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发表于 2009-3-5 01:07:46
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再更正一点,宏是可以以分号结束的,要看你用在什么地方:
#define NOOP() asm("nop");asm("nop");
#define MACRO1 a=0;
?:这个操作符存在C语言中的原因是它使得编译器能产生比if-then-else更优化的代码,这一说法似乎没有根据,它的存在应该是为了方便。下面是一个例证:
#include<stdio.h>
#include<windows.h>
//#define P (60 * 60 * 24 * 365)l
#define P (60 * 60 * 24 * 365L)
void main()
{
short int i;
int a, b, c, d;
i = P;
a = scanf("%d", &a);
b = scanf("%d", &b);
printf("Hello World!\n");
printf("%d",i);
MessageBox(NULL, "12", "", MB_OK);
c = (a > b) ? a*a : b*b;
MessageBox(NULL, "34", "", MB_OK);
if(a & b)
d = a*a;
else
d = b*b;
printf("c=%d d=%d\n", c, d);
}
::00401000:: 83EC 08 SUB ESP,8 \:BYCALL CallBy:004011A7,
::00401003:: 56 PUSH ESI
::00401004:: 8D4424 08 LEA EAX,DWORD PTR [ESP+8]
::00401008:: 57 PUSH EDI
::00401009:: 50 PUSH EAX
::0040100A:: 68 54804000 PUSH 408054 \->: %d
::0040100F:: E8 CD000000 CALL 004010E1 \:JMPDOWN
::00401014:: 8D4C24 10 LEA ECX,DWORD PTR [ESP+10]
::00401018:: 894424 14 MOV DWORD PTR [ESP+14],EAX
::0040101C:: 51 PUSH ECX
::0040101D:: 68 54804000 PUSH 408054 \->: %d
::00401022:: E8 BA000000 CALL 004010E1 \:JMPDOWN
::00401027:: 68 44804000 PUSH 408044 \->: Hello World!\x0A
::0040102C:: 894424 1C MOV DWORD PTR [ESP+1C],EAX
::00401030:: E8 7B000000 CALL 004010B0 \:JMPDOWN
::00401035:: 68 80330000 PUSH 3380
::0040103A:: 68 54804000 PUSH 408054 \->: %d
::0040103F:: E8 6C000000 CALL 004010B0 \:JMPDOWN
::00401044:: 8B3D B4704000 MOV EDI,DWORD PTR [4070B4]
::0040104A:: 83C4 1C ADD ESP,1C
::0040104D:: 6A 00 PUSH 0
::0040104F:: 68 08A94000 PUSH 40A908
::00401054:: 68 40804000 PUSH 408040 \->: 12
::00401059:: 6A 00 PUSH 0
::0040105B:: FFD7 CALL NEAR EDI
::0040105D:: 8B4424 0C MOV EAX,DWORD PTR [ESP+C]
::00401061:: 8B4C24 08 MOV ECX,DWORD PTR [ESP+8]
::00401065:: 3BC1 CMP EAX,ECX
::00401067:: 7E 07 JLE SHORT 00401070 \:JMPDOWN
::00401069:: 8BF0 MOV ESI,EAX
::0040106B:: 0FAFF0 IMUL ESI,EAX
::0040106E:: EB 05 JMP SHORT 00401075 \:JMPDOWN
::00401070:: 8BF1 MOV ESI,ECX \:BYJMP JmpBy:00401067,
::00401072:: 0FAFF1 IMUL ESI,ECX
::00401075:: 6A 00 PUSH 0 \:BYJMP JmpBy:0040106E,
::00401077:: 68 08A94000 PUSH 40A908
::0040107C:: 68 3C804000 PUSH 40803C \->: 34
::00401081:: 6A 00 PUSH 0
::00401083:: FFD7 CALL NEAR EDI
::00401085:: 8B4424 08 MOV EAX,DWORD PTR [ESP+8]
::00401089:: 8B4C24 0C MOV ECX,DWORD PTR [ESP+C]
::0040108D:: 85C1 TEST ECX,EAX
::0040108F:: 74 07 JE SHORT 00401098 \:JMPDOWN
::00401091:: 8BC1 MOV EAX,ECX
::00401093:: 0FAFC1 IMUL EAX,ECX
::00401096:: EB 03 JMP SHORT 0040109B \:JMPDOWN
::00401098:: 0FAFC0 IMUL EAX,EAX \:BYJMP JmpBy:0040108F,
::0040109B:: 50 PUSH EAX \:BYJMP JmpBy:00401096,
::0040109C:: 56 PUSH ESI
::0040109D:: 68 30804000 PUSH 408030 \->: c=%d d=%d\x0A
::004010A2:: E8 09000000 CALL 004010B0 \:JMPDOWN
::004010A7:: 83C4 0C ADD ESP,C
::004010AA:: 5F POP EDI
::004010AB:: 5E POP ESI
::004010AC:: 83C4 08 ADD ESP,8
::004010AF:: C3 RETN
从反汇编代码来看?:和if else 是一样的(::0040105D:: ~ ::00401072::是c = (a > b) ? a*a : b*b; ;::00401085:: ~ ::00401098::是 if(a & b) d = a*a; else d = b*b;)。 |
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