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bit_cast `memcpy` warnings with `-Wall -Wextra` #3000

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fkallen opened this issue Jul 23, 2022 · 4 comments

fkallen commented Jul 23, 2022 •

edited

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format.h: In instantiation of ‘To fmt::v9::detail::bit_cast(const From&) [with To = fmt::v9::detail::uint128_fallback; From = long double; typename std::enable_if<(sizeof (To) == sizeof (From)), int>::type = 0]’:
...

format.h:290:12: warning: ‘void* memcpy(void*, const void*, size_t)’ copying an object of non-trivial type ‘class fmt::v9::detail::uint128_fallback’ from an array of ‘const long double’ [-Wclass-memaccess]
290 | std::memcpy(&to, &from, sizeof(to));

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

Contributor

vitaut commented Jul 23, 2022

Could you provide a godbolt repro?

Author

fkallen commented Jul 24, 2022

The error does not appear on godbolt.

The reproducer is

#define FMT_HEADER_ONLY
#include <fmt/core.h>

When it is compiled with either
nvcc -std=c++14 -c -Xcompiler "-Wall" -Ifmt-master/include test1.cpp -o test1
or
g++ -std=c++14 -c -D__NVCC__ -Wall -I"fmt-master/include" test1.cpp -o test1
the warning occurs.
There is no warning with g++ -std=c++14 -c -Wall -I"fmt-master/include" test1.cpp -o test1.

nvcc version is V11.5.119 , g++ version is 9.4.0

The warning disappears if in format.h line https://github.com/fmtlib/fmt/blob/master/include/fmt/format.h#L290 is changed to std::memcpy((void*)&to, &from, sizeof(to)); From the gcc documentation:

Explicitly casting the pointer to the class object to void * or to a type that can be safely accessed by the raw memory function suppresses the warning.

Contributor

vitaut commented Jul 28, 2022

This looks like a bug in gcc because it should check whether the type is trivially copyable, not trivial (same as bit_cast). But I suppressed the warning anyway in e9ca7ea. Thanks for reporting.

vitaut closed this as completed

vitaut mentioned this issue

copying an object of non-trivial type error #2970

Closed

SoapGentoo commented Aug 2, 2022

Likely due to https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104855#c5

mtremer referenced this issue in ipfire/ipfire-2.x


          fmt: Update to version 9.1.0

90d43e5

- 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 <[email protected]>
Reviewed-by: Michael Tremer <[email protected]>

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