Accept the 80-bit long double in <xutility> #2073

smanna12 · 2021-07-22T13:27:02Z

The previous codes only work for complex header file by PR's:
#1316
#1728.

The _Bit_cast function in xutility header file does not allow 80 bits wide long double. The Intel C++ compiler uses /Qlong-double option to set the long double type to 80 bits wide.

This patch generalizes the 80 bit long double check in xutility header file using same logic from #1316.

stl/inc/xutility

celonymire

This probably needs tests.

Signed-off-by: Soumi Manna <soumi.manna@intel.com>

smanna12 · 2021-07-26T20:06:05Z

This probably needs tests.

Thanks @sam20908 for looking into the patch and the reviews.
The fix is same as #1316 to generalize the 80 bit long double check in xutility header file.
I do not see any test there, so no tests have been added in this patch.

celonymire

I just spotted one weird thing :)

stl/inc/xutility

statementreply · 2021-07-28T03:00:00Z

stl/inc/xutility

 template <class _Ty, enable_if_t<is_floating_point_v<_Ty>, int> = 0>
 _NODISCARD _CONSTEXPR_BIT_CAST bool _Is_finite(const _Ty _Xx) { // constexpr isfinite()
+#if defined(__LDBL_DIG__) && __LDBL_DIG__ == 18
+    return _CSTD _LDtest(&_Xx) == _FINITE;


I don't believe that _LDtest could properly handle 80-bit long double:

STL/stl/src/xldtest.cpp

Lines 10 to 12 in bd7adb4

_CRTIMP2_PURE short __CLRCALL_PURE_OR_CDECL _LDtest(long double* px) { // categorize *px -- 64-bit

return _Dtest(reinterpret_cast<double*>(px));

}

_LDtest is handling long double:

STL/stl/inc/complex

Lines 542 to 557 in bd7adb4

static bool _Isinf(_Ty _Left) { // test for infinity

#if defined(__LDBL_DIG__) && __LDBL_DIG__ == 18

return _CSTD _LDtest(&_Left) == _INFCODE;

#else // ^^^ 80-bit long double (not supported by MSVC in general, see GH-1316) / 64-bit long double vvv

const auto _Uint = _Bit_cast<uint64_t>(_Left);

return (_Uint & 0x7fffffffffffffffU) == 0x7ff0000000000000U;

#endif // ^^^ 64-bit long double ^^^

}

static _CONSTEXPR20 bool _Isnan(_Ty _Left) {

#if defined(__LDBL_DIG__) && __LDBL_DIG__ == 18

return _CSTD _LDtest(&_Left) == _NANCODE;

#else // ^^^ 80-bit long double (not supported by MSVC in general, see GH-1316) / 64-bit long double vvv

const auto _Uint = _Bit_cast<uint64_t>(_Left);

return (_Uint & 0x7fffffffffffffffU) > 0x7ff0000000000000U;

#endif // ^^^ 64-bit long double ^^^

I work with Intel C++ compiler team. The Intel C++ compiler uses /Qlong-double option to set the long double type to 80 bits wide. The _Bit_cast function in <xutility> header file does not allow 80 bits wide long double.

Intel C++ Compiler with /Qlong-double enabled, produces the following compilation error messages when we compile <complex> header file with /std:c++latest option:

