其他
SIMD系列-GATHER/SCATTER操作
1、Stried access跨步访问
float a[LARGE_DATA_SIZE];uint32_t STRIDE = 8;...for(int i = 0; i < PROBLEM_SIZE; i+=8) { SIMDVec<float, 8> vec; // Note that we have to scale the loop index. int offset = i*STRIDE; // 'load' the data to vec. vec.gather(&a[offset], STRIDE); // do something useful vec += 3.14; // store the result at original locations vec.scatter(&a[offset], STRIDE);}2、Indexed access索引访问
float a[LARGE_DATA_SIZE];int indices[PROBLEM_SIZE];uint32_t STRIDE = 4;...for(int i = 0; i < PROBLEM_SIZE; i+=8) { SIMDVec<float, 8> vec; // Here we are using precomputed indices, // but they can be computed on-the-fly if necessary. SIMDVec<uint32_t, 8> indices_vec(&indices[i]; // 'load' the data to vec. vec.gather(&a[0], indices_vec); // do something useful vec += 3.14; // store the result at original locations vec.scatter(&a[0], indices_vec);}3、确保有条件访问
float a[PROBLEM_SIZE], b[PROBLEM_SIZE];float c[LARGE_DATASET_SIZE];...for(int i = 0; i < PROBLEM_SIZE; i++) { // Here we are checking if for some reason one of the // values in (a[i],b[i]) pair is not determined properly. if (std::isfin(a[i] - b[i])) { // Calculate the index only if both 'a' and 'b' are well defined int index = int(a[i] - b[i]); // 'gather' single element at a time float temp = c[index]; // Do something with the value temp += 3.14; // Update the values of 'c' c[index] = temp; }}float a[PROBLEM_SIZE], b[PROBLEM_SIZE];float c[LARGE_DATASET_SIZE];...// For simplification we are assuming that: ((PROBLEM_SIZE % 8) == 0)for(int i = 0; i < PROBLEM_SIZE; i+= 8) { // Here we are checking if for some reason (e.g. a design decision) one // of the values in (a[i],b[i]) pair is not determined properly. SIMDVec<float, 8> a_vec(&a[i]), b_vec(&b[i]); SIMDVecMask<8> condition = (a_vec - b_vec).isfin(); // if (std::isfin(a[i] - b[i])) { SIMDVec<uint32_t, 8> indices = a_vec - b_vec; SIMDVec<float, 8> temp; temp.gather(&c[0], indices); // This is WRONG!!! temp.adda(condition, 3.14); // only change selected elements temp.scatter(&c[0], indices); // Again WRONG!!!}float a[PROBLEM_SIZE], b[PROBLEM_SIZE];float c[LARGE_DATASET_SIZE];...// For simplification we are assuming that: ((PROBLEM_SIZE % 8) == 0)for(int i = 0; i < PROBLEM_SIZE; i+= 8) { // Here we are checking if for some reason (e.g. by design?) one of the // values in (a[i],b[i]) pair is not determined properly. SIMDVec<float, 8> a_vec(&a[i]), b_vec(&b[i]); SIMDVecMask<8> condition = (a_vec - b_vec).isfin(); // if (std::isfin(a[i] - b[i])) { SIMDVec<uint32_t, 8> indices = a_vec - b_vec; SIMDVec<float, 8> temp; temp.gather(condition, &c[0], indices); // Now the read is CORRECT!!! temp += 3.14; // we don't have to mask this operation temp.scatter(condition, &c[0], indices); // Again no problems here.}