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Use a more efficient enable_if_t implementation #3071

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Use a more efficient enable_if_t implementation #3071

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77af435
Use a more efficient `enable_if_t` implementation
miscco Dec 6, 2024
0923df6
Work around mdspan bug
miscco Dec 6, 2024
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10 changes: 6 additions & 4 deletions libcudacxx/include/cuda/std/__mdspan/layout_stride.h
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Original file line number Diff line number Diff line change
Expand Up @@ -298,8 +298,9 @@ struct layout_stride
// nvcc cannot deduce this constructor when using _CCCL_REQUIRES
template <
class _IntegralTypes,
enable_if_t<_CCCL_TRAIT(is_convertible, const remove_const_t<_IntegralTypes>&, index_type), int> = 0,
enable_if_t<_CCCL_TRAIT(is_nothrow_constructible, index_type, const remove_const_t<_IntegralTypes>&), int> = 0>
typename enable_if<_CCCL_TRAIT(is_convertible, const remove_const_t<_IntegralTypes>&, index_type), int>::type = 0,
typename enable_if<_CCCL_TRAIT(is_nothrow_constructible, index_type, const remove_const_t<_IntegralTypes>&),
int>::type = 0>
Comment on lines +301 to +303
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Q: Why isn't this using enable_if_t? What bug is worked around here?

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Yeah layout_stride cannot deduce its arguments if we use the enable_if_t 🤷

That said, we seem to have more issues like this

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@bernhardmgruber bernhardmgruber Dec 6, 2024 •
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Sad. Consider adding a comment for those cases, so nobody is tempted to rewrite a enable_if<...>::type into enable_if_t and needs a whole CI run to find out it's a bug.

_LIBCUDACXX_HIDE_FROM_ABI constexpr mapping(
extents_type const& __e, _CUDA_VSTD::array<_IntegralTypes, extents_type::rank()> const& __s) noexcept
# ifndef _CCCL_HAS_NO_ATTRIBUTE_NO_UNIQUE_ADDRESS
Expand Down Expand Up @@ -327,8 +328,9 @@ struct layout_stride
// nvcc cannot deduce this constructor when using _CCCL_REQUIRES
template <
class _IntegralTypes,
enable_if_t<_CCCL_TRAIT(is_convertible, const remove_const_t<_IntegralTypes>&, index_type), int> = 0,
enable_if_t<_CCCL_TRAIT(is_nothrow_constructible, index_type, const remove_const_t<_IntegralTypes>&), int> = 0>
typename enable_if<_CCCL_TRAIT(is_convertible, const remove_const_t<_IntegralTypes>&, index_type), int>::type = 0,
typename enable_if<_CCCL_TRAIT(is_nothrow_constructible, index_type, const remove_const_t<_IntegralTypes>&),
int>::type = 0>
_LIBCUDACXX_HIDE_FROM_ABI constexpr mapping(
extents_type const& __e, _CUDA_VSTD::span<_IntegralTypes, extents_type::rank()> const& __s) noexcept
# ifndef _CCCL_HAS_NO_ATTRIBUTE_NO_UNIQUE_ADDRESS
Expand Down
23 changes: 22 additions & 1 deletion libcudacxx/include/cuda/std/__type_traits/enable_if.h
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Expand Up @@ -3,7 +3,7 @@
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
// SPDX-FileCopyrightText: Copyright (c) 2023 NVIDIA CORPORATION & AFFILIATES.
// SPDX-FileCopyrightText: Copyright (c) 2023-24 NVIDIA CORPORATION & AFFILIATES.
//
//===----------------------------------------------------------------------===//

Expand Down Expand Up @@ -31,9 +31,30 @@ struct _CCCL_TYPE_VISIBILITY_DEFAULT enable_if<true, _Tp>
typedef _Tp type;
};

#if _CCCL_COMPILER(NVHPC) || _CCCL_COMPILER(ICC) || _CCCL_COMPILER(GCC, <, 8)

template <bool _Bp, class _Tp = void>
using enable_if_t _CCCL_NODEBUG_ALIAS = typename enable_if<_Bp, _Tp>::type;

#else // ^^^ standard implementation ^^^ / vvv optimized implementation

// Optimized enable_if_t implementation that does not instantiate a type every time
template <bool>
struct __enable_if_t_impl
{};

template <>
struct __enable_if_t_impl<true>
{
template <class _Tp>
using type = _Tp;
};

template <bool _Bp, class _Tp = void>
using enable_if_t _CCCL_NODEBUG_ALIAS = typename __enable_if_t_impl<_Bp>::template type<_Tp>;

#endif // !_CCCL_COMPILER(NVHPC) && !_CCCL_COMPILER(ICC) && !_CCCL_COMPILER(GCC, <, 8)

_LIBCUDACXX_END_NAMESPACE_STD

#endif // _LIBCUDACXX___TYPE_TRAITS_ENABLE_IF_H
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