memcpy多字节字节赋值问题

以前没注意过多字节赋值问题,考虑的角度是充分利用 cpu 的 32 位带宽。一次复制 1 个字节和一次复制 4 个字节占用的 cpu 指令周期是一样的,既然我们的 cpu 能同时操作 32 位,为什么我们不能一次复制 4 个字节呢?如此一来,总运行的指令数将降低到原来的 1/4 !

于是有下面代码(henix转自http://www.embedded.com/columns/technicalinsights/19205567?_requestid=212290):

memcpy多字节字节赋值问题
文章图片
memcpy多字节字节赋值问题
文章图片
代码


1 void * memcpy( void * dest, void * src, size_t count)
2 {
3 size_t c = count >> 2 ; /* 相当于 count / 4 */
4 long * pt;
5 const long * ps;
6 if (c)
7 {
8 /* 首先按 4 个字节复制 */
9 pt = ( long * ) dest;
10 ps = ( const long * ) src;
11 while (c)
12 {
13 * pt = * ps;
14 pt ++ ;
15 ps ++ ;
16 c -- ;
17 }
18 }
19 c = count & 3 ; /* 得到 count 模 4 的余数 */
20 if (c)
21 {
22 /* 如果有剩下的,再按 1 个字节复制剩余的 */
23 char * pct = ( char * ) pt;
24 const char * pcs = ( const char * ) ps;
25 while (c)
26 {
27 * pct = * pcs;
28 pct ++ ;
29 pcs ++ ;
30 c -- ;
31 }
32 }
33 return dest;
34 } 通过使用 long 型指针,强行按 4 个字节复制。



还有对 3 种 memcpy 实现的对比,上述算法受到地址对齐的影响较为严重。在地址按4 字节对齐的时候,上述算法的效率比单字节 memcpy 实现高很多,但如果地址没有按4 字节对齐,则其效率并不高,有时甚至还比普通 memcpy 还低。这可能是因为,虽然上述算法减少了 cpu 的指令数,但内存的速度比 cpu 慢得多,速度的瓶颈还是在内存。


【memcpy多字节字节赋值问题】刚看了一份memcpy的实现,为了解决字节对齐的问题,从128开始,依次减少,直到1个字节。原文在http://code.google.com/p/dingoo-sdk/source/browse/trunk/dingoo_sdk/src/libc/memcpy.c?spec=svn180&r=180
贴出如下:

