I have found an interesting small article about optimization, so I've tried the code in C and D, and I have found strange results (the D code shows timings opposite of the article).
This is the article, look at the "Branch Prediction" section:
http://www.ddj.com/184405848

The C code:
http://codepad.org/QSGIije4
And its asm (MinGW 4.2.1):
http://codepad.org/c7ZRiXGI

The similar D code:
http://codepad.org/slhcSJEA
Its asm (DMD 1.036):
http://codepad.org/AjlraEs9

There is also about 2X performance difference.

Bye,
bearophile

            if (i % 4 == 1) {
                if (i % 4 == 0) {
                    counter1++;
                } else {
                    counter2++;
                }
            } else {
                if (i % 4 == 2) {
                    counter3++;
                } else {
                    counter4++;
                }
            }
this is incorrect

You didn't run your code, lol.

Kagamin:
> this is incorrect

Thank you for spotting the silly bug, I'll fix it now. (It seems it's more easy to leave bugs in such kind of code because it does nothing useful).

But note that in both programs:
#define FIRST
static if (1) {
So the first part only is run in both D and C code, not the wrong one...

So the code is like this:

#include "stdio.h"

int main() {
    int counter0 = 0, counter1 = 0, counter2 = 0, counter3 = 0;
    int i = 300000000;
    while (i--) {
        // 0.63 s
        if (i % 4 == 0) {
            counter0++;
        } else if (i % 4 == 1) {
            counter1++;
        } else if (i % 4 == 2) {
            counter2++;
        } else {
            counter3++;
        }
    }

    printf("%d %d %d %d\n", counter0, counter1, counter2, counter3);
    return 0;
}

So the problem and timings of the first part I have shown are correct still :-)

Bye,
bearophile

bearophile wrote:
> I have found an interesting small article about optimization, so I've tried the code in C and D, and I have found strange results (the D code shows timings opposite of the article).
> This is the article, look at the "Branch Prediction" section:
> http://www.ddj.com/184405848
> 
> The C code:
> http://codepad.org/QSGIije4
> And its asm (MinGW 4.2.1):
> http://codepad.org/c7ZRiXGI
> 
> The similar D code:
> http://codepad.org/slhcSJEA
> Its asm (DMD 1.036):
> http://codepad.org/AjlraEs9
> 
> There is also about 2X performance difference.
> 
> Bye,
> bearophile

Are you running it on a Pentium 4? Pentium 4 has *horrific* branch misprediction (minimum 24 cycles, 45 uops). No other processor is nearly as bad, eg it's 15 cycles on Core2; it was just 4 cycles on PMMX.

Don:
> Are you running it on a Pentium 4? Pentium 4 has *horrific* branch misprediction (minimum 24 cycles, 45 uops). No other processor is nearly as bad, eg it's 15 cycles on Core2; it was just 4 cycles on PMMX.

Sorry, I am using a Core2 @ 2GHz.
The fixed C code with timings:

#include "stdio.h"

//#define FIRST

int main() {
    int counter0 = 0, counter1 = 0, counter2 = 0, counter3 = 0;
    int i = 300000000;
    while (i--) {
        #ifdef FIRST
            // 0.63 s
            if (i % 4 == 0) {
                counter0++;
            } else if (i % 4 == 1) {
                counter1++;
            } else if (i % 4 == 2) {
                counter2++;
            } else {
                counter3++;
            }
        #else
            // 0.66 s
            if (i & 2) {
                if (i & 1) {
                    counter3++;
                } else {
                    counter2++;
                }
            } else {
                if (i & 1) {
                    counter1++;
                } else {
                    counter0++;
                }
            }
        #endif
    }

    printf("%d %d %d %d\n", counter0, counter1, counter2, counter3);
    return 0;
}


Fixed D code with timings:

void main() {
    int counter0, counter1, counter2, counter3;

    int i = 300000000;
    while (i--)
        static if (0) { // 1.24 s
            if (i % 4 == 0) {
                counter0++;
            } else if (i % 4 == 1) {
                counter1++;
            } else if (i % 4 == 2) {
                counter2++;
            } else {
                counter3++;
            }
        } else { // 1.01 s
            if (i & 2) {
                if (i & 1) {
                    counter3++;
                } else {
                    counter2++;
                }
            } else {
                if (i & 1) {
                    counter1++;
                } else {
                    counter0++;
                }
            }
        }

    printf("%d %d %d %d\n", counter0, counter1, counter2, counter3);
}

As you can see the C version (GCC 4.2.1-dw2) is twice faster than the D one, and it shows the scan as faster than the binary search, as says the article I have linked.

Bye,
bearophile