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[2025-02 LWG Motion 13] P2933R4 Extend <bit> header function with overloads for std::simd #7689

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tkoeppe merged 1 commit into from
Mar 15, 2025
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[2025-02 LWG Motion 13] P2933R4 Extend <bit> header function with overloads for std::simd #7689

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201 changes: 201 additions & 0 deletions source/numerics.tex
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Original file line number Diff line number Diff line change
Expand Up @@ -16125,6 +16125,11 @@

template<class V, class T> using @\exposidnc{make-compatible-simd-t} = \seebelownc@; // \expos

template<class V>
concept @\defexposconceptnc{simd-type}@ = // \expos
@\libconcept{same_as}@<V, basic_simd<typename V::value_type, typename V::abi_type>> &&
is_default_constructible_v<V>;

template<class V>
concept @\defexposconceptnc{simd-floating-point}@ = // \expos
@\libconcept{same_as}@<V, basic_simd<typename V::value_type, typename V::abi_type>> &&
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template<@\exposconcept{math-floating-point}@ V>
@\exposid{deduced-simd-t}@<V>
sph_neumann(const rebind_simd_t<unsigned, @\exposid{deduced-simd-t}@<V>>& n, const V& x);

// \ref{simd.bit}, Bit manipulation
template<@\exposconcept{simd-type}@ V> constexpr V byteswap(const V& v) noexcept;
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@jensmaurer jensmaurer Feb 23, 2025

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Out of curiosity: Why this "simd-type" formulation instead of taking a suitably-parameterized std::basic_simd parameter type?

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@mattkretz mattkretz Feb 27, 2025

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I think I asked that question in LWG but I don't recall an answer. What difference does it make (other than style)?

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@jensmaurer jensmaurer Feb 27, 2025

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None, as far as I know; that's why I'm asking. It just feels a bit backwards to say "this function takes arguments of any type, except satisfaction checking of constraints will then check whether we have a specialization of X" instead of "this function takes a specialization of X".

(Maybe compiling is faster one way or the other.)

template<@\exposconcept{simd-type}@ V> constexpr V bit_ceil(const V& v) noexcept;
template<@\exposconcept{simd-type}@ V> constexpr V bit_floor(const V& v) noexcept;

template<@\exposconcept{simd-type}@ V>
constexpr typename V::mask_type has_single_bit(const V& v) noexcept;

template<@\exposconcept{simd-type}@ V0, @\exposconcept{simd-type}@ V1>
constexpr V0 rotl(const V0& v, const V1& s) noexcept;
template<@\exposconcept{simd-type}@ V>
constexpr V rotl(const V& v, int s) noexcept;

template<@\exposconcept{simd-type}@ V0, @\exposconcept{simd-type}@ V1>
constexpr V0 rotr(const V0& v, const V1& s) noexcept;
template<@\exposconcept{simd-type}@ V>
constexpr V rotr(const V& v, int s) noexcept;

template<@\exposconcept{simd-type}@ V>
constexpr rebind_simd_t<make_signed_t<typename V::value_type>, V>
bit_width(const V& v) noexcept;
template<@\exposconcept{simd-type}@ V>
constexpr rebind_simd_t<make_signed_t<typename V::value_type>, V>
countl_zero(const V& v) noexcept;
template<@\exposconcept{simd-type}@ V>
constexpr rebind_simd_t<make_signed_t<typename V::value_type>, V>
countl_one(const V& v) noexcept;
template<@\exposconcept{simd-type}@ V>
constexpr rebind_simd_t<make_signed_t<typename V::value_type>, V>
countr_zero(const V& v) noexcept;
template<@\exposconcept{simd-type}@ V>
constexpr rebind_simd_t<make_signed_t<typename V::value_type>, V>
countr_one(const V& v) noexcept;
template<@\exposconcept{simd-type}@ V>
constexpr rebind_simd_t<make_signed_t<typename V::value_type>, V>
popcount(const V& v) noexcept;
}
\end{codeblock}

Expand Down Expand Up @@ -18471,6 +18513,165 @@
\tcode{ret.first}.
\end{itemdescr}

\rSec3[simd.bit]{\tcode{basic_simd} bit library}

\begin{itemdecl}
template<@\exposconcept{simd-type}@ V> constexpr V byteswap(const V& v) noexcept;
\end{itemdecl}

\begin{itemdescr}
\pnum
\constraints
The type \tcode{V::value_type} models \tcode{integral}.

\pnum
\returns
A \tcode{basic_simd} object where the $i^\text{th}$ element is initialized to
the result of \tcode{std::byteswap(v[$i$])} for all $i$ in the range
\range{0}{V::size()}.
\end{itemdescr}

\begin{itemdecl}
template<@\exposconcept{simd-type}@ V> constexpr V bit_ceil(const V& v) noexcept;
\end{itemdecl}

\begin{itemdescr}
\pnum
\constraints
The type \tcode{V::value_type} is an unsigned integer type\iref{basic.fundamental}.

\pnum
\expects
For every $i$ in the range \range{0}{V::size()}, the smallest power of 2
greater than or equal to \tcode{v[$i$]} is representable as a value of type
\tcode{V::value_type}.

\pnum
\returns
A \tcode{basic_simd} object where the $i^\text{th}$ element is initialized to
the result of \tcode{std::bit_ceil(v[$i$])} for all $i$ in the range
\range{0}{V::size()}.

\pnum
\remarks
A function call expression that violates the precondition in the \expects
element is not a core constant expression\iref{expr.const}.
\end{itemdescr}

\begin{itemdecl}
template<@\exposconcept{simd-type}@ V> constexpr V bit_floor(const V& v) noexcept;
\end{itemdecl}

\begin{itemdescr}
\pnum
\constraints
The type \tcode{V::value_type} is an unsigned integer type\iref{basic.fundamental}.

