What are the differences between using array offsets vs pointer incrementation?

Question

Given 2 functions, which should be faster, if there is any difference at all? Assume that the input data is very large

void iterate1(const char* pIn, int Size)
{         


        

Answer 1

Several years ago I asked this exact question. Someone in an interview was failing a candidate for picking the array notation because it was supposedly obviously slower. At that point I compiled both versions and looked at the disassembly. There was one opcode extra in the array notation. This was with Visual C++ (.net?). Based on what I saw I concluded that there is no appreciable difference.

Doing this again, here is what I found:

    iterate1(arr, 400); // array notation
011C1027  mov         edi,dword ptr [__imp__printf (11C20A0h)] 
011C102D  add         esp,0Ch 
011C1030  xor         esi,esi 
011C1032  movsx       ecx,byte ptr [esp+esi+8] <-- Loop starts here
011C1037  push        ecx  
011C1038  push        offset string "%c" (11C20F4h) 
011C103D  call        edi  
011C103F  inc         esi  
011C1040  add         esp,8 
011C1043  cmp         esi,190h 
011C1049  jl          main+32h (11C1032h) 

    iterate2(arr, 400); // pointer offset notation
011C104B  lea         esi,[esp+8] 
011C104F  nop              
011C1050  movsx       edx,byte ptr [esi] <-- Loop starts here
011C1053  push        edx  
011C1054  push        offset string "%c" (11C20F4h) 
011C1059  call        edi  
011C105B  inc         esi  
011C105C  lea         eax,[esp+1A0h] 
011C1063  add         esp,8 
011C1066  cmp         esi,eax 
011C1068  jne         main+50h (11C1050h)