Constexpr formatted_size

[marksantaniello]

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]

static seems unnecessary, please remove.

[vitaut]

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

[marksantaniello]

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

[marksantaniello]

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

[marksantaniello]

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]

Maybe I can work around this by using __builtin_bit_cast

[phprus]

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]

@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]

@marksantaniello
Yes.

[marksantaniello]

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]

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]

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]

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]

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]

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]

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

[marksantaniello]

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

[vitaut]

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

[vitaut]

Thanks!