Constexpr formatted_size #3026

marksantaniello · 2022-08-06T18:03:55Z

Minimal changes to make fmt::formatted_size constexpr in C++20. Adjust the existing unit test accordingly.

vitaut

Thanks for the PR! One minor comment inline, otherwise LGTM.

vitaut · 2022-08-08T17:34:25Z

test/compile-test.cc

+  static FMT_CONSTEXPR20 size_t s1 = fmt::formatted_size(FMT_COMPILE("{0}"), 42);
+  EXPECT_EQ(2, s1);
+  static FMT_CONSTEXPR20 size_t s2 = fmt::formatted_size(FMT_COMPILE("{0:<4.2f}"), 42.0);
+  EXPECT_EQ(5, s2);


static seems unnecessary, please remove.

vitaut · 2022-08-08T21:25:04Z

CI is failing: https://github.com/fmtlib/fmt/runs/7734651761?check_suite_focus=true

marksantaniello · 2022-08-08T22:07:19Z

Seems Clang-11 has some issue with memcpy not being constexpr. Trying to repro but for me clang 11 actually crashes...

marksantaniello · 2022-08-08T23:03:31Z

OK got a repro. I was missing that I needed to use libc++. Will investigate.

marksantaniello · 2022-08-09T00:32:47Z

Seems the problem is here:

https://github.com/fmtlib/fmt/blob/master/include/fmt/format.h#L295-L305

I think Clang 11 supports -std=c++20, but the included libc++ doesn't have __cpp_lib_bit_cast, so we end up trying to use memcpy in a constexpr function.

marksantaniello · 2022-08-09T03:02:57Z

Maybe I can work around this by using __builtin_bit_cast

phprus · 2022-08-09T07:08:54Z

For C++20 constexpr fmt needs a bit_cast.
bit_cast requires (https://en.cppreference.com/w/cpp/compiler_support):
GCC libstdc++: 11 (_GLIBCXX_RELEASE >= 11, based on: __builtin_bit_cast)
Clang libc++: 14 (_LIBCPP_VERSION >= 14000, based on: __builtin_bit_cast)
MSVC STL: 19.27

I suggest checking these conditions or __builtin_bit_cast when defining FMT_CONSTEXPR20.
cc @vitaut

@marksantaniello, You can enable Github Actions for your fmt fork

marksantaniello · 2022-08-09T13:55:47Z

@phprus

You can enable Github Actions for your fmt fork

Are you saying that there's a way to get all the same test workflows running on my fork?

phprus · 2022-08-09T14:11:50Z

@marksantaniello
Yes.

marksantaniello · 2022-08-09T14:59:29Z

Thanks. I'll have to figure do that if I make PRs in the future. The test matrix is pretty large.

Anyway, it looks like the current version using __builtin_bit_cast is passing the checks.

phprus · 2022-08-09T15:21:42Z

Current test matrix does not check:
gcc-10 + C++20 (defined FMT_CONSTEXPR20, but not support __builtin_bit_cast)
Clang libc++ < 14 + C++20

marksantaniello · 2022-08-09T16:09:01Z

Ah, OK. Maybe I should figure out the workflows now then. I enabled "Actions" but I'm not sure how to clone the checks from here. And then I guess I'd need to add at least two more?

phprus · 2022-08-09T16:34:01Z

Strategies are defined in the .github/workflows/linux.yml file:

fmt/.github/workflows/linux.yml

Lines 11 to 61 in 8bd02e9

    
           strategy: 
        
             matrix: 
        
               cxx: [g++-4.8, g++-10, clang++-9] 
        
               build_type: [Debug, Release] 
        
               std: [11] 
        
               os: [ubuntu-18.04] 
        
               include: 
        
