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[2025-02 LWG Motion 11] P3430R3 simd issues: explicit, unsequenced, identity-element position, and members of disabled simd #7687

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Mar 15, 2025
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[2025-02 LWG Motion 11] P3430R3 simd issues: explicit, unsequenced, identity-element position, and members of disabled simd #7687

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47 changes: 23 additions & 24 deletions source/numerics.tex
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Original file line number Diff line number Diff line change
Expand Up @@ -17204,7 +17204,7 @@
constexpr basic_simd() noexcept = default;

// \ref{simd.ctor}, \tcode{basic_simd} constructors
template<class U> constexpr basic_simd(U&& value) noexcept;
template<class U> constexpr explicit(@\seebelow@) basic_simd(U&& value) noexcept;
template<class U, class UAbi>
constexpr explicit(@\seebelow@) basic_simd(const basic_simd<U, UAbi>&) noexcept;
template<class G> constexpr explicit basic_simd(G&& gen) noexcept;
Expand Down Expand Up @@ -17310,7 +17310,7 @@

\pnum
\recommended
Implementations should support explicit conversions between specializations of
Implementations should support implicit conversions between specializations of
\tcode{basic_simd} and appropriate \impldef{conversions of \tcode{basic_simd}
from/to implementation-specific vector types} types.
\begin{note}
Expand All @@ -17321,7 +17321,7 @@
\rSec3[simd.ctor]{\tcode{basic_simd} constructors}

\begin{itemdecl}
template<class U> constexpr basic_simd(U&&) noexcept;
template<class U> constexpr explicit(@\seebelow@) basic_simd(U&& value) noexcept;
\end{itemdecl}

\begin{itemdescr}
Expand All @@ -17330,24 +17330,29 @@

\pnum
\constraints
\tcode{From} satisfies \tcode{convertible_to<value_type>}, and either
\tcode{value_type} satisfies \tcode{constructible_from<U>}.

\pnum
\effects
Initializes each element to the value of the argument after conversion to
\tcode{value_type}.

\pnum
\remarks
The expression inside \tcode{explicit} evaluates to \tcode{false} if and only if
\tcode{U} satisfies \tcode{convertible_to<value_type>}, and either
\begin{itemize}
\item
\tcode{From} is not an arithmetic type and does not satisfy
\exposconcept{constexpr-wrapper-like},
\item
\tcode{From} is an arithmetic type and the conversion from \tcode{From} to
\tcode{value_type} is value-preserving\iref{simd.general}, or
\item
\tcode{From} is not an arithmetic type and does not satisfy
\tcode{\exposconcept{constexpr-wrapper-like}}, or
\item
\tcode{From} satisfies \tcode{\exposconcept{constexpr-wrapper-like}},
\tcode{From} satisfies \exposconcept{constexpr-wrapper-like},
\tcode{remove_const_t<decltype(From::value)>} is an arithmetic type, and
\tcode{From::value} is representable by \tcode{value_type}.
\end{itemize}

\pnum
\effects
Initializes each element to the value of the argument after conversion to
\tcode{value_type}.
\end{itemdescr}

\begin{itemdecl}
Expand Down Expand Up @@ -17384,7 +17389,7 @@
\end{itemdescr}

\begin{itemdecl}
template<class G> constexpr explicit basic_simd(G&& gen) noexcept;
template<class G> constexpr explicit basic_simd(G&& gen);
\end{itemdecl}

\begin{itemdescr}
Expand All @@ -17408,10 +17413,7 @@

\pnum
\remarks
The calls to \tcode{gen} are unsequenced with respect to each other.
Vectorization-unsafe\iref{algorithms.parallel.defns} standard library functions
may not be invoked by \tcode{gen}.
\tcode{gen} is invoked exactly once for each $i$.
\tcode{gen} is invoked exactly once for each $i$, in increasing order of $i$.
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This wording is hand-wavy; is that ok because it's just a remark? Same for this wording added elsewhere...

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What is hand-wavy about it? And what would you say instead?

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What is "each $i$"? The closest wording prior to this paper is "if $N = 0$ or if for each $i$ where $0 \le i &lt; N$ in increasing order"

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The preceding paragraph says:

Initializes the $i^\text{th}$ element with
static_cast<value_type>(gen(integral_constant<simd-size-type, i>())) for all $i$ in the range of [0,size()).

Doesn't that sufficiently explain what "each $i$" means? (And there's no $N = 0$ case to consider here.)

\end{itemdescr}

\begin{itemdecl}
Expand Down Expand Up @@ -19180,7 +19182,7 @@
\tcode{basic_simd_mask<Bytes, Abi>} is trivially copyable.

\pnum
\recommended Implementations should support explicit conversions between
\recommended Implementations should support implicit conversions between
specializations of \tcode{basic_simd_mask} and appropriate \impldef{conversions
of \tcode{basic_simd_mask} from/to implementation-specific vector types} types.
\begin{note}
Expand Down Expand Up @@ -19218,7 +19220,7 @@
\end{itemdescr}

\begin{itemdecl}
template<class G> constexpr explicit basic_simd_mask(G&& gen) noexcept;
template<class G> constexpr explicit basic_simd_mask(G&& gen);
\end{itemdecl}

\begin{itemdescr}
Expand All @@ -19236,10 +19238,7 @@

\pnum
\remarks
The calls to \tcode{gen} are unsequenced with respect to each other.
Vectorization-unsafe\iref{algorithms.parallel.defns} standard library
functions may not be invoked by \tcode{gen}.
\tcode{gen} is invoked exactly once for each $i$.
\tcode{gen} is invoked exactly once for each $i$, in increasing order of $i$.
\end{itemdescr}

\rSec3[simd.mask.subscr]{\tcode{basic_simd_mask} subscript operator}
Expand Down