Pre increment/decrement in a loop vs post increment/decrement with gcc and Visual C

Question

I know there have been a number of questions asked regarding the performance boost of using a pre increment (decrement) vs post. And from what I can infer, the majority of C/C++ compilers will change

for (int i =0; i<N; i++)
{
...
}

to the following as an optimization

for (int i =0; i<N; ++i)
{
...
}

I have been doing a lot of code review lately, I find many instances where I could change i++ to ++i. The question is doing fixing just good form or in some cases it can make a difference in performance?

Meaning do the compilers I usually use: gcc and Visual C++ automatically make this optimization? Information is also welcome about other well used compiles as well.

I think even the compiler would have a more consistent, and good looking style. Also, just generate assembly from your sources with the different optimization options, and check it yourself. It's not that hard. — Iharob Al Asimi, Feb 16 '17 at 12:55
http://stackoverflow.com/questions/24886/is-there-a-performance-difference-between-i-and-i-in-c — Support Ukraine, Feb 16 '17 at 12:56
Note that `while (myTestVal < x++)` means something different than `while (myTestVal < ++x)` — Stephan Lechner, Feb 16 '17 at 12:57
http://stackoverflow.com/questions/1116735/i-less-efficient-than-i-how-to-show-this — Support Ukraine, Feb 16 '17 at 12:57
In your while loop are the cases for x++ and ++x NOT equivalent, because in the one you compare with "x" and ind the other with "x+1" — ab.o2c, Feb 16 '17 at 12:58
If you are going for a consistent style, you might also want to use `i != N` as the condition. — Bo Persson, Feb 16 '17 at 13:00
To a C compiler `; i++ ;` and `; ++i ;` means the very same thing. — Lundin, Feb 16 '17 at 15:10

score 1 · Answer 1 · answered Feb 16 '17 at 12:58

1

For build-in types like int or double you won't see any difference in performance.

For custom C++ types you may see a difference depending on how they have implemented the operators - only way to know is to read the code and profile it.

answered Feb 16 '17 at 12:58

Jesper Juhl

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Pre increment/decrement in a loop vs post increment/decrement with gcc and Visual C

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