# _mm256_castpd128_pd256

Typecasts 128-bit double-precision floating point values to 256-bit double-precision floating point values. No corresponding Intel® AVX instruction.

## Syntax

extern __m256d _mm256_castpd128_pd256(__m128d a);

## Arguments

a | 128-bit float64 vector

## Description

Performs a typecast operation from 128-bit double-precision floating point values to 256-bit double-precision floating point values.

The lower 128-bits of the 256-bit resulting vector contains the source vector values; the upper 128-bits of the resulting vector are undefined. This intrinsic does not introduce extra moves to the generated code.

## Returns

A vector with 256-bit double-precision floating point values. The upper bits of the resulting vector are undefined.

Parent topic: Support Intrinsics for Vector Typecasting Operations