CODEWITHSUNDEEP
ssesimd
Green goblin
Green goblin

Reputation: 9996

Shuffle 16 bit vectors SSE

I am working on SSE and a newbie here. I am trying to use shuffle instruction to shuffle a 16 bit vector like below:

Input: 

1 2 3 4 5 6 7 8

Output: 

1 5 2 6 3 7 4 8

How do I achieve the desired target? I am confused about the constant being used here and I don't see any 16 bit shuffle instruction. shuffle instruction is only available for 8 bit and 32 bit.

Upvotes: 3

Views: 5573

Answers (1)

Paul R
Paul R

Reputation: 212969

So long as you can assume SSSE3 then you can use pshufb aka _mm_shuffle_epi8:

#include <stdio.h>
#include <tmmintrin.h>  // SSSE3

int main()
{
    __m128i v_in = _mm_setr_epi16(1, 2, 3, 4, 5, 6, 7, 8);
    __m128i v_perm = _mm_setr_epi8(0, 1, 8, 9, 2, 3, 10, 11, 4, 5, 12, 13, 6, 7, 14, 15);
    __m128i v_out = _mm_shuffle_epi8(v_in, v_perm); // pshufb
    printf("v_in  = %vhd\n", v_in);
    printf("v_out = %vhd\n", v_out);
    return 0;
}

Compile and run:

$ gcc -Wall -mssse3 green_goblin.c 
$ ./a.out 
v_in  = 1 2 3 4 5 6 7 8
v_out = 1 5 2 6 3 7 4 8

Alternate solution which relies only on SSE2:

#include <stdio.h>
#include <emmintrin.h>  // SSE2

int main()
{
    __m128i v_in = _mm_setr_epi16(1, 2, 3, 4, 5, 6, 7, 8);
    __m128i v_out = _mm_shuffle_epi32(v_in, _MM_SHUFFLE(3, 1, 2, 0)); // pshufd
    v_out = _mm_shufflelo_epi16(v_out, _MM_SHUFFLE(3, 1, 2, 0));      // pshuflw
    v_out = _mm_shufflehi_epi16(v_out, _MM_SHUFFLE(3, 1, 2, 0));      // pshufhw
    printf("v_in  = %vhd\n", v_in);
    printf("v_out = %vhd\n", v_out);
    return 0;
}

Compile and run:

$ gcc -Wall -msse2 green_goblin_sse2.c 
$ ./a.out 
v_in  = 1 2 3 4 5 6 7 8
v_out = 1 5 2 6 3 7 4 8

Upvotes: 6

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