Which loop has better performance? Increment or decrement? [duplicate]

Answer 1

The compiler should optimize both code to the same assembly, so it doesn't make a difference. Both take the same time.

A more valid discussion would be whether

  for(int i=0;i<10;++i)  //preincrement
  {
  }

would be faster than

  for(int i=0;i<10;i++)  //postincrement
  {
  }

Because, theoretically, post-increment does an extra operation (returns a reference to the old value). However, even this should be optimized to the same assembly.

Without optimizations, the code would look like this:

   for ( int i = 0; i < 10 ; i++ )
0041165E  mov         dword ptr [i],0 
00411665  jmp         wmain+30h (411670h) 
00411667  mov         eax,dword ptr [i] 
0041166A  add         eax,1 
0041166D  mov         dword ptr [i],eax 
00411670  cmp         dword ptr [i],0Ah 
00411674  jge         wmain+68h (4116A8h) 

   for ( int i = 0; i < 10 ; ++i )
004116A8  mov         dword ptr [i],0 
004116AF  jmp         wmain+7Ah (4116BAh) 
004116B1  mov         eax,dword ptr [i] 
004116B4  add         eax,1 
004116B7  mov         dword ptr [i],eax 
004116BA  cmp         dword ptr [i],0Ah 
004116BE  jge         wmain+0B2h (4116F2h) 

   for ( int i = 9; i >= 0 ; i-- )
004116F2  mov         dword ptr [i],9 
004116F9  jmp         wmain+0C4h (411704h) 
004116FB  mov         eax,dword ptr [i] 
004116FE  sub         eax,1 
00411701  mov         dword ptr [i],eax 
00411704  cmp         dword ptr [i],0 
00411708  jl          wmain+0FCh (41173Ch) 

so even in this case, the speed is the same.