In file included from c:/Program files/Microsoft Visual Studio/2022/Preview/VC/Tools/MSVC/14.29.30130/include\iostream:11: In file included from c:/Program files/Microsoft Visual Studio/2022/Preview/VC/Tools/MSVC/14.29.30130/include\istream:11: In file included from c:/Program files/Microsoft Visual Studio/2022/Preview/VC/Tools/MSVC/14.29.30130/include\ostream:11: In file included from c:/Program files/Microsoft Visual Studio/2022/Preview/VC/Tools/MSVC/14.29.30130/include\ios:11: In file included from c:/Program files/Microsoft Visual Studio/2022/Preview/VC/Tools/MSVC/14.29.30130/include\xlocnum:12: In file included from c:/Program files/Microsoft Visual Studio/2022/Preview/VC/Tools/MSVC/14.29.30130/include\cmath:26: c:/Program files/Microsoft Visual Studio/2022/Preview/VC/Tools/MSVC/14.29.30130/include\xutility(5918,24): error: no matching function for call to '_Bit_cast' const auto _Bits = _Bit_cast<_Uint_type>(_Xx); ^~~~~~~~~~~~~~~~~~~~~ c:/Program files/Microsoft Visual Studio/2022/Preview/VC/Tools/MSVC/14.29.30130/include\xutility(5967,12): note: in instantiation of function template specialization 'std::_Float_abs_bits<long double, 0>' requested here return _Float_abs_bits(_Xx) < _Traits::_Shifted_exponent_mask; ^ c:/Program files/Microsoft Visual Studio/2022/Preview/VC/Tools/MSVC/14.29.30130/include\cmath(1307,31): note: in instantiation of function template specialization 'std::_Is_finite<long double, 0>' requested here const bool _T_is_finite = _Is_finite(_ArgT); ^ c:/Program files/Microsoft Visual Studio/2022/Preview/VC/Tools/MSVC/14.29.30130/include\cmath(1392,12): note: in instantiation of function template specialization 'std::_Common_lerp<long double>' requested here return _Common_lerp(_ArgA, _ArgB, _ArgT); ^ c:/Program files/Microsoft Visual Studio/2022/Preview/VC/Tools/MSVC/14.29.30130/include\xutility(67,36): note: candidate template ignored: requirement 'conjunction_v<std::integral_constant<bool, false>, std::is_trivially_copyable<unsigned long long>, std::is_trivially_copyable<long double>>' was not satisfied [with _To = unsigned long long, _From = long double] _NODISCARD _CONSTEXPR_BIT_CAST _To _Bit_cast(const _From& _Val) noexcept { ^ In file included from c:/Program files/Microsoft Visual Studio/2022/Preview/VC/Tools/MSVC/14.29.30130/include\iostream:11: In file included from c:/Program files/Microsoft Visual Studio/2022/Preview/VC/Tools/MSVC/14.29.30130/include\istream:11: In file included from c:/Program files/Microsoft Visual Studio/2022/Preview/VC/Tools/MSVC/14.29.30130/include\ostream:11: In file included from c:/Program files/Microsoft Visual Studio/2022/Preview/VC/Tools/MSVC/14.29.30130/include\ios:11: In file included from c:/Program files/Microsoft Visual Studio/2022/Preview/VC/Tools/MSVC/14.29.30130/include\xlocnum:12: c:/Program files/Microsoft Visual Studio/2022/Preview/VC/Tools/MSVC/14.29.30130/include\cmath(1308,25): error: no matching function for call to '_Is_finite' if (_T_is_finite && _Is_finite(_ArgA) && _Is_finite(_ArgB)) { ^~~~~~~~~~ c:/Program files/Microsoft Visual Studio/2022/Preview/VC/Tools/MSVC/14.29.30130/include\cmath(1392,12): note: in instantiation of function template specialization 'std::_Common_lerp<long double>' requested here return _Common_lerp(_ArgA, _ArgB, _ArgT); ^ c:/Program files/Microsoft Visual Studio/2022/Preview/VC/Tools/MSVC/14.29.30130/include\xutility(5965,37): note: candidate template ignored: substitution failure [with _Ty = long double, $1 = 0] _NODISCARD _CONSTEXPR_BIT_CAST bool _Is_finite(const _Ty _Xx) { // constexpr isfinite() ^ In file included from c:/Program files/Microsoft Visual Studio/2022/Preview/VC/Tools/MSVC/14.29.30130/include\iostream:11: In file included from c:/Program files/Microsoft Visual Studio/2022/Preview/VC/Tools/MSVC/14.29.30130/include\istream:11: In file included from c:/Program files/Microsoft Visual Studio/2022/Preview/VC/Tools/MSVC/14.29.30130/include\ostream:11: In file included from c:/Program files/Microsoft Visual Studio/2022/Preview/VC/Tools/MSVC/14.29.30130/include\ios:11: In file included from c:/Program files/Microsoft Visual Studio/2022/Preview/VC/Tools/MSVC/14.29.30130/include\xlocnum:12: c:/Program files/Microsoft Visual Studio/2022/Preview/VC/Tools/MSVC/14.29.30130/include\cmath(1308,46): error: no matching function for call to '_Is_finite' if (_T_is_finite && _Is_finite(_ArgA) && _Is_finite(_ArgB)) { ^~~~~~~~~~ c:/Program files/Microsoft Visual Studio/2022/Preview/VC/Tools/MSVC/14.29.30130/include\xutility(5965,37): note: candidate template ignored: substitution failure [with _Ty = long double, $1 = 0] _NODISCARD _CONSTEXPR_BIT_CAST bool _Is_finite(const _Ty _Xx) { // constexpr isfinite() ^ c:/Program files/Microsoft Visual Studio/2022/Preview/VC/Tools/MSVC/14.29.30130/include\xutility(5941,12): error: no matching function for call to '_Float_abs_bits' return _Float_abs_bits(_Xx) > _Traits::_Shifted_exponent_mask; ^~~~~~~~~~~~~~~ c:/Program files/Microsoft Visual Studio/2022/Preview/VC/Tools/MSVC/14.29.30130/include\cmath(1337,13): note: in instantiation of function template specialization 'std::_Is_nan<long double, 0>' requested here if (_Is_nan(_ArgA)) { ^ c:/Program files/Microsoft Visual Studio/2022/Preview/VC/Tools/MSVC/14.29.30130/include\cmath(1392,12): note: in instantiation of function template specialization 'std::_Common_lerp<long double>' requested here return _Common_lerp(_ArgA, _ArgB, _ArgT); ^ c:/Program files/Microsoft Visual Studio/2022/Preview/VC/Tools/MSVC/14.29.30130/include\xutility(5915,37): note: candidate template ignored: substitution failure [with _Ty = long double, $1 = 0] _NODISCARD _CONSTEXPR_BIT_CAST auto _Float_abs_bits(const _Ty& _Xx) { ^ 4 errors generated

I think the problem is that _Bit_cast is unable to work on a long double (probably because sizeof(long double) > sizeof(unsigned long long)?).