#include "string.h" #include inline void _memcpy_1x1(void* dst, const void* src) { asm( "lb $t0, 0(%0)\n\t" "sb $t0, 0(%1)\n\t" : : "r"(dst), "r"(src) ); } inline void _memcpy_2x1(void* dst, const void* src) { asm( "lh $t0, 0(%0)\n\t" "sh $t0, 0(%1)\n\t" : : "r"(dst), "r"(src) ); } inline void _memcpy_4x1(void* dst, const void* src) { asm( "lw $t0, 0(%0)\n\t" "sw $t0, 0(%1)\n\t" : : "r"(dst), "r"(src) ); } inline void _memcpy_4x2(void* dst, const void* src) { asm( "lw $t0, 0(%0)\n\t" "lw $t1, 4(%0)\n\t" "sw $t0, 0(%1)\n\t" "sw $t1, 4(%1)\n\t" : : "r"(dst), "r"(src) ); } inline void _memcpy_4x4(void* dst, const void* src) { asm( "lw $t0,0(%0)\n\t" "lw $t1,4(%0)\n\t" "lw $t2,8(%0)\n\t" "lw $t3, 12(%0)\n\t" "sw $t0,0(%1)\n\t" "sw $t1,4(%1)\n\t" "sw $t2,8(%1)\n\t" "sw $t3, 12(%1)\n\t" : : "r"(dst), "r"(src) ); } inline void _memcpy_4x8(void* dst, const void* src) { asm( "lw $t0,0(%0)\n\t" "lw $t1,4(%0)\n\t" "lw $t2,8(%0)\n\t" "lw $t3, 12(%0)\n\t" "lw $t4, 16(%0)\n\t" "lw $t5, 20(%0)\n\t" "lw $t6, 24(%0)\n\t" "lw $t7, 28(%0)\n\t" "sw $t0,0(%1)\n\t" "sw $t1,4(%1)\n\t" "sw $t2,8(%1)\n\t" "sw $t3, 12(%1)\n\t" "sw $t4, 16(%1)\n\t" "sw $t5, 20(%1)\n\t" "sw $t6, 24(%1)\n\t" "sw $t7, 28(%1)\n\t" : : "r"(dst), "r"(src) ); } inline void _memcpy_4x16(void* dst, const void* src) { asm( "lw $t0,0(%0)\n\t" "lw $t1,4(%0)\n\t" "lw $t2,8(%0)\n\t" "lw $t3, 12(%0)\n\t" "lw $t4, 16(%0)\n\t" "lw $t5, 20(%0)\n\t" "lw $t6, 24(%0)\n\t" "lw $t7, 28(%0)\n\t" "lw $t8, 32(%0)\n\t" "lw $t9, 36(%0)\n\t" "sw $t0,0(%1)\n\t" "lw $t0, 40(%0)\n\t" "sw $t1,4(%1)\n\t" "lw $t1, 44(%0)\n\t" "sw $t2,8(%1)\n\t" "lw $t2, 48(%0)\n\t" "sw $t3, 12(%1)\n\t" "lw $t3, 52(%0)\n\t" "sw $t4, 16(%1)\n\t" "lw $t4, 56(%0)\n\t" "sw $t5, 20(%1)\n\t" "lw $t5, 60(%0)\n\t" "sw $t6, 24(%1)\n\t" "sw $t7, 28(%1)\n\t" "sw $t8, 32(%1)\n\t" "sw $t9, 36(%1)\n\t" "sw $t0, 40(%1)\n\t" "sw $t1, 44(%1)\n\t" "sw $t2, 48(%1)\n\t" "sw $t3, 52(%1)\n\t" "sw $t4, 56(%1)\n\t" "sw $t5, 60(%1)\n\t" : : "r"(dst), "r"(src) ); } inline void _memcpy_4x32(void* dst, const void* src) { asm( "lw $t0,0(%0)\n\t" "lw $t1,4(%0)\n\t" "lw $t2,8(%0)\n\t" "lw $t3,12(%0)\n\t" "lw $t4,16(%0)\n\t" "lw $t5,20(%0)\n\t" "lw $t6,24(%0)\n\t" "lw $t7,28(%0)\n\t" "lw $t8,32(%0)\n\t" "lw $t9,36(%0)\n\t" "sw $t0,0(%1)\n\t" "lw $t0,40(%0)\n\t" "sw $t1,4(%1)\n\t" "lw $t1,44(%0)\n\t" "sw $t2,8(%1)\n\t" "lw $t2,48(%0)\n\t" "sw $t3,12(%1)\n\t" "lw $t3,52(%0)\n\t" "sw $t4,16(%1)\n\t" "lw $t4,56(%0)\n\t" "sw $t5,20(%1)\n\t" "lw $t5,60(%0)\n\t" "sw $t6,24(%1)\n\t" "lw $t6,64(%0)\n\t" "sw $t7,28(%1)\n\t" "lw $t7,68(%0)\n\t" "sw $t8,32(%1)\n\t" "lw $t8,72(%0)\n\t" "sw $t9,36(%1)\n\t" "lw $t9,76(%0)\n\t" "sw $t0,40(%1)\n\t" "lw $t0,80(%0)\n\t" "sw $t1,44(%1)\n\t" "lw $t1,84(%0)\n\t" "sw $t2,48(%1)\n\t" "lw $t2,88(%0)\n\t" "sw $t3,52(%1)\n\t" "lw $t3,92(%0)\n\t" "sw $t4,56(%1)\n\t" "lw $t4,96(%0)\n\t" "sw $t5,60(%1)\n\t" "lw $t5, 100(%0)\n\t" "sw $t6,64(%1)\n\t" "lw $t6, 104(%0)\n\t" "sw $t7,68(%1)\n\t" "lw $t7, 108(%0)\n\t" "sw $t8,72(%1)\n\t" "lw $t8, 112(%0)\n\t" "sw $t9,76(%1)\n\t" "lw $t9, 116(%0)\n\t" "sw $t0,80(%1)\n\t" "lw $t0, 120(%0)\n\t" "sw $t1,84(%1)\n\t" "lw $t1, 124(%0)\n\t" "sw $t2,88(%1)\n\t" "sw $t3,92(%1)\n\t" "sw $t4,96(%1)\n\t" "sw $t5, 100(%1)\n\t" "sw $t6, 104(%1)\n\t" "sw $t7, 108(%1)\n\t" "sw $t8, 112(%1)\n\t" "sw $t9, 116(%1)\n\t" "sw $t0, 120(%1)\n\t" "sw $t1, 124(%1)\n\t" : : "r"(dst), "r"(src) ); } inline void _memcpy_down_1(uint8_t* dst, const uint8_t* src, uint8_t* dst_end) { if(((uintptr_t)dst_end - (uintptr_t)dst) >= 1) { _memcpy_1x1(dst, src); dst += 1; src += 1; } } inline void _memcpy_down_2(uint8_t* dst, const uint8_t* src, uint8_t* dst_end) { if(((uintptr_t)dst_end - (uintptr_t)dst) >= 2) { _memcpy_2x1(dst, src); dst += 2; src += 2; } _memcpy_down_1(dst, src, dst_end); } inline void _memcpy_down_4(uint8_t* dst, const