Memcpy 函数受指针对齐的速度影响

Memcpy 函数受指针对齐的速度影响

Memcpy 函数原型

• Defined in header <cstring>
• void* memcpy( void* dest, const void* src, std::size_t count );
• Functions: Copies count bytes from the object pointed to by src to the object pointed to by dest. Both objects are reinterpreted as arrays of unsigned char.

Parameters

dest - pointer to the memory location to copy to
src - pointer to the memory location to copy from
count - number of bytes to copy
Return value
dest

测试条件

IAR 8.30  , Simulater ，Corex-m3, 优化-O0

测试代码及结果

在“start" 和“end”分别打断点，观察断点“end”处 CCSTEP 值，并记录到Cycle speed中。

#include "stdint.h"

/*
memcpy function speed test:


------------------------------------------------------------------------
|Test List  |   Dest pointer       |   Src  pointer      |  Cycle speed  |
--------------------------------------------------------------------------
|  0        | 4 byte aligned       | 4 byte aligned      |      2203
--------------------------------------------------------------------------
|  1        | 4 byte aligned       | 4 byte aligned  +1  |      2765
--------------------------------------------------------------------------
|  2        | 4 byte aligned +1    | 4 byte aligned      |      2783
---------------------------------------------------------------------------
|  3        | 4 byte aligned +1    | 4 byte aligned +1   |      2229
--------------------------------------------------------------------------
|  4        | 4 byte aligned +2    | 4 byte non-aligned  |      2773
--------------------------------------------------------------------------
|  5        | 4 byte aligned +3    | 4 byte non-aligned  |      2763
--------------------------------------------------------------------------
*/
uint32_t buf[5000];

void * p_s;
void * p_d;

#define TEST_LIST    5

void main( void )
{

#if TEST_LIST == 0
   p_s = (void *)buf;
   p_d =  (void *)((uint8_t *)(buf+2000));
#elif TEST_LIST == 1
   p_s = (void *)((uint8_t *)(buf) +1 );
   p_d =  (void *)((uint8_t *)(buf+2000));
#elif TEST_LIST == 2
   p_s = (void *)buf;
   p_d =  (void *)((uint8_t *)(buf+2000) +1 );
#elif TEST_LIST == 3
   p_s = (void *)((uint8_t *)(buf) +1 );
   p_d =  (void *)((uint8_t *)(buf+2000) +1 );
#elif TEST_LIST == 4
   p_s = (void *)((uint8_t *)(buf) +1 );
   p_d =  (void *)((uint8_t *)(buf+2000) +2 );
#elif TEST_LIST == 5
   p_s = (void *)((uint8_t *)(buf) +1 );
   p_d =  (void *)((uint8_t *)(buf+2000) +3 );
#endif
   
   printf("start\r\n");
   memcpy(p_s,p_d,1000);
   printf("end\r\n");
}

结论

1. 目标及源都是4byte对齐的时候，速度最快。
2. 如果目标及源对于4byte对齐都有相同的偏移，那么速度是第二，应该是memcpy先做单字节拷贝，然后再整体4字节拷贝。
3. 其他情况速度都比1慢，时间增加25%