NSIMD documentation

C++20 concepts

As of C++20, concepts are available. We quote en.cppreference.com to introduce concepts.

Class templates, function templates, and non-template functions (typically members of class templates) may be associated with a constraint, which specifies the requirements on template arguments, which can be used to select the most appropriate function overloads and template specializations.

Named sets of such requirements are called concepts. Each concept is a predicate, evaluated at compile time, and becomes a part of the interface of a template where it is used as a constraint

Concepts provided by NSIMD

All concepts provided by NSIMD comes in two forms:

The native C++20 form in the nsimd namespace
As a macro for keeping the compatibility with older versions of C++

The following tables list all concepts and is exhaustive. Native concepts are accessible through the nsimd namespace. They take only one argument. Their macro counterparts take no argument as they are meant to be used as constraint placeholder types. When compiling for older C++ versions NSIMD concepts macros are simply read as typename by the compiler.

Table for base C and C++ APIs:

Native concept	Macro	Description
`simd_ext_c`	`NSIMD_CONCEPT_SIMD_EXT`	Valid SIMD extension
`simd_value_type_c`	`NSIMD_CONCEPT_VALUE_TYPE`	Valid NSIMD underlying value type
`simd_value_type_or_bool_c`	`NSIMD_CONCEPT_VALUE_TYPE_OR_BOOL`	Valid NSIMD underlying value type or `bool`
`alignment_c`	`NSIMD_CONCEPT_ALIGNMENT`	Valid NSIMD alignment `aligned` or `unaligned`

Table for advanced C++ API:

Native concept	Macro	Description
`is_pack_c`	`NSIMD_CONCEPT_PACK`	Valid NSIMD pack
`is_packl_c`	`NSIMD_CONCEPT_PACKL`	Valid NSIMD packl
`is_packx1_c`	`NSIMD_CONCEPT_PACKX1`	Valid NSIMD packx1
`is_packx2_c`	`NSIMD_CONCEPT_PACKX2`	Valid NSIMD packx2
`is_packx3_c`	`NSIMD_CONCEPT_PACKX3`	Valid NSIMD packx3
`is_packx4_c`	`NSIMD_CONCEPT_PACKX4`	Valid NSIMD packx4
`any_pack_c`	`NSIMD_CONCEPT_ANY_PACK`	Any of the above pack

Expressing C++20 constraints

Expressing constraints can of course be done with the requires keyword. But for compatibility with older C++ versions NSIMD provides NSIMD_REQUIRES which take as onyl argument the constraints.

template <typename T, typename S>
NSIMD_REQUIRES(sizeof(T) == sizeof(S))
void foo(T, S);

It is advised to use doubled parenthesis as coma in the constraints expression can be interpreted as argument separators for the macro itself.

template <typename T, typename S>
NSIMD_REQUIRES((std::is_same<T, S>))
void foo(T, S);

Note that when expressing constraints using nsimd::sizeof_v's prefer the NSIMD definition of sizeof for the following reason: when dealing with float16's one cannot know the underlying representation of such a type as it is non-portable and non-standard, but NSIMD provides helper functions to transparently deal with float16's as if they were 16-bits wide. Therefore expressing sizeof equality should be done with nsimd::sizeof_v.

template <typename T, typename S>
NSIMD_REQUIRES((nsimd::sizeof_v<T> == nsimd::sizeof_v<S>))
void foo(T, S);