debug版本:
do
{
sum++;
01321471 mov eax,dword ptr [sum]
01321474 add eax,1
01321477 mov dword ptr [sum],eax
} while (sum < a);
0132147A mov eax,dword ptr [sum]
0132147D cmp eax,dword ptr [a]
01321480 jl main+51h (01321471h)
do_begin
执行循环体
;影响标记位的指令
jxx do_begin
while (sum < a)
00BF1471 mov eax,dword ptr [sum]
00BF1474 cmp eax,dword ptr [a]
00BF1477 jge main+64h (0BF1484h)
{
sum++;
00BF1479 mov eax,dword ptr [sum]
00BF147C add eax,1
00BF147F mov dword ptr [sum],eax
}
00BF1482 jmp main+51h (0BF1471h)
while_begin
;jxx 影响标志位的指令
执行循环体
jmp while_begin
while_end
for (int sum = 0; sum < a; sum++)
//计数器初始化
0039146A mov dword ptr [ebp-18h],0
//步长
00391471 jmp main+5Ch (039147Ch)
00391473 mov eax,dword ptr [ebp-18h]
00391476 add eax,1
00391479 mov dword ptr [ebp-18h],eax
//判断执行条件
0039147C mov eax,dword ptr [ebp-18h]
0039147F cmp eax,dword ptr [a]
00391482 jge main+7Dh (039149Dh)
//执行循环体
{
printf("hehhe");
00391484 mov esi,esp
00391486 push 39585Ch
0039148B call dword ptr ds:[399114h]
00391491 add esp,4
00391494 cmp esi,esp
00391496 call __RTC_CheckEsp (0391140h)
}
0039149B jmp main+53h (0391473h)
从中可以看出debug版本do...while的效率要比while的效率高,而while的效率要比for循环高
编译器在进行优化时,是将while循环和for循环转化为do...while循环
while循环的优化
if(XXX)
{
do
{
do something
}while(XXX)
}
for循环的优化
for(int i = 0; XXX; ++I)
{
do something
}
int i = 0;
while(XXX)
{
do something
++i
}
int i = 0
if(XXX)
{
do
{
do something
++i
}while(XXX);
}