VC++中结构体赋值和memcpy的比较
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// Test1 typedef struct { int nValue; BYTE nValue2[4]; }ST_TEST;
int main() { ST_TEST sTest1 = {0}; ST_TEST sTest2 = {0}; sTest2 = sTest1; memcpy(&sTest2, &sTest1, sizeof(sTest1)); return 0; }
// Disassembly typedef struct { int nValue; BYTE nValue2[4]; }ST_TEST;
int main() { 004196B0 push ebp 004196B1 mov ebp,esp 004196B3 sub esp,0E4h 004196B9 push ebx 004196BA push esi 004196BB push edi 004196BC lea edi,[ebp-0E4h] 004196C2 mov ecx,39h 004196C7 mov eax,0CCCCCCCCh 004196CC rep stos dword ptr es:[edi] 004196CE mov eax,dword ptr [___security_cookie (428080h)] 004196D3 xor eax,ebp 004196D5 mov dword ptr [ebp-4],eax ST_TEST sTest1 = {0}; 004196D8 mov dword ptr [ebp-10h],0 004196DF xor eax,eax 004196E1 mov dword ptr [ebp-0Ch],eax ST_TEST sTest2 = {0}; 004196E4 mov dword ptr [ebp-20h],0 004196EB xor eax,eax 004196ED mov dword ptr [ebp-1Ch],eax sTest2 = sTest1; 004196F0 mov eax,dword ptr [ebp-10h] 004196F3 mov dword ptr [ebp-20h],eax 004196F6 mov ecx,dword ptr [ebp-0Ch] 004196F9 mov dword ptr [ebp-1Ch],ecx memcpy(&sTest2, &sTest1, sizeof(sTest1)); 004196FC push 8 004196FE lea eax,[ebp-10h] 00419701 push eax 00419702 lea ecx,[ebp-20h] 00419705 push ecx 00419706 call @ILT+980(_memcpy) (4113D9h) 0041970B add esp,0Ch
return 0; 0041970E xor eax,eax }
// Test2 typedef struct { int nValue; BYTE bValue[4092]; }ST_TEST;
int main() { ST_TEST sTest1 = {0}; ST_TEST sTest2 = {0}; sTest2 = sTest1; memcpy(&sTest2, &sTest1, sizeof(sTest1));
return 0; }
// Disassembly typedef struct { int nValue; BYTE bValue[4092]; }ST_TEST;
int main() { 004196B0 push ebp 004196B1 mov ebp,esp 004196B3 mov eax,20D4h 004196B8 call @ILT+415(__chkstk) (4111A4h) 004196BD push ebx 004196BE push esi 004196BF push edi 004196C0 lea edi,[ebp-20D4h] 004196C6 mov ecx,835h 004196CB mov eax,0CCCCCCCCh 004196D0 rep stos dword ptr es:[edi] 004196D2 mov eax,dword ptr [___security_cookie (428080h)] 004196D7 xor eax,ebp 004196D9 mov dword ptr [ebp-4],eax ST_TEST sTest1 = {0}; 004196DC mov dword ptr [ebp-1008h],0 004196E6 push 0FFCh 004196EB push 0 004196ED lea eax,[ebp-1004h] 004196F3 push eax 004196F4 call @ILT+510(_memset) (411203h) 004196F9 add esp,0Ch ST_TEST sTest2 = {0}; 004196FC mov dword ptr [ebp-2010h],0 00419706 push 0FFCh 0041970B push 0 0041970D lea eax,[ebp-200Ch] 00419713 push eax 00419714 call @ILT+510(_memset) (411203h) 00419719 add esp,0Ch sTest2 = sTest1; 0041971C push 1000h 00419721 lea eax,[ebp-1008h] 00419727 push eax 00419728 lea ecx,[ebp-2010h] 0041972E push ecx 0041972F call @ILT+980(_memcpy) (4113D9h) 00419734 add esp,0Ch memcpy(&sTest2, &sTest1, sizeof(sTest1)); 00419737 push 1000h 0041973C lea eax,[ebp-1008h] 00419742 push eax 00419743 lea ecx,[ebp-2010h] 00419749 push ecx 0041974A call @ILT+980(_memcpy) (4113D9h) 0041974F add esp,0Ch
return 0; 00419752 xor eax,eax }
通过上面的的两份测试代码,我们可以看出,结构的直接赋值(默认结构体赋值函数)与memcpy之间的异同. 例1中,结构体小于4096时,赋值是通过结构体中每个变量的赋值来完成的. 例2中,结构体大于等于4096时,结构体赋值与memcpy的效果是一样的.因为我们通过代码的反汇编,可以看出编译器已经将代码优化成一样的汇编代码了. 所以,理论上来说,针对结构体调用memcpy的效果与直接赋值基本上是一样的. 而且按照汇编的结果,直接赋值的汇编代码少于memcpy的汇编代码,所以更推荐使用结构体的直接赋值(注意,这里的结构体是指没有重载赋值函数的) |