foreach is slower than for?

[Stanislav Blinov]

Tomek Sowiński wrote:
> Dnia 08-09-2010 o 23:12:55 Stanislav Blinov <stanislav.blinov at gmail.com> 
> napisał(a):
> 
>> I was wondering about it for some time:
>>
>> void foreach_loop(int arr[100])
>> {
>>      foreach(i,a; arr)
>>      {
>>          auto e = a;
>>      }
>> }
>>
>> void for_loop(int arr[100])
>> {
>>      for (int i = 0; i < 100; ++i)
>>      {
>>          auto e = arr[i];
>>      }
>> }
>>
>>
>> compiled with -O -release yields this:
>>
>> .text.void main.foreach_loop(int[100])  segment
>>          assume  CS:.text.void main.foreach_loop(int[100])
>> void main.foreach_loop(int[100]):
>>                  push    EBP
>>                  mov     EBP,ESP
>>                  sub     ESP,8
>>                  mov     EAX,064h
>>                  lea     ECX,8[EBP]
>>                  xor     EDX,EDX
>>                  mov     -8[EBP],EAX
>>                  mov     -4[EBP],ECX
>>                  cmp     -8[EBP],EDX
>>                  je      L23
>> L1B:            add     EDX,1
>>                  cmp     EDX,-8[EBP]
>>                  jb      L1B
>> L23:            mov     ESP,EBP
>>                  pop     EBP
>>                  ret     0190h
>>                  nop
>>                  nop
>>                  nop
>> .text.void main.foreach_loop(int[100])  ends
>> .text.void main.for_loop(int[100])      segment
>>          assume  CS:.text.void main.for_loop(int[100])
>> void main.for_loop(int[100]):
>>                  push    EBP
>>                  mov     EBP,ESP
>>                  xor     EAX,EAX
>> L5:             inc     EAX
>>                  cmp     EAX,064h
>>                  jb      L5
>>                  pop     EBP
>>                  ret     0190h
>>                  nop
>> .text.void main.for_loop(int[100])      ends
>>
>> Without -O -release the difference is even greater
>>
>> Is this normal? I mean, shouldn't the compiler optimize foreach a bit 
>> more? Or is it that I simply misunderstand generated code?
> 
> Yes, it should; that was the point of having foreach in the language:
> http://www.d-programming-language.org/faq.html#foreach
> 
> I think it's just that there's always plenty of more urgent things with 
> DMD.
> 
> 
> Tomek

Thanks, I hoped I'd see this kind of answer. :)