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The latest version of this topic can be found at __m128d.

Microsoft Specific
The __m128d data type, for use with the Streaming SIMD Extensions 2 instructions intrinsics, is defined in emmintrin.h.

// data_types__m128d.cpp  
#include <emmintrin.h>  
int main() {  
   __m128d x;  

You should not access the __m128d fields directly. You can, however, see these types in the debugger. A variable of type __m128 maps to the XMM[0-7] registers.

Variables of type _m128d are automatically aligned on 16-byte boundaries.

The __m128d data type is not supported on ARM processors.

Fundamental Types
Data Type Ranges