\pnum
\returns
A \tcode{basic_simd} object where the $i^\text{th}$ element is initialized to
the result of \tcode{std::bit_floor(v[$i$])} for all $i$ in the range
\range{0}{V::size()}.
\end{itemdescr}

\begin{itemdecl}
template<@\exposconcept{simd-type}@ V>
constexpr typename V::mask_type has_single_bit(const V& v) noexcept;
\end{itemdecl}

\begin{itemdescr}
\pnum
\constraints
The type \tcode{V::value_type} is an unsigned integer type\iref{basic.fundamental}.

\pnum
\returns
A \tcode{basic_simd_mask} object where the $i^\text{th}$ element is initialized
to the result of \tcode{std::has_single_bit(v[$i$])} for all $i$ in the range
\range{0}{V::size()}.
\end{itemdescr}

\begin{itemdecl}
template<@\exposconcept{simd-type}@ V0, @\exposconcept{simd-type}@ V1>
constexpr V0 rotl(const V0& v0, const V1& v1) noexcept;
template<@\exposconcept{simd-type}@ V0, @\exposconcept{simd-type}@ V1>
constexpr V0 rotr(const V0& v0, const V1& v1) noexcept;
\end{itemdecl}

\begin{itemdescr}
\pnum
\constraints
\begin{itemize}
\item
The type \tcode{V0::value_type} is an unsigned integer type\iref{basic.fundamental},
\item
the type \tcode{V1::value_type} models \tcode{integral},
\item
\tcode{V0::size() == V1::size()} is \tcode{true}, and
\item
\tcode{sizeof(typename V0::value_type) == sizeof(typename V1::value_type)} is \tcode{true}.
\end{itemize}

\pnum
\returns
A \tcode{basic_simd} object where the $i^\text{th}$ element is initialized to
the result of \tcode{\placeholder{bit-func}(v0[$i$],
static_cast<int>(v1[$i$]))} for all $i$ in the range \range{0}{V0::size()},
where \placeholder{bit-func} is the corresponding scalar function from \libheader{bit}.
\end{itemdescr}

\begin{itemdecl}
template<@\exposconcept{simd-type}@ V> constexpr V rotl(const V& v, int s) noexcept;
template<@\exposconcept{simd-type}@ V> constexpr V rotr(const V& v, int s) noexcept;
\end{itemdecl}

\begin{itemdescr}
\pnum
\constraints
The type \tcode{V::value_type} is an unsigned integer type\iref{basic.fundamental}

\pnum
\returns
A \tcode{basic_simd} object where the $i^\text{th}$ element is initialized to
the result of \tcode{\placeholder{bit-func}(v[$i$], s)} for all $i$ in the
range \range{0}{V::size()}, where \placeholder{bit-func} is the corresponding
scalar function from \libheader{bit}.
\end{itemdescr}

\begin{itemdecl}
template<@\exposconcept{simd-type}@ V>
constexpr rebind_simd_t<make_signed_t<typename V::value_type>, V>
bit_width(const V& v) noexcept;
template<@\exposconcept{simd-type}@ V>
constexpr rebind_simd_t<make_signed_t<typename V::value_type>, V>
countl_zero(const V& v) noexcept;
template<@\exposconcept{simd-type}@ V>
constexpr rebind_simd_t<make_signed_t<typename V::value_type>, V>
countl_one(const V& v) noexcept;
template<@\exposconcept{simd-type}@ V>
constexpr rebind_simd_t<make_signed_t<typename V::value_type>, V>
countr_zero(const V& v) noexcept;
template<@\exposconcept{simd-type}@ V>
constexpr rebind_simd_t<make_signed_t<typename V::value_type>, V>
countr_one(const V& v) noexcept;
template<@\exposconcept{simd-type}@ V>
constexpr rebind_simd_t<make_signed_t<typename V::value_type>, V>
popcount(const V& v) noexcept;
\end{itemdecl}

\begin{itemdescr}
\pnum
\constraints
The type \tcode{V::value_type} is an unsigned integer type\iref{basic.fundamental}

\pnum
\returns
A \tcode{basic_simd} object where the $i^\text{th}$ element is initialized to
the result of \tcode{\placeholder{bit-func}(v[$i$])} for all $i$ in the range
\range{0}{V::size()}, where \placeholder{bit-func} is the corresponding scalar
function from \libheader{bit}.
\end{itemdescr}

\rSec2[simd.mask.class]{Class template \tcode{basic_simd_mask}}

\rSec3[simd.mask.overview]{Class template \tcode{basic_simd_mask} overview}
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2 changes: 1 addition & 1 deletion source/support.tex
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Expand Up @@ -799,7 +799,7 @@
#define @\defnlibxname{cpp_lib_shared_ptr_weak_type}@ 201606L // also in \libheader{memory}
#define @\defnlibxname{cpp_lib_shared_timed_mutex}@ 201402L // also in \libheader{shared_mutex}
#define @\defnlibxname{cpp_lib_shift}@ 202202L // also in \libheader{algorithm}
#define @\defnlibxname{cpp_lib_simd}@ 202411L // also in \libheader{simd}
#define @\defnlibxname{cpp_lib_simd}@ 202502L // also in \libheader{simd}
#define @\defnlibxname{cpp_lib_smart_ptr_for_overwrite}@ 202002L // also in \libheader{memory}
#define @\defnlibxname{cpp_lib_smart_ptr_owner_equality}@ 202306L // also in \libheader{memory}
#define @\defnlibxname{cpp_lib_source_location}@ 201907L // freestanding, also in \libheader{source_location}
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