[simd.expos.abi]

# 29 Numerics library [[numerics]](./#numerics)

## 29.10 Data-parallel types [[simd]](simd#expos.abi)

### 29.10.2 Exposition-only types, variables, and concepts [[simd.expos]](simd.expos#abi)

#### 29.10.2.2 simd ABI tags [simd.expos.abi]

[🔗](#itemdecl:1)

`template<class T> using native-abi = see below;
template<class T, simd-size-type N> using deduce-abi-t = see below;
`

[1](#1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/numerics.tex#L16425)

An [*ABI tag*](#def:ABI_tag "29.10.2.2 simd ABI tags [simd.expos.abi]") is a type that indicates a choice of size and binary
representation for objects of data-parallel type[.](#1.sentence-1)

[*Note [1](#note-1)*:

The intent is for the size and binary representation to depend on the target
architecture and compiler flags[.](#1.sentence-2)

The ABI tag, together with a given element type, implies the width[.](#1.sentence-3)

— *end note*]

[2](#2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/numerics.tex#L16434)

[*Note [2](#note-2)*:

The ABI tag is orthogonal to selecting the machine instruction set[.](#2.sentence-1)

The selected machine instruction set limits the usable ABI tag types, though
(see [[simd.overview]](simd.overview "29.10.7.1 Class template basic_­vec overview"))[.](#2.sentence-2)

The ABI tags enable users to safely pass objects of data-parallel type between
translation unit boundaries (e.g., function calls or I/O)[.](#2.sentence-3)

— *end note*]

[3](#3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/numerics.tex#L16443)

An implementation defines ABI tag types as necessary for the following aliases[.](#3.sentence-1)

[4](#4)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/numerics.tex#L16446)

*deduce-abi-t*<T, N> is defined if

- [(4.1)](#4.1)

T is a vectorizable type,

- [(4.2)](#4.2)

N is greater than zero, and

- [(4.3)](#4.3)

N is not larger than an implementation-defined maximum[.](#4.sentence-1)

The implementation-defined maximum forN is not smaller than 64 and can differ depending on T[.](#4.sentence-2)

[5](#5)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/numerics.tex#L16456)

Where present, *deduce-abi-t*<T, N> names an ABI tag type such
that

- [(5.1)](#5.1)

*simd-size-v*<T, *deduce-abi-t*<T, N>> equals N,

- [(5.2)](#5.2)

basic_vec<T, *deduce-abi-t*<T, N>> is
 enabled ([[simd.overview]](simd.overview "29.10.7.1 Class template basic_­vec overview")), and

- [(5.3)](#5.3)

basic_mask<sizeof(T), *deduce-abi-t*<*integer-from*<sizeof(T)>, N>> is enabled[.](#5.sentence-1)

[6](#6)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/numerics.tex#L16469)

*native-abi*<T> is an implementation-defined alias for an ABI tag[.](#6.sentence-1)

basic_vec<T, *native-abi*<T>> is an enabled specialization[.](#6.sentence-2)

[*Note [3](#note-3)*:

The intent is to use the ABI tag producing the most efficient data-parallel
execution for the element type T on the currently targeted system[.](#6.sentence-3)

For target architectures with ISA extensions, compiler flags can change the type
of the *native-abi*<T> alias[.](#6.sentence-4)

— *end note*]

[*Example [1](#example-1)*:

Consider a target architecture supporting the ABI tags __simd128 and__simd256, where hardware support for __simd256 exists only for
floating-point types[.](#6.sentence-5)

The implementation therefore defines *native-abi*<T> as an alias
for

- [(6.1)](#6.1)

__simd256 if T is a floating-point type, and

- [(6.2)](#6.2)

__simd128 otherwise[.](#6.sentence-6)

— *end example*]