                 - cxx: g++-4.8 
        
                   install: sudo apt install g++-4.8 
        
                   os: ubuntu-18.04 
        
                 - cxx: g++-8 
        
                   build_type: Debug 
        
                   std: 14 
        
                   install: sudo apt install g++-8 
        
                   os: ubuntu-18.04 
        
                 - cxx: g++-8 
        
                   build_type: Debug 
        
                   std: 17 
        
                   install: sudo apt install g++-8 
        
                   os: ubuntu-18.04 
        
                 - cxx: g++-10 
        
                   build_type: Debug 
        
                   std: 17 
        
                   os: ubuntu-18.04 
        
                 - cxx: g++-11 
        
                   build_type: Debug 
        
                   std: 20 
        
                   os: ubuntu-20.04 
        
                   install: sudo apt install g++-11 
        
                 - cxx: clang++-8 
        
                   build_type: Debug 
        
                   std: 17 
        
                   cxxflags: -stdlib=libc++ 
        
                   os: ubuntu-18.04 
        
                   install: sudo apt install clang-8 libc++-8-dev libc++abi-8-dev 
        
                 - cxx: clang++-9 
        
                   build_type: Debug 
        
                   fuzz: -DFMT_FUZZ=ON -DFMT_FUZZ_LINKMAIN=ON 
        
                   std: 17 
        
                   os: ubuntu-18.04 
        
                 - cxx: clang++-11 
        
                   build_type: Debug 
        
                   std: 20 
        
                   os: ubuntu-20.04 
        
                 - cxx: clang++-11 
        
                   build_type: Debug 
        
                   std: 20 
        
                   cxxflags: -stdlib=libc++ 
        
                   os: ubuntu-20.04 
        
                   install: sudo apt install libc++-11-dev libc++abi-11-dev 
        
                 - shared: -DBUILD_SHARED_LIBS=ON

I think for the test you need to add:
gcc-9,gcc-10 + C++20

vitaut · 2022-08-09T21:23:33Z

FMT_CONSTEXPR20 should definitely not be changed. I suggest making the test conditional on whether __cpp_lib_bit_cast is defined, removing the bit_cast changes and new CI configs (we already have way too many).
Then everything that used to work will continue to work and if bit_cast is constexpr and there is C++20 constexpr support then formatted_size will usable in constexpr.

vitaut · 2022-08-09T22:10:36Z

test/compile-test.cc

 TEST(compile_test, formatted_size) {
-  EXPECT_EQ(2, fmt::formatted_size(FMT_COMPILE("{0}"), 42));
-  EXPECT_EQ(5, fmt::formatted_size(FMT_COMPILE("{0:<4.2f}"), 42.0));
+#ifdef __cpp_lib_bit_cast
+  FMT_CONSTEXPR20 
+#endif 
+  size_t s1 = fmt::formatted_size(FMT_COMPILE("{0}"), 42);
+  EXPECT_EQ(2, s1);
+#ifdef __cpp_lib_bit_cast
+  FMT_CONSTEXPR20 
+#endif 
+size_t s2 = fmt::formatted_size(FMT_COMPILE("{0:<4.2f}"), 42.0);
+  EXPECT_EQ(5, s2);
 }


Let's conditionally compile the test case itself because the whole purpose of it is to test constexpr formatted_size. Also formatted_size -> contexpr_formatted_size to make it clear what we are testing here.

#ifdef __cpp_lib_bit_cast TEST(compile_test, constexpr_formatted_size) { FMT_CONSTEXPR20 size_t s1 = fmt::formatted_size(FMT_COMPILE("{0}"), 42); EXPECT_EQ(2, s1); FMT_CONSTEXPR20 size_t s2 = fmt::formatted_size(FMT_COMPILE("{0:<4.2f}"), 42.0); EXPECT_EQ(5, s2); } #endif

vitaut · 2022-08-09T22:11:14Z

.github/workflows/linux.yml

+          - cxx: g++-9
+            build_type: Debug
+            std: 20
+            install: sudo apt install g++-9
+            os: ubuntu-18.04
          - cxx: g++-10
            build_type: Debug
            std: 17
            os: ubuntu-18.04
+          - cxx: g++-10
+            build_type: Debug
+            std: 20
+            os: ubuntu-18.04


As commented earlier, let's not add new CI configs.

Sorry, yes -- I'm just crap at git. Will try again.

vitaut · 2022-08-10T02:40:51Z

Looks good but there are some merge conflicts, please rebase.

vitaut · 2022-08-10T16:36:15Z

Thanks!

This reverts commit fd93b63.

