`isfinite` declared `constexpr` for c++20 #3745

hchataing · 2023-12-08T22:52:56Z

std::isfinite is not declared constexpr before c++23, but fmt::isfinite is declared constexpr for c++20
(https://en.cppreference.com/w/cpp/numeric/math/isfinite).

https://github.com/fmtlib/fmt/blob/master/include/fmt/core.h#L108:

#if ((FMT_CPLUSPLUS >= 202002L) &&                            \
     (!defined(_GLIBCXX_RELEASE) || _GLIBCXX_RELEASE > 9)) || \
    (FMT_CPLUSPLUS >= 201709L && FMT_GCC_VERSION >= 1002)
#  define FMT_CONSTEXPR20 constexpr
#else
#  define FMT_CONSTEXPR20
#endif

https://github.com/fmtlib/fmt/blob/master/include/fmt/format.h#L2753:

template <typename T, FMT_ENABLE_IF(std::is_floating_point<T>::value&&
                                        has_isfinite<T>::value)>
FMT_CONSTEXPR20 bool isfinite(T value) {
  constexpr T inf = T(std::numeric_limits<double>::infinity());
  if (is_constant_evaluated())
    return !detail::isnan(value) && value < inf && value > -inf;
  return std::isfinite(value);
}

Resulting compilation error:

external/fmtlib/test/format-test.cc:94:24: error: constexpr variable 'result' must be initialized by a constant expression
  FMT_CONSTEXPR20 auto result = isfinite(Float(limits::infinity()));
                       ^        ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
external/fmtlib/test/format-test.cc:103:3: note: in instantiation of function template specialization 'check_isfinite<double>' requested here
  check_isfinite<double>();
  ^
external/fmtlib/include/fmt/format.h:2751:10: note: non-constexpr function 'isfinite<double>' cannot be used in a constant expression
  return std::isfinite(value);
         ^
external/fmtlib/test/format-test.cc:94:33: note: in call to 'isfinite(INF)'
  FMT_CONSTEXPR20 auto result = isfinite(Float(limits::infinity()));
                                ^
external/libcxx/include/math.h:439:1: note: declared here
isfinite(_A1 __lcpp_x) _NOEXCEPT
^
1 error generated.
22:46:14 ninja failed with: exit status 1

Compiler version (Android 🤷 )

Android (11039501, +pgo, +bolt, +lto, +mlgo, based on r498229b) clang version 17.0.4 (https://android.googlesource.com/toolchain/llvm-project e34ed7d63863b45858e74126edaa738e75887800)
Target: x86_64-unknown-linux-gnu
Thread model: posix
InstalledDir: /usr/local/google/home/henrichataing/Projects/android/aosp-master/prebuilts/clang/host/linux-x86/clang-r498229b/bin

The text was updated successfully, but these errors were encountered:

hchataing · 2023-12-08T23:08:27Z

Locally fixed by defining

#if (FMT_CPLUSPLUS >= 202302L)
#  define FMT_CONSTEXPR23 constexpr
#else
#  define FMT_CONSTEXPR23
#endif

and using this define in the declaration of isfinite

template <typename T, FMT_ENABLE_IF(std::is_floating_point<T>::value&&
                                        has_isfinite<T>::value)>
FMT_CONSTEXPR23 bool isfinite(T value) {
  constexpr T inf = T(std::numeric_limits<double>::infinity());
  if (is_constant_evaluated())
    return !detail::isnan(value) && value < inf && value > -inf;
  return std::isfinite(value);
}

I can propose a patch if you are happy with this solution.

vitaut · 2023-12-09T00:25:02Z

I can propose a patch if you are happy with this solution.

Please do.

Fix fmtlib#3745

Dependent function std::isfinite is constexpr for c++23. https://en.cppreference.com/w/cpp/numeric/math/isfinite Fix fmtlib#3745

phprus · 2023-12-09T06:52:50Z

Which version of libc++ or libstdc++ are you using?

std::isfinite is only used in non-constexpr contexts.

In a constexpr context, the condition and code must be executed:

if (is_constant_evaluated())
  return !detail::isnan(value) && value < inf && value > -inf;

phprus · 2023-12-09T07:29:46Z

When using Clang-17 and libc++-17 in c++20 mode, compilation and tests pass without errors:

https://github.com/phprus/fmt/actions/runs/7149828208/job/19472458664
https://github.com/phprus/fmt/actions/runs/7149828208/job/19472458718

hchataing · 2023-12-10T00:56:16Z

Will confirm the details on monday, but from my knowledge: the code is compiled with std=gnu++20 but the library supports c++17 only

hchataing · 2023-12-12T16:45:24Z

Turns out the standard library does not provide std::is_constant_evaluated, or at least it is not in context, thus making the function implementation non constexpr compatible.
That's the what we get for having different versions for the compiler and stl 😿

