通过指针进行迭代和通过索引进行迭代是否产生相同代码

编译器对通过指针进行迭代和通过索引进行迭代是否产生相同代码？问题来自于《The C++ Program Language》第五章习题8。

 

测试代码如下：（采用vc.net向导自动生成控制台应用程序）

#include "stdafx.h"

void fi (char v[]) // line 5

{

    for (int i = 0; v[i] != 0; ++i) {

        v[i] += 1;

    }

}

 

void fp (char v[]) // line 12

{

    for (char *p=v; *p != 0; ++p) {

         *pa += 1;

    }

}

 

int _tmain(int argc, _TCHAR* argv[]) // line 19

{

    char
a[8] = {'a', 'b', 'c', 'd', 'e', 'f', 'g' , 'h'};

    fp (a);

    fi (a);

    return 0;

}

 

在vc++7.net上使用参数FA(仅列出程序集)，产生的汇编代码如下（未优化）：

 

; Listing generated by Microsoft (R) Optimizing Compiler Version 13.10.3077  

       TITLE       ./5_8.cpp       .386Pinclude listing.inc... ... ... ...INCLUDELIB LIBCDINCLUDELIB OLDNAMES 

PUBLIC       ?fi@@YAXQAD@Z                              ; fi，函数fi的代码开始EXTRN       __RTC_InitBase:NEAREXTRN       __RTC_Shutdown:NEAR;      COMDAT rtc$IMZ; File d:/vsproject/5_8/5_8.cpprtc$IMZ       SEGMENT__RTC_InitBase.rtc$IMZ DD FLAT:__RTC_InitBasertc$IMZ  ENDS;      COMDAT rtc$TMZrtc$TMZ       SEGMENT__RTC_Shutdown.rtc$TMZ DD FLAT:__RTC_Shutdown; Function compile flags: /Odt /RTCsu /ZIrtc$TMZ ENDS;       COMDAT ?fi@@YAXQAD@Z_TEXT       SEGMENT_i$10435 = -8                                    ; size = 4_v$ = 8                                             ; size = 4?fi@@YAXQAD@Z PROC NEAR                                 ; fi, COMDAT; Line 6       push       ebp       mov ebp, esp       sub  esp, 204                       ; 000000ccH       push       ebx       push       esi       push       edi       lea   edi, DWORD PTR [ebp-204]       mov ecx, 51                                ; 00000033H       mov eax, -858993460                         ; ccccccccH       rep stosd; Line 7  // ********************************       mov       DWORD PTR _i$10435[ebp], 0       jmp       SHORT $L10436$L10437:       mov eax, DWORD PTR _i$10435[ebp]       add  eax, 1       mov       DWORD PTR _i$10435[ebp], eax$L10436:       mov eax, DWORD PTR _v$[ebp]       add  eax, DWORD PTR _i$10435[ebp]       movsx       ecx, BYTE PTR [eax]       test  ecx, ecx       je       SHORT $L10434; Line 8       mov eax, DWORD PTR _v$[ebp]       add  eax, DWORD PTR _i$10435[ebp]       movsx       ecx, BYTE PTR [eax]       add  ecx, 1       mov edx, DWORD PTR _v$[ebp]       add  edx, DWORD PTR _i$10435[ebp]       mov BYTE PTR [edx], cl; Line 9       jmp       SHORT $L10437$L10434:; Line 10       pop  edi       pop  esi       pop  ebx       mov esp, ebp       pop  ebp       ret   0?fi@@YAXQAD@Z ENDP                                  ; fi_TEXT   ENDS 

