之前照着天书夜读,用Delphi来弄了下循环体,现在就来弄一下条件判断吧. 首先肯定是我们经常看见的IF语句咯.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 Var I: Integer; Begin I:= 99 ; If (I> 0 )And (I< 0 ) Then Writeln('I>0' ) Else If (I> 10 )And (I< 100 ) Then Writeln('I>10 and I<100' ) Else Writeln('I>100' ); End .
反汇编出来会是什么样子的呢?
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 Project5.dpr.12: I:= 99; 004AC44C B863000000 mov eax,$00000063 Project5.dpr.13: If (I> 0)And (I< 0) Then 004AC451 85C0 test eax,eax 004AC453 7E1F jle $004ac474 004AC455 85C0 test eax,eax 004AC457 7D1B jnl $004ac474 Project5.dpr.14: Writeln('I>0') 004AC459 A1DC314B00 mov eax,[$004b31dc] 004AC45E BAC4C44A00 mov edx,$004ac4c4 004AC463 E874BAF5FF call @Write0UString 004AC468 E8BF89F5FF call @WriteLn 004AC46D E83E7FF5FF call @_IOTest 004AC472 EB3E jmp $004ac4b2 Project5.dpr.16: If (I> 10)And (I< 100) Then 004AC474 83F80A cmp eax,$0a 004AC477 7E20 jle $004ac499 004AC479 83F864 cmp eax,$64 004AC47C 7D1B jnl $004ac499 Project5.dpr.17: Writeln('I>10 and I<100') 004AC47E A1DC314B00 mov eax,[$004b31dc] 004AC483 BAD8C44A00 mov edx,$004ac4d8 004AC488 E84FBAF5FF call @Write0UString 004AC48D E89A89F5FF call @WriteLn 004AC492 E8197FF5FF call @_IOTest 004AC497 EB19 jmp $004ac4b2 Project5.dpr.19: Writeln('I>100'); 004AC499 A1DC314B00 mov eax,[$004b31dc] 004AC49E BA04C54A00 mov edx,$004ac504 004AC4A3 E834BAF5FF call @Write0UString 004AC4A8 E87F89F5FF call @WriteLn 004AC4AD E8FE7EF5FF call @_IOTest
我们这里就容易得出来Delphi里面IF语句的结构了. 判断1 不满足就跳————————-| 满足条件代码 | [无条件Jmp到所有判断外] | [判断2] <——| ……………………………………….. 就是这样的 在IF的反汇编上面 和 书中的VC反汇编出来的代码差不多. 接下来就是看switch语句(Delphi里面的Case):
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 Var I: Integer; Begin I:= 9 ; Case I Of 0 : Writeln('0' ); 1 : Writeln('1' ); 2 : Writeln('2' ); 3 : Writeln('3' ); 4 : Writeln('4' ); 5 : Writeln('5' ); 6 : Writeln('6' ); 7 : Writeln('7' ); Else Writeln('0' ); End ; End .