The implementation failed to be constexpr in the situation where: __cplusplus == 202002L && !defined(__cpp_lib_is_constant_evaluated) Fix fmtlib#3745

vitaut · 2023-12-13T15:09:05Z

@hchataing, could you check if #3754 fixes the issue?

hchataing · 2023-12-13T16:24:12Z

I checked the compiler pre-defined variables, and it is not going to be sufficient.
For reference:

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#define __cpp_template_auto 201606L
#define __cpp_threadsafe_static_init 200806L
#define __cpp_unicode_characters 200704L
#define __cpp_unicode_literals 200710L
#define __cpp_user_defined_literals 200809L
#define __cpp_using_enum 201907L
#define __cpp_variable_templates 201304L
#define __cpp_variadic_templates 200704L
#define __cpp_variadic_using 201611L
#define __gnu_linux__ 1
#define __k8 1
#define __k8__ 1
#define __linux 1
#define __linux__ 1
#define __llvm__ 1
#define __pic__ 2
#define __pie__ 2
#define __private_extern__ extern
#define __seg_fs __attribute__((address_space(257)))
#define __seg_gs __attribute__((address_space(256)))
#define __tune_k8__ 1
#define __unix 1
#define __unix__ 1
#define __x86_64 1
#define __x86_64__ 1

vitaut · 2023-12-13T16:49:18Z

_LIBCPP_VERSION should be defined by the library, not the compiler. Are you sure that it is not defined?

vitaut · 2023-12-13T16:53:34Z

You could check with something like

#ifndef _LIBCPP_VERSION
#  error _LIBCPP_VERSION undefined
#endif

vitaut · 2023-12-17T16:35:12Z

Should be fixed by #3754 but if you still get this issue feel free to reopen and provide more details, most importantly print the actual value of _LIBCPP_VERSION.

hchataing · 2023-12-18T16:12:22Z

Got it, thanks for the help !

vitaut added the help wanted label Dec 8, 2023

hchataing mentioned this issue Dec 9, 2023

Update isfinite declaration to be constexpr for c++23 only #3746

Closed

hchataing added a commit to hchataing/fmt that referenced this issue Dec 9, 2023

Update isfinite declaration to be constexpr for c++23 only

2b23e4e

Fix fmtlib#3745

hchataing added a commit to hchataing/fmt that referenced this issue Dec 9, 2023

Update isfinite declaration to be constexpr for c++23 only

f009126

Dependent function std::isfinite is constexpr for c++23. https://en.cppreference.com/w/cpp/numeric/math/isfinite Fix fmtlib#3745

hchataing added a commit to hchataing/fmt that referenced this issue Dec 9, 2023

Update isfinite declaration to be constexpr for c++23 only

04338ce

Dependent function std::isfinite is constexpr for c++23. https://en.cppreference.com/w/cpp/numeric/math/isfinite Fix fmtlib#3745

hchataing added a commit to hchataing/fmt that referenced this issue Dec 12, 2023

Fix isfinite implementation to be constexpr

f68bff6

The implementation failed to be constexpr in the situation where: __cplusplus == 202002L && !defined(__cpp_lib_is_constant_evaluated) Fix fmtlib#3745

phprus mentioned this issue Dec 13, 2023

Add stdlib version check for C++20 #3754

Merged

vitaut closed this as completed in #3754 Dec 17, 2023

BrewTestBot mentioned this issue Jan 1, 2024

fmt 10.2.0 Homebrew/homebrew-core#158708

Merged

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`isfinite` declared `constexpr` for c++20 #3745

`isfinite` declared `constexpr` for c++20 #3745

hchataing commented Dec 8, 2023 •

edited

hchataing commented Dec 8, 2023

vitaut commented Dec 9, 2023

phprus commented Dec 9, 2023

phprus commented Dec 9, 2023

hchataing commented Dec 10, 2023

hchataing commented Dec 12, 2023

vitaut commented Dec 13, 2023

hchataing commented Dec 13, 2023

vitaut commented Dec 13, 2023

vitaut commented Dec 13, 2023

vitaut commented Dec 17, 2023

hchataing commented Dec 18, 2023

isfinite declared constexpr for c++20 #3745

isfinite declared constexpr for c++20 #3745

Comments

hchataing commented Dec 8, 2023 • edited

hchataing commented Dec 8, 2023

vitaut commented Dec 9, 2023

phprus commented Dec 9, 2023

phprus commented Dec 9, 2023

hchataing commented Dec 10, 2023

hchataing commented Dec 12, 2023

vitaut commented Dec 13, 2023

hchataing commented Dec 13, 2023

vitaut commented Dec 13, 2023

vitaut commented Dec 13, 2023

vitaut commented Dec 17, 2023

hchataing commented Dec 18, 2023

`isfinite` declared `constexpr` for c++20 #3745

`isfinite` declared `constexpr` for c++20 #3745

hchataing commented Dec 8, 2023 •

edited