Bitwise, Byte Foolish?

February 9, 2009 at 11:22 pm (JavaScript)
Tags: bits, bitwise, farrah, logan, math, shifting

 

To avoid Carnival, Logan is trying to get some plastic surgery from Farrah Fawcett.

In the post-apocalyptic world of Logan’s Run, humans live out cramped lives in a bunker. Because of the crowded conditions, all citizens must submit to Carnival on their 30th birthdays. What do you do on your Carnival day? You and all the other 30 year olds spiral up into the air and get zapped to death.

Sound like a bad deal? It gets worse. The radiation outside has long since subsided, and the rules being enforced are no longer necessary.

I’ve been immersed in JavaScript for the last few years. Every time I’ve read about JavaScript’s bit operations (and it has a very nice set of ops), I’ve been warned that they are “a hack,” and “slow.” This has always bummed me out. I love bits.

But I got to thinking the other day, all these warnings sound like a sort of a Logan’s Run passed-down message from days gone by. With all the cool things that have been going on with JavaScript lately (thanks to Google’s V8 and the various JIT subsystems going into browsers), maybe, just maybe, bitwise operations have sped up enough to be worthwhile. Since some of the JIT compilers are doing type inference, a series of bit operations could, in theory, be done at full speed, without costly implicit type conversions.

Who Needs Bits?

When I started programming (on 8-bit processors), I mostly worked with integers. When floating point numbers became available, I avoided them for years because they were slow. We did as much math as we could with integers and bit operations. Shift left once, and that’s a multiplication by 2. If you wanted to multiply by 10, you’d do a shift left by one (times two) and a shift left by 3 (times eight) and add those results together.

You wouldn’t do math like that any more, but bits are still good as flags. A single JavaScript number (when treated bitwise) can hold 32 boolean flags!

Here’s what I found out. Just about everything I tried was faster with bitwise operations than with other solutions (even in Internet Explorer). No, you can’t usefully replace one multiplication with a series of adds and shifts, but by no means are bitwise operators too slow to be useful.

        k=~~i; //this is much faster
        k=Math.floor(i); //than this

You can lop off the decimal part of a number not just with ~~, but with ^0 and <<0, too. And they are ALL significantly faster, in all modern major browsers, than calling Math.floor().

//break a number in the range 0-65535 into
// low and high bytes

// this is significantly faster
    lo = i & 255;
    hi = i >> 8;

//than this
    lo = i % 256;
    hi = (i-lo)/256;

Don’t discriminate against JavaScript’s bitwise operators. But keep these things in mind…

1. You have 32 bits to work with when you treat a JavaScript variable as bits. And that top bit is a sign bit (meaning any number with that top bit set is a negative number).
2. Don’t mistake bitwise for logical operators.
3. Learn the difference between “-” and “~”.
4. Remember that >> shifts the sign bit down while >>> shifts a zero into the top bit.

My favorite place to learn about JavaScript bitwise facilities is here: Bitwise Operators.

When should you be using bitwise operators? Obviously, when you want to pack a bunch of flags into one variable. That should be much faster than having an array of flags.

Dealing with the red, blue, and green parts of a color call out for bitwise operators.

And think about using a shift when you want to multiply or divide by a power of 2, while at the same time converting a floating point number into an integer. Two operations for the price of one!