反汇编一下:
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 Project5.dpr.12: I:= 9; 004AC44C B809000000 mov eax,$00000009 Project5.dpr.13: Case I Of 004AC451 83F807 cmp eax,$07 004AC454 0F8703010000 jnbe $004ac55d 004AC45A FF248561C44A00 jmp dword ptr [eax*4+$4ac461] 004AC461 81C44A009EC4 add esp,$c49e004a 004AC467 4A dec edx 004AC468 00BBC44A00D8 add [ebx-$27ffb53c],bh 004AC46E C44A00 les ecx,[edx+$00] 004AC471 F5 cmc 004AC472 C44A00 les ecx,[edx+$00] 004AC475 0FC54A0029 pextrw ecx,qword ptr [edx+$00],$29 004AC47A C54A00 lds ecx,[edx+$00] 004AC47D 43 inc ebx 004AC47E C54A00 lds ecx,[edx+$00] Project5.dpr.15: Writeln('0'); 004AC481 A1DC314B00 mov eax,[$004b31dc] 004AC486 66BA3000 mov dx,$0030 004AC48A E8DDADF5FF call @Write0WChar 004AC48F E89889F5FF call @WriteLn 004AC494 E8177FF5FF call @_IOTest 004AC499 E9D7000000 jmp $004ac575 Project5.dpr.17: Writeln('1'); 004AC49E A1DC314B00 mov eax,[$004b31dc] 004AC4A3 66BA3100 mov dx,$0031 004AC4A7 E8C0ADF5FF call @Write0WChar 004AC4AC E87B89F5FF call @WriteLn 004AC4B1 E8FA7EF5FF call @_IOTest 004AC4B6 E9BA000000 jmp $004ac575 Project5.dpr.19: Writeln('2'); 004AC4BB A1DC314B00 mov eax,[$004b31dc] 004AC4C0 66BA3200 mov dx,$0032 004AC4C4 E8A3ADF5FF call @Write0WChar 004AC4C9 E85E89F5FF call @WriteLn 004AC4CE E8DD7EF5FF call @_IOTest 004AC4D3 E99D000000 jmp $004ac575 Project5.dpr.21: Writeln('3'); 004AC4D8 A1DC314B00 mov eax,[$004b31dc] 004AC4DD 66BA3300 mov dx,$0033 004AC4E1 E886ADF5FF call @Write0WChar 004AC4E6 E84189F5FF call @WriteLn 004AC4EB E8C07EF5FF call @_IOTest 004AC4F0 E980000000 jmp $004ac575 Project5.dpr.23: Writeln('4'); 004AC4F5 A1DC314B00 mov eax,[$004b31dc] 004AC4FA 66BA3400 mov dx,$0034 004AC4FE E869ADF5FF call @Write0WChar 004AC503 E82489F5FF call @WriteLn 004AC508 E8A37EF5FF call @_IOTest 004AC50D EB66 jmp $004ac575 Project5.dpr.25: Writeln('5'); 004AC50F A1DC314B00 mov eax,[$004b31dc] 004AC514 66BA3500 mov dx,$0035 004AC518 E84FADF5FF call @Write0WChar 004AC51D E80A89F5FF call @WriteLn 004AC522 E8897EF5FF call @_IOTest 004AC527 EB4C jmp $004ac575 Project5.dpr.27: Writeln('6'); 004AC529 A1DC314B00 mov eax,[$004b31dc] 004AC52E 66BA3600 mov dx,$0036 004AC532 E835ADF5FF call @Write0WChar 004AC537 E8F088F5FF call @WriteLn 004AC53C E86F7EF5FF call @_IOTest 004AC541 EB32 jmp $004ac575 Project5.dpr.29: Writeln('7'); 004AC543 A1DC314B00 mov eax,[$004b31dc] 004AC548 66BA3700 mov dx,$0037 004AC54C E81BADF5FF call @Write0WChar 004AC551 E8D688F5FF call @WriteLn 004AC556 E8557EF5FF call @_IOTest 004AC55B EB18 jmp $004ac575 Project5.dpr.31: Writeln('0'); 004AC55D A1DC314B00 mov eax,[$004b31dc] 004AC562 66BA3000 mov dx,$0030 004AC566 E801ADF5FF call @Write0WChar 004AC56B E8BC88F5FF call @WriteLn 004AC570 E83B7EF5FF call @_IOTest
