Question

我想知道如何将这个C代码转换为C ++以进行内存对齐。

float *pResult = (float*) _aligned_malloc(length * sizeof(float), 16);

我确实看了here，然后我尝试了这个 float *pResult = (float*) __attribute__((aligned(16)));

和这个

float *pResult = __attribute__((aligned(16)));

但两者都有类似的错误。

error: expected primary-expression before '__attribute__'|
error: expected ',' or ';' before '__attribute__'|

完整代码

#include "stdafx.h"
#include <xmmintrin.h>  // Need this for SSE compiler intrinsics
#include <math.h>       // Needed for sqrt in CPU-only version
#include "stdio.h"

int main(int argc, char* argv[])
{
    printf("Starting calculation...\n");

    const int length = 64000;

    // We will be calculating Y = Sin(x) / x, for x = 1->64000

    // If you do not properly align your data for SSE instructions, you may take a huge performance hit.
    float *pResult = (float*) __attribute__((aligned(16))); // align to 16-byte for SSE
    __m128 x;
    __m128 xDelta = _mm_set1_ps(4.0f);      // Set the xDelta to (4,4,4,4)
    __m128 *pResultSSE = (__m128*) pResult;


    const int SSELength = length / 4;

    for (int stress = 0; stress < 100000; stress++) // lots of stress loops so we can easily use a stopwatch
    {
#define TIME_SSE    // Define this if you want to run with SSE
#ifdef TIME_SSE
        x = _mm_set_ps(4.0f, 3.0f, 2.0f, 1.0f); // Set the initial values of x to (4,3,2,1)
        for (int i=0; i < SSELength; i++)
        {
            __m128 xSqrt = _mm_sqrt_ps(x);
            // Note! Division is slow. It's actually faster to take the reciprocal of a number and multiply
            // Also note that Division is more accurate than taking the reciprocal and multiplying

#define USE_DIVISION_METHOD
#ifdef USE_FAST_METHOD
            __m128 xRecip = _mm_rcp_ps(x);
            pResultSSE[i] = _mm_mul_ps(xRecip, xSqrt);
#endif //USE_FAST_METHOD
#ifdef USE_DIVISION_METHOD
            pResultSSE[i] = _mm_div_ps(xSqrt, x);
#endif  // USE_DIVISION_METHOD

            // NOTE! Sometimes, the order in which things are done in SSE may seem reversed.
            // When the command above executes, the four floating elements are actually flipped around
            // We have already compensated for that flipping by setting the initial x vector to (4,3,2,1) instead of (1,2,3,4)

            x = _mm_add_ps(x, xDelta);  // Advance x to the next set of numbers
        }
#endif  // TIME_SSE
#ifndef TIME_SSE
        float xFloat = 1.0f;
        for (int i=0 ; i < length; i++)
        {
            pResult[i] = sqrt(xFloat) / xFloat; // Even though division is slow, there are no intrinsic functions like there are in SSE
            xFloat += 1.0f;
        }
#endif  // !TIME_SSE
    }

    // To prove that the program actually worked
    for (int i=0; i < 20; i++)
    {
        printf("Result[%d] = %f\n", i, pResult[i]);
    }

    // Results for my particular system
    // 23.75 seconds for SSE with reciprocal/multiplication method
    // 38.5 seconds for SSE with division method
    // 301.5 seconds for CPU

    return 0;
}

Answer 1

使用C ++ 11，你可以使用类似的东西：

struct aligned_float
{
    alignas(16) float f[4];
};

static_assert(sizeof(aligned_float) == 4 * sizeof(float), "padding issue");

int main()
{
    const int length = 64000;
    std::vector<aligned_float> pResult(length / sizeof(aligned_float));

    return 0;
}

Answer 2

对齐的属性仅适用于编译/链接的方式。它没有运行时效果。

我知道解决这个问题的唯一可移植方法是使用一个实际分配超过必要值的包装器，然后屏蔽低位以确保它所支持的内容符合充分的对齐。

Answer 3

见这里：

http://www.gnu.org/software/libc/manual/html_node/Aligned-Memory-Blocks.html

Glibc提供了aligned_alloc（）。

在C ++中SSE的内存对齐，_aligned_malloc等价？

3 个答案: