Many programmers are aware of a special case where you can use a bitwise shift for multiplication or division when you’re multiplying or dividing by a power of two. For example, you can replace `i * 2` with `i << 1` and `i / 2` with `i >> 1`. A lesser-known trick works for modulus. But are these alternatives actually any faster? Today's article puts them to the test!

Here are today's competitors:

```// Multiplication
i * 8; // normal
i << 3; // bitwise [8 = 2^3, so use 3]

// Division
i / 16; // normal
i >> 4; // bitwise [16 = 2^4, so use 4]

// Modulus
i % 4; // normal
i & 3; // bitwise [4 = 1 << 2, apply ((1 << 2) - 1), so use 3]```

Here's a performance test that performs each of these operations a lot of times:

```package
{
import flash.display.*;
import flash.utils.*;
import flash.text.*;

public class FasterDivMod extends Sprite
{
private var __logger:TextField = new TextField();
private function row(...cols): void
{
__logger.appendText(cols.join(",")+"\n");
}

public function FasterDivMod()
{
stage.align = StageAlign.TOP_LEFT;
stage.scaleMode = StageScaleMode.NO_SCALE;

__logger.autoSize = TextFieldAutoSize.LEFT;

init();
}

private function init(): void
{
var beforeTime:int;
var afterTime:int;
var i:int;
var REPS:int = 100000000;
var absInt:int;

row("Method", "Time");

beforeTime = getTimer();
for (i = 0; i < REPS; ++i)
{
absInt = i / 4;
}
afterTime = getTimer();
row("Div: i / 4", (afterTime-beforeTime), absInt);

beforeTime = getTimer();
for (i = 0; i < REPS; ++i)
{
absInt = i >> 2;
}
afterTime = getTimer();
row("Div: i >> 2", (afterTime-beforeTime), absInt);

beforeTime = getTimer();
for (i = 0; i < REPS; ++i)
{
absInt = i * 4;
}
afterTime = getTimer();
row("Mul: i * 4", (afterTime-beforeTime), absInt);

beforeTime = getTimer();
for (i = 0; i < REPS; ++i)
{
absInt = i << 2;
}
afterTime = getTimer();
row("Mul: i << 2", (afterTime-beforeTime), absInt);

beforeTime = getTimer();
for (i = 0; i < REPS; ++i)
{
absInt = i % 4;
}
afterTime = getTimer();
row("Mod: i % 4", (afterTime-beforeTime), absInt);

beforeTime = getTimer();
for (i = 0; i < REPS; ++i)
{
absInt = i & 3; // ((1 << 2) - 1) == 3
}
afterTime = getTimer();
row("Mod: i & 3", (afterTime-beforeTime), absInt);
}
}
}```

I ran this test app in the following environment:

• Flex SDK (MXMLC) 4.6.0.23201, compiling in release mode (no debugging or verbose stack traces)
• Release version of Flash Player 11.3.300.271
• 2.3 Ghz Intel Core i7
• Mac OS X 10.8.0

And here are the results I got:

Method Time
Div: i / 4 358
Div: i >> 2 224
Mul: i * 4 206
Mul: i << 2 255
Mod: i % 4 918
Mod: i & 3 254

The above results validate the bitwise versions in two out of three tests: division and modulus. In the multiplication case, the normal version actually performs about 20% faster than the bitwise equivalent. On the other hand, division is nearly twice as fast with the bitwise shift and the bitwise modulus (really just an `&`) is more than three times faster! So if you're got a lot of divides or mods in your performance-critical code, swap them over to the bitwise versions!

Spot a bug? Know why multiplication is faster than a left bitwise shift and want to fill us all in? Post a comment!