Altivec：_mm_sad_epu8（）的模拟

Question

我试图移植一个SSE函数，它可以得到两个8位无符号整数数组的绝对差值。 它看起来像：

uint64_t AbsDiffSum(const uint8_t * a, const uint8_t * b, size_t size) 
{ 
    assert(size%16 == 0); 
    __m128i _sum = _mm_setzero_si128(); 
    for(size_t i = 0; i < size; i += 16) 
    { 
     const __m128i _a = _mm_loadu_si128((__m128i*)(a + i)); 
     const __m128i _b = _mm_loadu_si128((__m128i*)(b + i)); 
     _sum = _mm_add_epi64(_sum, _mm_sad_epu8(_a, _b)); 
    } 
    return _mm_cvtsi128_si64(_mm_add_epi64(_sum, _mm_srli_si128(_sum, 8))); 
} 

主要工作是通过内部函数_mm_sad_epu8进行（）。

Altivec有类似吗？

Answer 1

不幸的是，Altivec没有内在函数_mm_sad_epu8的直接模拟。 但有可能模仿它：

typedef __vector uint8_t uint8x16_t; 
typedef __vector uint32_t uint32x4_t; 
const uint8_t K8_01 = {1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1}; 

uint64_t AbsDiffSum(const uint8_t * a, const uint8_t * b, size_t size) 
{ 
    uint32x4_t _sum = {0, 0, 0, 0}; 
    for(size_t i = 0; i < size; i += 16) 
    { 
     // Aligned loading of 128-bit vector 
     uint8x16_t _a = vec_ld(a + i); 
     // Aligned loading of 128-bit vector 
     uint8x16_t _b = vec_ld(b + i); 
     // Find absolute difference of two 8-bit unsigned 
     uint8x16_t absDifference = vec_sub(vec_max(a, b), vec_min(a, b)); 
     // Sum result with using of vec_msum 
     _sum = vec_msum(absDifference, K8_01, _sum); 
    } 
    return vec_extract(_sum, 0) + vec_extract(_sum, 1) + 
      vec_extract(_sum, 2) + vec_extract(_sum, 3); 
}