I am not sure whether _Bit_cast needs to be fixed, or whether we should be avoiding trying to call it in the first place (generalizes the 80 bit long double check in xutility header file using same logic #1316)

It would be very helpful if we could get any input or assistance in figuring out the right path forward to fix the problem.
Tagging @CaseyCarter, @statementreply, @StephanTLavavej, @sam20908, @simmse for any suggestion. Thank you.

Adding @AaronBallman, @erichkeane for any more information.

I believe there's a separate correctness issue here, because this codepath was being used for 32-bit float as well, and that won't be properly handled by _LDtest (regardless of whether it's the original 64-bit implementation or a replaced 80-bit implementation).

stl/inc/xutility

Signed-off-by: Soumi Manna <soumi.manna@intel.com>

StephanTLavavej · 2021-07-28T21:45:05Z

The maintainer team talked about this at our weekly meeting, and we believe that this falls under our project's Non-Goals; 80-bit long double is essentially a different platform (it is certainly ABI-incompatible with MSVC), and as a result we can't devote any significant time towards maintaining it. In a couple of previous PRs, we were willing to make targeted changes to restore old code that was being changed, and to generalize the check for 80-bit long double, as this was quick to review and non-destabilizing. However, we can't keep adding machinery for 80-bit long double endlessly; that's essentially providing support which we have never claimed.

As always, you're welcome to maintain a fork of our repo and add support for 80-bit long double there; our concerns are simply with the resources needed to upstream such support.

smanna12 added 2 commits July 22, 2021 06:25

Accept the 80-bit long double in <xutility> for Clang

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fix error

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smanna12 added 2 commits July 26, 2021 12:01

update patch

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update patch

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smanna12 changed the title ~~Accept the 80-bit long double in <xutility> for Clang~~ Accept the 80-bit long double in <xutility> Jul 26, 2021

smanna12 added 2 commits July 26, 2021 12:19

fix errors

33a77c2

Signed-off-by: Soumi Manna <soumi.manna@intel.com>

update patch with #if defined

af0b826

smanna12 requested a review from CaseyCarter July 26, 2021 20:07

smanna12 marked this pull request as ready for review July 26, 2021 20:08

smanna12 requested a review from a team as a code owner July 26, 2021 20:08

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fix typo

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Add _FINITE macro for _Dtest RETURN

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smanna12 requested a review from CaseyCarter July 27, 2021 18:46

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remove ^^^ from #endif

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Signed-off-by: Soumi Manna <soumi.manna@intel.com>

StephanTLavavej closed this Jul 28, 2021

frederick-vs-ja mentioned this pull request Jul 10, 2025

<complex>: _LDtest looks quite questionable #5642

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Accept the 80-bit long double in <xutility> #2073

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	_CRTIMP2_PURE short __CLRCALL_PURE_OR_CDECL _LDtest(long double* px) { // categorize *px -- 64-bit
	return _Dtest(reinterpret_cast<double*>(px));
	}

	static bool _Isinf(_Ty _Left) { // test for infinity
	#if defined(__LDBL_DIG__) && __LDBL_DIG__ == 18
	return _CSTD _LDtest(&_Left) == _INFCODE;
	#else // ^^^ 80-bit long double (not supported by MSVC in general, see GH-1316) / 64-bit long double vvv
	const auto _Uint = _Bit_cast<uint64_t>(_Left);
	return (_Uint & 0x7fffffffffffffffU) == 0x7ff0000000000000U;
	#endif // ^^^ 64-bit long double ^^^
	}

	static _CONSTEXPR20 bool _Isnan(_Ty _Left) {
	#if defined(__LDBL_DIG__) && __LDBL_DIG__ == 18
	return _CSTD _LDtest(&_Left) == _NANCODE;
	#else // ^^^ 80-bit long double (not supported by MSVC in general, see GH-1316) / 64-bit long double vvv
	const auto _Uint = _Bit_cast<uint64_t>(_Left);
	return (_Uint & 0x7fffffffffffffffU) > 0x7ff0000000000000U;
	#endif // ^^^ 64-bit long double ^^^

Accept the 80-bit long double in <xutility> #2073

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