# Intrinsics Returning Vectors of Undefined Values

These intrinsics generate vectors of undefined values. The result of the intrinsics is usually used as an argument to another intrinsic that requires all operands to be initialized, and when the content of a particular argument does not matter.

To use these intrinsics, include the immintrin.h file as follows:

    #include <immintrin.h>

For example, you can use such an intrinsic when you need to calculate a sum of packed double-precision floating-point values located in the xmm register. To avoid unnecessary moves, you can use the following code to obtain the required result at the low 64 bits:

    __m128d HILO	= doSomeWork();
    __m128d HI = _mm_unpackhi_pd(HILO, _mm_undefined_pd());
    __m128d result	= _mm_add_sd(HI, HILO);

## _mm_undefined_pd

extern __m128d _mm_undefined_pd(void);

Returns a vector of two double precision floating point elements. The content of the vector is not specified.

## _mm_undefined_si128

extern __m128i _mm_undefined_si128(void);

Returns a vector of four packed doubleword integer elements. The content of the vector is not specified.

Parent topic: Miscellaneous Functions and Intrinsics

## See Also

_mm256_undefined_si128() Returns a vector of eight packed doubleword integer elements. No corresponding Intel® AVX instruction.
_mm256_undefined_pd() Returns a vector of four double precision floating point elements. No corresponding Intel® AVX instruction.