使用SIMD內在函數進行高效的行列轉換

Question

我是SIMD編程的初學者。我想處理我的數據如下：

考慮我有4個SIMD變量（__m128i）與數據如下：

__m128i a = {a1, a2, a3, a4} 
__m128i b = {b1, b2, b3, b4} 
__m128i c = {c1, c2, c3, c4} 
__m128i d = {d1, d2, d3, d4} 

現在我想如下初始化熙：

__m128i x1 = {a1, b1, c1, d1} 
__m128i x2 = {a2, b2, c2, d2} 
__m128i x3 = {a3, b3, c3, d3} 
__m128i x4 = {a4, b4, c4, d4} 

有人可以向我建議我如何有效地做到這一點？

Answer 1

我假設在您的問題中存在拼寫錯誤，並且您確實想要執行4x4轉置。如果是這樣，那麼你可以做一個4×4與8個指令轉，像這樣：

#include "emmintrin.h" 

inline void Transpose_4_4(
    __m128i &v0,    // a1, a2, a3, a4 => a1, b1, c1, d1 
    __m128i &v1,    // b1, b2, b3, b4 => a2, b2, c2, d2 
    __m128i &v2,    // c1, c2, c3, c4 => a3, b3, c3, d3 
    __m128i &v3)    // d1, d2, d3, d4 => a4, b4, c4, d4 
{ 
    __m128i w0 = _mm_unpacklo_epi32(v0, v1); 
    __m128i w1 = _mm_unpackhi_epi32(v0, v1); 
    __m128i w2 = _mm_unpacklo_epi32(v2, v3); 
    __m128i w3 = _mm_unpackhi_epi32(v2, v3); 
    v0 = _mm_unpacklo_epi64(w0, w2); 
    v1 = _mm_unpackhi_epi64(w0, w2); 
    v2 = _mm_unpacklo_epi64(w1, w3); 
    v3 = _mm_unpackhi_epi64(w1, w3); 
} 

演示：

// 
// tranpose_4_4.cpp 
// 

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

inline void Transpose_4_4(
    __m128i &v0,    // a1, a2, a3, a4 => a1, b1, c1, d1 
    __m128i &v1,    // b1, b2, b3, b4 => a2, b2, c2, d2 
    __m128i &v2,    // c1, c2, c3, c4 => a3, b3, c3, d3 
    __m128i &v3)    // d1, d2, d3, d4 => a4, b4, c4, d4 
{ 
    __m128i w0 = _mm_unpacklo_epi32(v0, v1); 
    __m128i w1 = _mm_unpackhi_epi32(v0, v1); 
    __m128i w2 = _mm_unpacklo_epi32(v2, v3); 
    __m128i w3 = _mm_unpackhi_epi32(v2, v3); 
    v0 = _mm_unpacklo_epi64(w0, w2); 
    v1 = _mm_unpackhi_epi64(w0, w2); 
    v2 = _mm_unpacklo_epi64(w1, w3); 
    v3 = _mm_unpackhi_epi64(w1, w3); 
} 

int main(void) 
{ 
    int32_t buff[4][4] __attribute__ ((aligned(16))); 
    int i, j; 
    int k = 0; 

    // init buff 
    for (i = 0; i < 4; ++i) 
    { 
     for (j = 0; j < 4; ++j) 
     { 
      buff[i][j] = k++; 
     } 
    } 

    // print buff 
    printf("\nBEFORE:\n"); 
    for (i = 0; i < 4; ++i) 
    { 
     for (j = 0; j < 4; ++j) 
     { 
      printf("%4d", buff[i][j]); 
     } 
     printf("\n"); 
    } 

    // transpose 
    Transpose_4_4(*(__m128i *)buff[0], *(__m128i *)buff[1], *(__m128i *)buff[2], *(__m128i *)buff[3]); 

    // print buff 
    printf("\nAFTER:\n"); 
    for (i = 0; i < 4; ++i) 
    { 
     for (j = 0; j < 4; ++j) 
     { 
      printf("%4d", buff[i][j]); 
     } 
     printf("\n"); 
    } 

    return 0; 
} 

編譯並運行：

$ g++ -Wall -msse3 transpose_4_4.cpp && ./a.out 

BEFORE: 
    0 1 2 3 
    4 5 6 7 
    8 9 10 11 
    12 13 14 15 

AFTER: 
    0 4 8 12 
    1 5 9 13 
    2 6 10 14 
    3 7 11 15 
$