uint8_t* src, uint8_t* dst_end) { if(((uintptr_t)dst_end - (uintptr_t)dst) >= 4) { _memcpy_4x1(dst, src); dst += 4; src += 4; } _memcpy_down_2(dst, src, dst_end); } inline void _memcpy_down_8(uint8_t* dst, const uint8_t* src, uint8_t* dst_end) { if(((uintptr_t)dst_end - (uintptr_t)dst) >= 8) { _memcpy_4x2(dst, src); dst += 8; src += 8; } _memcpy_down_4(dst, src, dst_end); } inline void _memcpy_down_16(uint8_t* dst, const uint8_t* src, uint8_t* dst_end) { if(((uintptr_t)dst_end - (uintptr_t)dst) >= 16) { _memcpy_4x4(dst, src); dst += 16; src += 16; } _memcpy_down_8(dst, src, dst_end); } inline void _memcpy_down_32(uint8_t* dst, const uint8_t* src, uint8_t* dst_end) { if(((uintptr_t)dst_end - (uintptr_t)dst) >= 32) { _memcpy_4x8(dst, src); dst += 32; src += 32; } _memcpy_down_16(dst, src, dst_end); } inline void _memcpy_down_64(uint8_t* dst, const uint8_t* src, uint8_t* dst_end) { if(((uintptr_t)dst_end - (uintptr_t)dst) >= 64) { _memcpy_4x16(dst, src); dst += 64; src += 64; } _memcpy_down_32(dst, src, dst_end); } inline void _memcpy_down_128(uint8_t* dst, const uint8_t* src, uint8_t* dst_end) { if(((uintptr_t)dst_end - (uintptr_t)dst) >= 128) { _memcpy_4x32(dst, src); dst += 128; src += 128; } _memcpy_down_64(dst, src, dst_end); } inline void _memcpy_1(uint8_t* dst, const uint8_t* src, uint8_t* dst_end) { for(; dst <= (dst_end - 1); dst += 1, src += 1) _memcpy_1x1(dst, src); } inline void _memcpy_2(uint8_t* dst, const uint8_t* src, uint8_t* dst_end) { for(; dst <= (dst_end - 2); dst += 2, src += 2) _memcpy_2x1(dst, src); _memcpy_down_1(dst, src, dst_end); } inline void _memcpy_4(uint8_t* dst, const uint8_t* src, uint8_t* dst_end) { for(; dst <= (dst_end - 4); dst += 4, src += 4) _memcpy_4x1(dst, src); _memcpy_down_2(dst, src, dst_end); } inline void _memcpy_8(uint8_t* dst, const uint8_t* src, uint8_t* dst_end) { for(; dst <= (dst_end - 8); dst += 8, src += 8) _memcpy_4x2(dst, src); _memcpy_down_4(dst, src, dst_end); } inline void _memcpy_16(uint8_t* dst, const uint8_t* src, uint8_t* dst_end) { for(; dst <= (dst_end - 16); dst += 16, src += 16) _memcpy_4x4(dst, src); _memcpy_down_8(dst, src, dst_end); } inline void _memcpy_32(uint8_t* dst, const uint8_t* src, uint8_t* dst_end) { for(; dst <= (dst_end - 32); dst += 32, src += 32) _memcpy_4x8(dst, src); _memcpy_down_16(dst, src, dst_end); } inline void _memcpy_64(uint8_t* dst, const uint8_t* src, uint8_t* dst_end) { for(; dst <= (dst_end - 64); dst += 64, src += 64) _memcpy_4x16(dst, src); _memcpy_down_32(dst, src, dst_end); } inline void _memcpy_128(uint8_t* dst, const uint8_t* src, uint8_t* dst_end) { for(; dst <= (dst_end - 128); dst += 128, src += 128) _memcpy_4x32(dst, src); _memcpy_down_64(dst, src, dst_end); } void* _memcpy_fast(void* dst, const void* src, uintptr_t size) { if((dst == NULL) || (src =https://www.it610.com/article/= NULL)) return NULL; uint8_t*_dst = (uint8_t*)dst; uint8_t*_dst_end = &_dst[size]; const uint8_t* _src = (const uint8_t*)src; if((((uintptr_t)_dst & 1) != 0) && (((uintptr_t)_dst & 1) == ((uintptr_t)_src & 1)) && ((_dst + 1) <= _dst_end)) { _memcpy_1x1(_dst, _src); _dst += 1; _src += 1; } if(((uintptr_t)_dst & 1) == 0) { if((((uintptr_t)_dst & 3) != 0) && (((uintptr_t)_dst & 3) == ((uintptr_t)_src & 3)) && ((_dst + 2) <= _dst_end)) { _memcpy_2x1(_dst, _src); _dst += 2; _src += 2; } if(((uintptr_t)_dst & 3) == 0) { if((((uintptr_t)_dst & 7) != 0) && (((uintptr_t)_dst & 7) == ((uintptr_t)_src & 7)) && ((_dst + 4) <= _dst_end)) { _memcpy_4x1(_dst, _src); _dst += 4; _src += 4; } if(((uintptr_t)_dst & 7) == 0) { _memcpy_128(_dst, _src, _dst_end); } else _memcpy_4(_dst, _src, _dst_end); } else _memcpy_2(_dst, _src, _dst_end); } else _memcpy_1(_dst, _src, _dst_end); return dst; }












转载于:https://www.cnblogs.com/shapherd/archive/2010/05/19/1739628.html

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