- Update from version 9.0.0 to 9.1.0 - Update of rootfile - Changelog 9.1.0 - 2022-08-27 * ``fmt::formatted_size`` now works at compile time `#3026 <https://github.com/fmtlib/fmt/pull/3026>`_ For example (`godbolt <https://godbolt.org/z/1MW5rMdf8>`__): .. code:: c++ #include <fmt/compile.h> int main() { using namespace fmt::literals; constexpr size_t n = fmt::formatted_size("{}"_cf, 42); fmt::print("{}\n", n); // prints 2 } * Fixed handling of invalid UTF-8 `#3038 <https://github.com/fmtlib/fmt/pull/3038>`_, `#3044 <https://github.com/fmtlib/fmt/pull/3044>`_, `#3056 <https://github.com/fmtlib/fmt/pull/3056>`_ * Improved Unicode support in ``ostream`` overloads of ``print`` `#2994 <https://github.com/fmtlib/fmt/pull/2994>`_, `#3001 <https://github.com/fmtlib/fmt/pull/3001>`_, `#3025 <https://github.com/fmtlib/fmt/pull/3025>`_ * Fixed handling of the sign specifier in localized formatting on systems with 32-bit ``wchar_t`` `#3041 <https://github.com/fmtlib/fmt/issues/3041>`_). * Added support for wide streams to ``fmt::streamed`` `#2994 <https://github.com/fmtlib/fmt/pull/2994>`_ * Added the ``n`` specifier that disables the output of delimiters when formatting ranges `#2981 <https://github.com/fmtlib/fmt/pull/2981>`_, `#2983 <https://github.com/fmtlib/fmt/pull/2983>`_ For example (`godbolt <https://godbolt.org/z/roKqGdj8c>`__): .. code:: c++ #include <fmt/ranges.h> #include <vector> int main() { auto v = std::vector{1, 2, 3}; fmt::print("{:n}\n", v); // prints 1, 2, 3 } * Worked around problematic ``std::string_view`` constructors introduced in C++23 `#3030 <https://github.com/fmtlib/fmt/issues/3030>`_, `#3050 <https://github.com/fmtlib/fmt/issues/3050>`_ * Improve handling (exclusion) of recursive ranges `#2968 <https://github.com/fmtlib/fmt/issues/2968>`_, `#2974 <https://github.com/fmtlib/fmt/pull/2974>`_ * Improved error reporting in format string compilation `#3055 <https://github.com/fmtlib/fmt/issues/3055>`_ * Improved the implementation of `Dragonbox <https://github.com/jk-jeon/dragonbox>`_, the algorithm used for the default floating-point formatting `#2984 <https://github.com/fmtlib/fmt/pull/2984>`_ * Fixed issues with floating-point formatting on exotic platforms. * Improved the implementation of chrono formatting `#3010 <https://github.com/fmtlib/fmt/pull/3010>`_ * Improved documentation `#2966 <https://github.com/fmtlib/fmt/pull/2966>`_, `#3009 <https://github.com/fmtlib/fmt/pull/3009>`_, `#3020 <https://github.com/fmtlib/fmt/issues/3020>`_, `#3037 <https://github.com/fmtlib/fmt/pull/3037>`_ * Improved build configuration `#2991 <https://github.com/fmtlib/fmt/pull/2991>`_, `#2995 <https://github.com/fmtlib/fmt/pull/2995>`_, `#3004 <https://github.com/fmtlib/fmt/issues/3004>`_, `#3007 <https://github.com/fmtlib/fmt/pull/3007>`_, `#3040 <https://github.com/fmtlib/fmt/pull/3040>`_ * Fixed various warnings and compilation issues `#2969 <https://github.com/fmtlib/fmt/issues/2969>`_, `#2971 <https://github.com/fmtlib/fmt/pull/2971>`_, `#2975 <https://github.com/fmtlib/fmt/issues/2975>`_, `#2982 <https://github.com/fmtlib/fmt/pull/2982>`_, `#2985 <https://github.com/fmtlib/fmt/pull/2985>`_, `#2988 <https://github.com/fmtlib/fmt/issues/2988>`_, `#3000 <https://github.com/fmtlib/fmt/issues/3000>`_, `#3006 <https://github.com/fmtlib/fmt/issues/3006>`_, `#3014 <https://github.com/fmtlib/fmt/issues/3014>`_, `#3015 <https://github.com/fmtlib/fmt/issues/3015>`_, `#3021 <https://github.com/fmtlib/fmt/pull/3021>`_, `#3023 <https://github.com/fmtlib/fmt/issues/3023>`_, `#3024 <https://github.com/fmtlib/fmt/pull/3024>`_, `#3029 <https://github.com/fmtlib/fmt/pull/3029>`_, `#3043 <https://github.com/fmtlib/fmt/pull/3043>`_, `#3052 <https://github.com/fmtlib/fmt/issues/3052>`_, `#3053 <https://github.com/fmtlib/fmt/pull/3053>`_, `#3054 <https://github.com/fmtlib/fmt/pull/3054>`_ Signed-off-by: Adolf Belka <adolf.belka@ipfire.org> Reviewed-by: Michael Tremer <michael.tremer@ipfire.org>

Constexpr formatted_size

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vitaut reviewed Aug 8, 2022

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marksantaniello added 2 commits August 8, 2022 13:05

Constexpr formatted_size

4767e72

Merge branch 'master' of https://github.com/marksantaniello/fmt

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marksantaniello added 2 commits August 8, 2022 20:27

Merge branch 'master' of https://github.com/marksantaniello/fmt

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Merge branch 'master' of https://github.com/marksantaniello/fmt

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Merge branch 'fmtlib:master' into master

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Add C++20 tests for gcc 9 and 10

39c52e4

marksantaniello added 2 commits August 9, 2022 14:43

Adjust unit test to require __cpp_lib_bit_cast

003ccb4

Merge branch 'master' of https://github.com/marksantaniello/fmt

b519f0c

vitaut requested changes Aug 9, 2022

View reviewed changes

marksantaniello and others added 2 commits August 9, 2022 16:32

Address feedback from Victor

a703799

Merge branch 'fmtlib:master' into master

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vitaut approved these changes Aug 10, 2022

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vitaut merged commit fd93b63 into fmtlib:master Aug 10, 2022

dimztimz added a commit to dimztimz/fmt that referenced this pull request Aug 10, 2022

Revert "Constexpr formatted_size (fmtlib#3026)"

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This reverts commit fd93b63.

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Constexpr formatted_size #3026

Constexpr formatted_size #3026

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Constexpr formatted_size #3026

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