用宏也不见得效率高——max宏使用方法说明

很多人认为使用宏，在预处理时候就完成了宏替换，消除了函数调用的代价，因此效率很高。其实也不见得。比如max宏一般是这样定义的：

#define max(a,b) ((a)>(b)?(a):(b))

使用它来比较两个看不出什么问题，我们不妨用它来比较一下a, b, c, d的大小：
max(a, max(b, max(c, d)))
宏替换后的结果是这样的：
((a)>(((b)>(((c)>(d)?(c):(d)))?(b):(((c)>(d)?(c):(d)))))?(a):(((b)>(((c)>(d)?(c):(d)))?(b):(((c)>(d)?(c):(d))))))
够晕够复杂吧？

如果这样用试试看：
max(max(a, b), max(c, d))
同样还是使用三次max宏，但是效果就大大不同了：
((((a)>(b)?(a):(b)))>(((c)>(d)?(c):(d)))?(((a)>(b)?(a):(b))):(((c)>(d)?(c):(d))))

看样子是短了点了，可是也不见得效率高！其实还真不如这样搞：

max_num = a;
if (max_num<b) max_num = b;
if (max_num<c) max_num = c;
if (max_num<d) max_num = d;

这样做又清爽又直观难道不好么？所以，兵器有多种，如何发挥作用还在于用的人。:p

该方法在Andrew Koenig的《C陷阱与缺陷》中有提到。

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这么明显的差异看不到？单单是代码中少做了2次比较就已经差的很多了。

13:       max_num =  max(a, max(b, max(c, d)));
0040104A   mov         eax,dword ptr [ebp-0Ch]
0040104D   cmp         eax,dword ptr [ebp-10h]
00401050   jle         main+4Ah (0040105a)
00401052   mov         ecx,dword ptr [ebp-0Ch]
00401055   mov         dword ptr [ebp-18h],ecx
00401058   jmp         main+50h (00401060)
0040105A   mov         edx,dword ptr [ebp-10h]
0040105D   mov         dword ptr [ebp-18h],edx
00401060   mov         eax,dword ptr [ebp-8]
00401063   cmp         eax,dword ptr [ebp-18h]
00401066   jle         main+60h (00401070)
00401068   mov         ecx,dword ptr [ebp-8]
0040106B   mov         dword ptr [ebp-1Ch],ecx
0040106E   jmp         main+7Ch (0040108c)
00401070   mov         edx,dword ptr [ebp-0Ch]
00401073   cmp         edx,dword ptr [ebp-10h]
00401076   jle         main+70h (00401080)
00401078   mov         eax,dword ptr [ebp-0Ch]
0040107B   mov         dword ptr [ebp-20h],eax
0040107E   jmp         main+76h (00401086)
00401080   mov         ecx,dword ptr [ebp-10h]
00401083   mov         dword ptr [ebp-20h],ecx
00401086   mov         edx,dword ptr [ebp-20h]
00401089   mov         dword ptr [ebp-1Ch],edx
0040108C   mov         eax,dword ptr [ebp-4]
0040108F   cmp         eax,dword ptr [ebp-1Ch]
00401092   jle         main+8Ch (0040109c)
00401094   mov         ecx,dword ptr [ebp-4]
00401097   mov         dword ptr [ebp-24h],ecx
0040109A   jmp         main+0D4h (004010e4)
0040109C   mov         edx,dword ptr [ebp-0Ch]
0040109F   cmp         edx,dword ptr [ebp-10h]
004010A2   jle         main+9Ch (004010ac)
004010A4   mov         eax,dword ptr [ebp-0Ch]
004010A7   mov         dword ptr [ebp-28h],eax
004010AA   jmp         main+0A2h (004010b2)
004010AC   mov         ecx,dword ptr [ebp-10h]
004010AF   mov         dword ptr [ebp-28h],ecx
004010B2   mov         edx,dword ptr [ebp-8]
004010B5   cmp         edx,dword ptr [ebp-28h]
004010B8   jle         main+0B2h (004010c2)
004010BA   mov         eax,dword ptr [ebp-8]
004010BD   mov         dword ptr [ebp-2Ch],eax
004010C0   jmp         main+0CEh (004010de)
004010C2   mov         ecx,dword ptr [ebp-0Ch]
004010C5   cmp         ecx,dword ptr [ebp-10h]
004010C8   jle         main+0C2h (004010d2)
004010CA   mov         edx,dword ptr [ebp-0Ch]
004010CD   mov         dword ptr [ebp-30h],edx
004010D0   jmp         main+0C8h (004010d8)
004010D2   mov         eax,dword ptr [ebp-10h]
004010D5   mov         dword ptr [ebp-30h],eax
004010D8   mov         ecx,dword ptr [ebp-30h]
004010DB   mov         dword ptr [ebp-2Ch],ecx
004010DE   mov         edx,dword ptr [ebp-2Ch]
004010E1   mov         dword ptr [ebp-24h],edx
004010E4   mov         eax,dword ptr [ebp-24h]
004010E7   mov         dword ptr [ebp-14h],eax
14:       max_num =  max(max(a, b), max(c, d));
004010EA   mov         ecx,dword ptr [ebp-4]
004010ED   cmp         ecx,dword ptr [ebp-8]
004010F0   jle         main+0EAh (004010fa)
004010F2   mov         edx,dword ptr [ebp-4]
004010F5   mov         dword ptr [ebp-34h],edx
004010F8   jmp         main+0F0h (00401100)
004010FA   mov         eax,dword ptr [ebp-8]
004010FD   mov         dword ptr [ebp-34h],eax
00401100   mov         ecx,dword ptr [ebp-0Ch]
00401103   cmp         ecx,dword ptr [ebp-10h]
00401106   jle         main+100h (00401110)
00401108   mov         edx,dword ptr [ebp-0Ch]
0040110B   mov         dword ptr [ebp-38h],edx
0040110E   jmp         main+106h (00401116)
00401110   mov         eax,dword ptr [ebp-10h]
00401113   mov         dword ptr [ebp-38h],eax
00401116   mov         ecx,dword ptr [ebp-34h]
00401119   cmp         ecx,dword ptr [ebp-38h]
0040111C   jle         main+12Ch (0040113c)
0040111E   mov         edx,dword ptr [ebp-4]
00401121   cmp         edx,dword ptr [ebp-8]
00401124   jle         main+11Eh (0040112e)
00401126   mov         eax,dword ptr [ebp-4]
00401129   mov         dword ptr [ebp-3Ch],eax
0040112C   jmp         main+124h (00401134)
0040112E   mov         ecx,dword ptr [ebp-8]
00401131   mov         dword ptr [ebp-3Ch],ecx
00401134   mov         edx,dword ptr [ebp-3Ch]
00401137   mov         dword ptr [ebp-40h],edx
0040113A   jmp         main+148h (00401158)
0040113C   mov         eax,dword ptr [ebp-0Ch]
0040113F   cmp         eax,dword ptr [ebp-10h]
00401142   jle         main+13Ch (0040114c)
00401144   mov         ecx,dword ptr [ebp-0Ch]
00401147   mov         dword ptr [ebp-44h],ecx
0040114A   jmp         main+142h (00401152)
0040114C   mov         edx,dword ptr [ebp-10h]
0040114F   mov         dword ptr [ebp-44h],edx
00401152   mov         eax,dword ptr [ebp-44h]
00401155   mov         dword ptr [ebp-40h],eax
00401158   mov         ecx,dword ptr [ebp-40h]
0040115B   mov         dword ptr [ebp-14h],ecx

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