PUBLIC       ?fp@@YAXQAD@Z                                  ; fp，函数fp的代码开始; Function compile flags: /Odt /RTCsu /ZI;       COMDAT ?fp@@YAXQAD@Z_TEXT       SEGMENT_p$10442 = -8                                          ; size = 4_v$ = 8                                             ; size = 4?fp@@YAXQAD@Z PROC NEAR                                 ; fp, COMDAT; Line 13       push       ebp       m
fc31
ov ebp, esp       sub  esp, 204                       ; 000000ccH       push       ebx       push       esi       push       edi       lea   edi, DWORD PTR [ebp-204]       mov ecx, 51                                ; 00000033H       mov eax, -858993460                         ; ccccccccH       rep stosd; Line 14       // ******************************       mov eax, DWORD PTR _v$[ebp]       mov       DWORD PTR _p$10442[ebp], eax       jmp       SHORT $L10443$L10444:       mov eax, DWORD PTR _p$10442[ebp]       add  eax, 1       mov       DWORD PTR _p$10442[ebp], eax$L10443:       mov eax, DWORD PTR _p$10442[ebp]       movsx       ecx, BYTE PTR [eax]       test  ecx, ecx       je       SHORT $L10441; Line 15       mov eax, DWORD PTR _p$10442[ebp]       movsx       ecx, BYTE PTR [eax]       add  ecx, 1       mov edx, DWORD PTR _p$10442[ebp]       mov BYTE PTR [edx], cl; Line 16       jmp       SHORT $L10444$L10441:; Line 17       pop  edi       pop  esi       pop  ebx       mov esp, ebp       pop  ebp       ret   0?fp@@YAXQAD@Z ENDP                               ; fp_TEXT   ENDS 

PUBLIC  _mainEXTRN       @_RTC_CheckStackVars@8:NEAREXTRN       __RTC_CheckEsp:NEAR; Function compile flags: /Odt /RTCsu /ZI;      COMDAT _main... ... ... ...END注意上面Line8和Line15前后的汇编语句，就是一模一样的。也就是说，在没有任何优化的情况下，通过指针进行迭代和通过索引进行迭代将产生相同代码。

如果进行全面的优化，结果会怎么样呢？对同一个文件，在vc7.net采用RELEASE,打开全局优化，产生的汇编代码如下。可以看到，两个函数的代码仍然是相同的。

; Listing generated by Microsoft (R) Optimizing Compiler Version 13.10.3077  

       TITLE       ./5_8.cpp       .386Pinclude listing.inc... ... ... ...INCLUDELIB LIBCINCLUDELIB OLDNAMES 

PUBLIC       ?fi@@YAXQAD@Z                              ; fi，函数fi的代码开始; Function compile flags: /Ogty;       COMDAT ?fi@@YAXQAD@Z_TEXT       SEGMENT_v$ = 8                                             ; size = 4?fi@@YAXQAD@Z PROC NEAR                                 ; fi, COMDAT; File d:/vsproject/5_8/5_8.cpp; Line 7       mov eax, DWORD PTR _v$[esp-4]       cmp BYTE PTR [eax], 0       je       SHORT $L9624       npad       7$L9622:; Line 8       inc   BYTE PTR [eax]       mov cl, BYTE PTR [eax+1]       inc   eax       test  cl, cl       jne       SHORT $L9622$L9624:; Line 10       ret   0?fi@@YAXQAD@Z ENDP                                  ; fi_TEXT   ENDS 

PUBLIC       ?fp@@YAXQAD@Z                                  ; fp，函数fp的代码开始; Function compile flags: /Ogty;       COMDAT ?fp@@YAXQAD@Z_TEXT       SEGMENT_v$ = 8                                             ; size = 4?fp@@YAXQAD@Z PROC NEAR                                 ; fp, COMDAT; Line 14       mov eax, DWORD PTR _v$[esp-4]       cmp BYTE PTR [eax], 0       je       SHORT $L9631       npad       7$L9629:; Line 15       inc   BYTE PTR [eax]       mov cl, BYTE PTR [eax+1]       inc   eax       test  cl, cl       jne       SHORT $L9629$L9631:; Line 17       ret   0?fp@@YAXQAD@Z ENDP                               ; fp_TEXT   ENDS 

PUBLIC  _mainEXTRN       ___security_cookie:DWORDEXTRN       @__security_check_cookie@4:NEAR; Function compile flags: /Ogty;      COMDAT _main... ... ... ...END