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[2026-06 LWG Motion 11] P3793R2 Better shifting #9068

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8ec3c1f
P3793R2 Better shifting
eisenwave Jun 13, 2026
13abe7f
Fixup: paper typo in synopsis
eisenwave Jun 13, 2026
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66 changes: 66 additions & 0 deletions source/numerics.tex
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Original file line number Diff line number Diff line change
Expand Up @@ -17391,6 +17391,16 @@
template<@\exposconcept{simd-vec-type}@ V>
constexpr typename V::mask_type has_single_bit(const V& v) noexcept;

template<@\exposconcept{simd-vec-type}@ VX, @\exposconcept{simd-vec-type}@ VS>
constexpr VX shl(const VX& x, const VS& s) noexcept;
template<@\exposconcept{simd-vec-type}@ V, class S>
constexpr V shl(const V& x, S s) noexcept;

template<@\exposconcept{simd-vec-type}@ VX, @\exposconcept{simd-vec-type}@ VS>
constexpr VX shr(const VX& x, const VS& s) noexcept;
template<@\exposconcept{simd-vec-type}@ V, class S>
constexpr V shr(const V& x, S s) noexcept;

template<@\exposconcept{simd-vec-type}@ V0, @\exposconcept{simd-vec-type}@ V1>
constexpr V0 rotl(const V0& v, const V1& s) noexcept;
template<@\exposconcept{simd-vec-type}@ V>
Expand Down Expand Up @@ -17592,6 +17602,8 @@
using simd::bit_ceil;
using simd::bit_floor;
using simd::has_single_bit;
using simd::shl;
using simd::shr;
using simd::rotl;
using simd::rotr;
using simd::bit_width;
Expand Down Expand Up @@ -20638,6 +20650,60 @@
\range{0}{V::size()}.
\end{itemdescr}

\indexlibrarymember{shl}{simd}
\indexlibrarymember{shr}{simd}
\begin{itemdecl}
template<@\exposconcept{simd-vec-type}@ VX, @\exposconcept{simd-vec-type}@ VS>
constexpr VX shl(const VX& x, const VS& s) noexcept;
template<@\exposconcept{simd-vec-type}@ VX, @\exposconcept{simd-vec-type}@ VS>
constexpr VX shr(const VX& x, const VS& s) noexcept;
\end{itemdecl}

\begin{itemdescr}
\pnum
\constraints
\begin{itemize}
\item
Each of \tcode{VX::value_type} and \tcode{VS::value_type}
is a signed or unsigned integer type\iref{basic.fundamental},
\item
\tcode{VX::size() == VS::size()} is \tcode{true}, and
\item
\tcode{sizeof(typename VX::value_type) == sizeof(typename VS::value_type)}
is \tcode{true}.
\end{itemize}

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

\indexlibrarymember{shl}{simd}
\indexlibrarymember{shr}{simd}
\begin{itemdecl}
template<@\exposconcept{simd-vec-type}@ V, class S>
constexpr V shl(const V& x, S s) noexcept;
template<@\exposconcept{simd-vec-type}@ V, class S>
constexpr V shr(const V& x, S s) noexcept;
\end{itemdecl}

\begin{itemdescr}
\pnum
\constraints
Each of \tcode{VX::value_type} and \tcode{S}
is a signed or unsigned integer type\iref{basic.fundamental}.

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

\indexlibrarymember{rotl}{simd}
\indexlibrarymember{rotr}{simd}
\begin{itemdecl}
Expand Down
3 changes: 2 additions & 1 deletion source/support.tex
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Expand Up @@ -602,7 +602,7 @@
#define @\defnlibxname{cpp_lib_bind_back}@ 202306L // freestanding, also in \libheader{functional}
#define @\defnlibxname{cpp_lib_bind_front}@ 202306L // freestanding, also in \libheader{functional}
#define @\defnlibxname{cpp_lib_bit_cast}@ 201806L // freestanding, also in \libheader{bit}
#define @\defnlibxname{cpp_lib_bitops}@ 201907L // freestanding, also in \libheader{bit}
#define @\defnlibxname{cpp_lib_bitops}@ 202606L // freestanding, also in \libheader{bit}
#define @\defnlibxname{cpp_lib_bitset}@ 202306L // also in \libheader{bitset}
#define @\defnlibxname{cpp_lib_bool_constant}@ 201505L // freestanding, also in \libheader{type_traits}
#define @\defnlibxname{cpp_lib_bounded_array_traits}@ 201902L // freestanding, also in \libheader{type_traits}
Expand Down Expand Up @@ -838,6 +838,7 @@
#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}@ 202603L // also in \libheader{simd}
#define @\defnlibxname{cpp_lib_simd_bitops}@ 202606L // also in \libheader{simd}
#define @\defnlibxname{cpp_lib_simd_complex}@ 202502L // also in \libheader{simd}
#define @\defnlibxname{cpp_lib_simd_permutations}@ 202506L // also in \libheader{simd}
#define @\defnlibxname{cpp_lib_smart_ptr_for_overwrite}@ 202002L // also in \libheader{memory}
Expand Down
48 changes: 48 additions & 0 deletions source/utilities.tex
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Expand Up @@ -16316,6 +16316,12 @@
template<class T>
constexpr int bit_width(T x) noexcept;

// \ref{bit.shift}, shifting
template<class T, class S>
constexpr T shl(T x, S s) noexcept;
template<class T, class S>
constexpr T shr(T x, S s) noexcept;

// \ref{bit.rotate}, rotating
template<class T>
constexpr T rotl(T x, int s) noexcept;
Expand Down Expand Up @@ -16524,6 +16530,48 @@

\end{itemdescr}

\rSec2[bit.shift]{Shifting}

\pnum
Within this subclause,
the result $r$ of an arithmetic operation is
first represented in a hypothetical integer type with sufficient range,
then converted to \tcode{T} as if by \tcode{static_cast<T>(r)}.

\indexlibraryglobal{shl}%
\begin{itemdecl}
template<class T, class S>
constexpr T shl(T x, S s) noexcept;
\end{itemdecl}

\begin{itemdescr}
\pnum
\constraints
Each of \tcode{T} and \tcode{S}
is a signed or unsigned integer type\iref{basic.fundamental}.

\pnum
\returns
$x \times 2^s$ rounded towards negative infinity.
\end{itemdescr}

\indexlibraryglobal{shr}%
\begin{itemdecl}
template<class T, class S>
constexpr T shr(T x, S s) noexcept;
\end{itemdecl}

\begin{itemdescr}
\pnum
\constraints
Each of \tcode{T} and \tcode{S}
is a signed or unsigned integer type\iref{basic.fundamental}.

\pnum
\returns
$x \times 2^{-s}$ rounded towards negative infinity.
\end{itemdescr}

\rSec2[bit.rotate]{Rotating}

\pnum
Expand Down
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