如何使以下code快 [英] How to make the following code faster

问题描述

int u1, u2;  
unsigned long elm1[20], _mulpre[16][20], res1[40], res2[40]; 64 bits long     
res1, res2 initialized to zero.  

l = 60;  
while (l)  
{  
    for (i = 0; i < 20; i += 2)  
    {  
        u1 = (elm1[i] >> l) & 15;  
        u2 = (elm1[i + 1] >> l) & 15;

        for (k = 0; k < 20; k += 2)  
        {  
            simda = _mm_load_si128 ((__m128i *) &_mulpre[u1][k]);  
            simdb = _mm_load_si128 ((__m128i *) &res1[i + k]);  
            simdb = _mm_xor_si128  (simda, simdb);  
            _mm_store_si128 ((__m128i *)&res1[i + k], simdb);  

            simda = _mm_load_si128 ((__m128i *)&_mulpre[u2][k]);  
            simdb = _mm_load_si128 ((__m128i *)&res2[i + k]);  
            simdb = _mm_xor_si128  (simda, simdb);  
            _mm_store_si128 ((__m128i *)&res2[i + k], simdb);  
        } 
    }
    l -= 4;
    All res1, res2 values are left shifted by 4 bits.  
}

以上提到的code是我的程序多次调用（分析器显示98％）。

The above mentioned code is called many times in my program (profiler shows 98%).

编辑：在内环，RES1 [1 + k]的值被加载多次为相同第（i + k）的值。我这个尝试的while循环中，我加载完所有RES1值到SIMD寄存器（阵列）和使用数组元素最内层的内循环更新数组元素。一旦双方的for循环完成后，我存储在数组值回RES1，RE2。但是，计算时间与此增加。任何想法，我错了吗？这个想法似乎是正确的。

In the inner loop, res1[i + k] values are loaded many times for same (i + k) values. I tried with this inside the while loop, I loaded all the res1 values into simd registers (array) and use array elements inside the innermost for loop to update array elements . Once both for loops are done, I stored the array values back to the res1, re2. But computation time increases with this. Any idea where I got wrong? The idea seemed to be correct

任何建议，使其更快是值得欢迎的。

Any suggestion to make it faster is welcome.

推荐答案

不幸的是，最明显的优化可能已经被编译器完成：

Unfortunately the most obvious optimisations are probably already being done by the compiler:

• 您可以拉＆放大器; _mul pre [U1] 和＆放大器; MUL pre [U2] 内环我们。


• 您可以拉＆放大器; RES1 [I] 内环我们


• 使用不同变量的两个内部操作，重新排列它们，可能会实现更好的流水线。

• You can pull &_mulpre[u1] and &mulpre[u2] our of the inner loop.
• You can pull &res1[i] our of the inner loop.
• Using different variables for the two inner operations, and reordering them, might allow for better pipelining.

可能交换外循环就会提高缓存的区域性 elm1 。

Possibly swapping the outer loops would improve cache locality on elm1.

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