_NO_BUILTIN_IS_CONSTANT_EVALUATED)",
   }, {
     "name": "__cpp_lib_is_final",
     "values": { "c++14": 201402 },
     "headers": ["type_traits"],
   }, {
     "name": "__cpp_lib_is_invocable",
     "values": { "c++17": 201703 },
     "headers": ["type_traits"],