我可以处理__m128i中包含的4 int32_t吗？

Question

I would like to have a __m128i variable, and do some operation like this:

unsigned char* myArray;
__m128i fourValues;

//Do some strange reference assignment, e.g.:
//int32_t& a = *((int32_t*) &fourValues);
//int32_t& b = *(((int32_t*) &fourValues) + 1);
//int32_t& c = *(((int32_t*) &fourValues) + 2);
//int32_t& d = *(((int32_t*) &fourValues) + 3);

for (int i =0; i < someSize; i+=4) {
  a = d + myArray[i];
  b = a + myArray[i+1];
  c = b + myArray[i+2];
  d = c + myArray[i+3];
  //Do something with fourValues;
  }

where a,b,c,d are (or behave as) int32_t variables, and are the first, second, third and fourth 32bits of fourValues. Is that possible?

Answer 1

You can use

int _mm_extract_epi32( 
   __m128i a,
   const int ndx 
);

see - http://msdn.microsoft.com/en-us/library/bb531460%28v=vs.90%29.aspx

EDIT: Note that this only works for int SSE vectors, I see there's quite a lot of debate going around here - Get member of __m128 by index? , which refers to a __m128, but in your case it should be possible to use that. Still, always worth to get a heads up that this isn't generic enough for floats.

For writing you can use the equivalient _mm_insert_epi32, as you point out.