在这里我们看到了一个Delphi的优化,它首先把I和7比,如果大于就直接跳到else语句那里. 如果小于等于,我们看到是无条件Jmp到一个I为等间距偏移的地方,也就是每个writeln语句是等大小的. 要是我们改成不是01234567呢?大家自己试一下,不是等距case代码还是没有变. 而VC里面会怎么样? cmp->je->cmp->je->cmp->je 就是这样不停的对比,如果相等就跳.这样符合我们思维一些. 转自https://www.cnblogs.com/huangjacky/archive/2010/02/04/1663847.html
For循环
1 2 3 4 5 6 7 8 9 10 11 12 13 14 Function TestFor ( a, b: Integer ) : Integer ;Var I: Integer ; Begin Result := a + b ; For I := 0 To 49 Do Result := Result + I ; End ;{$R *.dfm} Procedure TForm1 .btn1Click ( Sender: TObject ) ;Begin ShowMessage( IntToStr( TestFor( 1 , 2 ) ) ) ; End ;
反汇编代码:
1 2 3 4 5 6 7 8 9 10 11 12 13 Unit4.pas.28: Result := a + b ; 00523AE0 03D0 add edx,eax //执行a+b,但是我很奇怪为什么不用add eax,edx 这样最后就不用执行那句mov eax,edx了,节约一句 Unit4.pas.29: For I := 0 To 49 Do 00523AE2 33C0 xor eax,eax //清除eax,来充当I这个循环变量 Unit4.pas.30: Result := Result + I ; 00523AE4 03D0 add edx,eax //累计和,并把循环变量+1,这里就是循环体的执行代码 00523AE6 40 inc eax Unit4.pas.29: For I := 0 To 49 Do 00523AE7 83F832 cmp eax,$32 //边界比较,不等于就跳回去,继续执行循环体代码. 00523AEA 75F8 jnz $00523ae4 //看来还是再写个函数来看看 Unit4.pas.31: End ; 00523AEC 8BC2 mov eax,edx 00523AEE C3 ret
对比书中C的反汇编代码,Delphi反汇编出来的代码很精简,虽然声明了局部变量,但是函数直接用寄存器代替了.很好. 书中C循环体先直接用Jmp跳过去与临界变量比较,然后再来看是否执行循环体,而改变循环变量它用了add eax,1,然后再把eax传给栈上面的I. 后来我尝试改成0 to -1 Delphi编译器会识别出来,然后不会添加任何循环代码到函数中去的.
接下来是do循环,也就是Repeat
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 function TestRepeat (a,b:Integer) : Integer ;var I:Integer; begin Result:=a + b; I:=0 ; repeat Result:= Result + I; Inc(I); until I = 50 ; end ;反汇编代码: ``` asm Unit4.pas.30 : Result:=a + b; 00523 AE0 03 D0 add edx,eaxUnit4.pas.31 : I:=0 ; 00523 AE2 33 C0 xor eax,eaxUnit4.pas.33 : Result:= Result + I; 00523 AE4 03 D0 add edx,eaxUnit4.pas.34 : Inc(I); 00523 AE6 40 inc eaxUnit4.pas.35 : until I = 50 ; 00523 AE7 83 F832 cmp eax,$32 00523 AEA 75 F8 jnz $00523ae4 Unit4.pas.36 : end ; 00523 AEC 8 BC2 mov eax,edx00523 AEE C3 ret
我们发现反汇编代码和For是一样的. 这段代码就和VC反编译出来的差不多了,先执行循环体然后再比较边界. 好,看看while呢?
1 2 3 4 5 6 7 8 9 10 11 12 Function TestWhile (A, B: Integer) : Integer;Var I: Integer; Begin Result:= A+ B; I:= 0 ; While I< 50 Do Begin Result:= Result+ I; Inc(I); End ; End ;
反汇编代码:
1 2 3 4 5 6 7 8 9 10 11 12 13 14 Unit4.pas.38: Result:= A+ B; 00523AE0 03D0 add edx,eax Unit4.pas.39: I:= 0; 00523AE2 33C0 xor eax,eax Unit4.pas.42: Result:= Result+ I; 00523AE4 03D0 add edx,eax Unit4.pas.43: Inc(I); 00523AE6 40 inc eax Unit4.pas.40: While I< 50 Do 00523AE7 83F832 cmp eax,$32 00523AEA 7CF8 jl $00523ae4 Unit4.pas.45: End; 00523AEC 8BC2 mov eax,edx 00523AEE C3 ret
我晕,居然还是一样的.也就是Delphi里面3种循环执行效率是一样的吧. 而VC同样是先初始化循环变量,然后无条件跳转到边界检测,然后才执行循环体代码. 循环就这么多,以后该是条件判断