c++ - 与 ARM Neon vtbx 的字节序混淆

Question

我有一个 16 字节的置换掩码 uint8_t[16] 和一个 16 字节的数据数组 uint32_t[4]。我想像这样使用 vtbl 来“洗牌”这个数据数组:

          0   1   2   3   4   5   6   7    8   9    A    B    C    D    E    F
  Data ||0x0,0x0,0x1,0x2|0x0,0x3,0x0,0x4||0x5,0x6, 0x7, 0x8| 0x0, 0x0, 0x0, 0x9||

 SMask ||0x2,0x3,0x5,0x6|0x7,0x8,0x9,0xA||0xB,0xF,0x10,0x10|0x10,0x10,0x10,0x10||

Result ||0x1,0x2,0x3,0x0|0x4,0x5,0x6,0x7||0x8,0x9, 0x0, 0x0| 0x0, 0x0, 0x0, 0x0||

到目前为止，这是我的代码:

#include <iostream>
#include <arm_neon.h>

inline uint8x16_t Shuffle(const uint8x16_t & src, const uint8x16_t & shuffle) {
  return vcombine_u8(
          vtbl2_u8(
            (const uint8x8x2_t &)src, 
            vget_low_u8(shuffle)
          ),
          vtbl2_u8(
            (const uint8x8x2_t &)src, 
          vget_high_u8(shuffle)
          )
        );
}

int main() {
   uint32_t* data32 = new uint32_t[4];
   data32[0] = 258;             // [0x00 0x00 0x01 0x02]
   data32[1] = 196612;          // [0x00 0x03 0x00 0x04]
   data32[2] = 84281096;        // [0x05 0x06 0x07 0x08]
   data32[3] = 9;               // [0x00 0x00 0x00 0x09]    
   /*load structure*/
   uint32x4_t data32Vec = vld1q_u32(data32);

   uint8_t* sMask = new uint8_t[16];
   sMask[0] = 2;
   sMask[1] = 3;
   sMask[2] = 5;
   sMask[3] = 6;
   sMask[4] = 7;
   sMask[5] = 8;
   sMask[6] = 9;
   sMask[7] = 10;
   sMask[8] = 11;
   sMask[9] = 15;
   sMask[10] = 16;
   sMask[11] = 16;
   sMask[12] = 16;
   sMask[13] = 16;
   sMask[14] = 16;
   sMask[15] = 16;
   /*load permutationmask into vector register*/
   uint8x16_t shuffleMask = vld1q_u8(sMask);

   uint8_t* comprData = new uint8_t[16];
   /*shuffle the data with the mask and store it into an uint8_t[16]*/
   vst1q_u8(comprData, Shuffle(vreinterpretq_u8_u32(data32Vec),shuffleMask));
   for(int i = 0; i < 16; ++i) {
      std::cout << (unsigned)comprData[i] << "   " ;
   }
   std::cout << std::endl;
   delete[] comprData;
   delete[] sMask;
   delete[] data32;
   return 0;
}

输出如下所示:

0   0   0   3   0   8   7   6   5   0   0   0   0   0   0   0

应该是这样的:

1   2   3   0   4   5   6   7   8   9   0   0   0   0   0   0

我认为这与字节序有关，但看不出问题所在。有没有人有提示？

我根据 ErmIg 的回答更新了代码。主要问题是，我混淆了 vtbx 和 vtbl。

真诚的

Answer 1

可能对您有帮助(我使用这些函数在 Arm NEON 的 vector 中随机排列字节):

    inline uint8x16_t Shuffle(const uint8x16_t & src, const uint8x16_t & shuffle)
    {
        return vcombine_u8(
            vtbl2_u8((const uint8x8x2_t &)src, vget_low_u8(shuffle)),
            vtbl2_u8((const uint8x8x2_t &)src, vget_high_u8(shuffle)));
    }

    inline uint8x16_t Shuffle(const uint8x16x2_t & src, const uint8x16_t & shuffle)
    {
        return vcombine_u8(
            vtbl4_u8((const uint8x8x4_t &)src, vget_low_u8(shuffle)), 
            vtbl4_u8((const uint8x8x4_t &)src, vget_high_u8(shuffle)));
    }