SIMD library

Merged into ISO C++ The functionality described on this page was merged into the mainline ISO C++ standard as of 11/2024; see the data-parallel types (SIMD) (since C++26)

The SIMD library provides portable types for explicitly stating data-parallelism and structuring data for more efficient SIMD access.

An object of type simd<T> behaves analogue to objects of type T. But while T stores and manipulates one value, simd<T> stores and manipulates multiple values (called width but identified as size for consistency with the rest of the standard library; cf. simd_size).

Every operator and operation on simd<T> acts element-wise (except for horizontal operations, which are clearly marked as such). This simple rule expresses data-parallelism and will be used by the compiler to generate SIMD instructions and/or independent execution streams.

The width of the types simd<T> and native_simd<T> is determined by the implementation at compile-time. In contrast, the width of the type fixed_size_simd<T, N> is fixed by the developer to a certain size.

A recommended pattern for using a mix of different SIMD types with high efficiency uses native_simd and rebind_simd:

#include <experimental/simd>
namespace stdx = std::experimental;
 
using floatv  = stdx::native_simd<float>;
using doublev = stdx::rebind_simd_t<double, floatv>;
using intv    = stdx::rebind_simd_t<int, floatv>;

This ensures that the set of types all have the same width and thus can be interconverted. A conversion with mismatching width is not defined because it would either drop values or have to invent values. For resizing operations, the SIMD library provides the split and concat functions.

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