Is BitArray faster in C# for getting a bit value than a simple conjuction with bitwise shift?

问题

1). var bitValue = (byteValue & (1 << bitNumber)) != 0;

2). using System.Collections.BitArray with a Get(int index) method

What is faster?
In what situations for the .NET projects BitArray could be more useful than a simple conjunction with the bitwise shift?

回答1:

BitArray is going to be able to handle an arbitrary number of boolean values, whereas a byte will hold only 8, int only 32, etc. This is going to be the biggest difference between the two.

Also, BitArray implements IEnumerable, where a integral type obviously does not. So it all depends on the requirements of your project; if you need an IEnumerable or array-like interface, then go with the BitArray.

I would actually use a bool[] over either solution, simply because it is more explicit in what kind of data you're keeping track of. T

BitArray or bitfield will use approximately 1/8th the space of a bool[] because they "pack" 8 boolean values into a single byte, whereas a bool by itself will take up the whole 8-bit byte. The space advantage of using a bitfield or BitArray isn't going to matter though until you being storing lots of bools. (The math is left up to the reader :-))

Benchmark

Results: For my primitive test environment, it appears that BitArray is a bit faster, but is on the same order of magnitude as doing it yourself with an integral type. Also tested was a bool[], which was unsurprisingly the fastest. Accessing single bytes in memory is going to be less complex than accessing individual bits in different bytes.

Testing with 10000000 operations:
   A UInt32 bitfield took 808 ms.
   A BitArray (32) took 574 ms.
   A List<bool>(32) took 436 ms.

Code:

class Program
{
    static void Main(string[] args)
    {
        Random r = new Random();
        r.Next(1000);

        const int N = 10000000;

        Console.WriteLine("Testing with {0} operations:", N);

        Console.WriteLine("   A UInt32 bitfield took {0} ms.", TestBitField(r, N));
        Console.WriteLine("   A BitArray (32) took {0} ms.", TestBitArray(r, N));
        Console.WriteLine("   A List<bool>(32) took {0} ms.", TestBoolArray(r, N));

        Console.Read();
    }


    static long TestBitField(Random r, int n)
    {
        UInt32 bitfield = 0;

        var sw = Stopwatch.StartNew();
        for (int i = 0; i < n; i++) {

            SetBit(ref bitfield, r.Next(32), true);
            bool b = GetBit(bitfield, r.Next(32));
            SetBit(ref bitfield, r.Next(32), b);
        }
        sw.Stop();
        return sw.ElapsedMilliseconds;
    }

    static bool GetBit(UInt32 x, int bitnum) {
        if (bitnum < 0 || bitnum > 31)
            throw new ArgumentOutOfRangeException("Invalid bit number");

        return (x & (1 << bitnum)) != 0;
    }

    static void SetBit(ref UInt32 x, int bitnum, bool val)
    {
        if (bitnum < 0 || bitnum > 31)
            throw new ArgumentOutOfRangeException("Invalid bit number");

        if (val)
            x |= (UInt32)(1 << bitnum);
        else
            x &= ~(UInt32)(1 << bitnum);
    }



    static long TestBitArray(Random r, int n)
    {
        BitArray b = new BitArray(32, false);     // 40 bytes

        var sw = Stopwatch.StartNew();
        for (int i = 0; i < n; i++) {

            b.Set(r.Next(32), true);
            bool v = b.Get(r.Next(32));
            b.Set(r.Next(32), v);
        }
        sw.Stop();
        return sw.ElapsedMilliseconds;
    }



    static long TestBoolArray(Random r, int n)
    {
        bool[] ba = new bool[32];

        var sw = Stopwatch.StartNew();
        for (int i = 0; i < n; i++) {

            ba[r.Next(32)] = true;
            bool v = ba[r.Next(32)];
            ba[r.Next(32)] = v;
        }
        sw.Stop();
        return sw.ElapsedMilliseconds;
    }
}

回答2:

@Jonathon Reinhart,

your benchmark is unfortunately inconclusive. It does not take into account the effects of possible lazy-loading, caching and/or prefetching (by the CPU, the host OS and/or the .NET runtime).

Shuffle the order of the tests (or call the test methods multiple times) and you might notice different time measurments.

I did your original benchmark built with "Any CPU" platform target and .NET 4.0 client profile, running on my machine with a i7-3770 CPU and 64-bit Windows 7.

What i got was this:

Testing with 10000000 operations:
   A UInt32 bitfield took 484 ms.
   A BitArray (32) took 459 ms.
   A List<bool>(32) took 393 ms.

which is pretty much in line with your observations.

However, executing the BitArray test before the UInt32 test yielded this:

Testing with 10000000 operations:
   A BitArray (32) took 513 ms.
   A UInt32 bitfield took 456 ms.
   A List<bool>(32) took 417 ms.

By looking at the times for the UInt32 and BitArray tests you will notice that the measured time does not seem to be connected to the tests themselves, but rather to the order in which the tests are run.

To alleviate these side effects at least a little bit, i executed the test methods twice in each program run with the following results.

Test order UInt32, BitArray, BoolArray, UInt32, BitArray, BoolArray:

Testing with 10000000 operations:
   A UInt32 bitfield took 476 ms.
   A BitArray (32) took 448 ms.
   A List<bool>(32) took 367 ms.

   A UInt32 bitfield took 419 ms.  <<-- Watch this.
   A BitArray (32) took 444 ms.    <<-- Watch this.
   A List<bool>(32) took 388 ms.

Test order BitArray, UInt32, BoolArray, BitArray, UInt32, BoolArray:

Testing with 10000000 operations:
   A BitArray (32) took 514 ms.
   A UInt32 bitfield took 413 ms.
   A List<bool>(32) took 379 ms.

   A BitArray (32) took 444 ms.    <<-- Watch this.
   A UInt32 bitfield took 413 ms.  <<-- Watch this.
   A List<bool>(32) took 381 ms.

Looking at the second invocations of the test methods, it appears that at least on i7 CPUs with up-to-date .NET runtime, the UInt32 test is faster than the BitArray test, while the BoolArray test is still being the fastest.

(I apologize that i had to write my response to Jonathon's benchmark as an answer, but as a new SO user i am not allowed to comment...)

EDIT:

Instead of shuffling the order of test methods, you might try putting a Thread.Sleep(5000) or similar right before calling the first test...

Also the original test seems to put the UInt32 test at disadvantage, because it includes a boundary check "if (bitnum < 0 || bitnum > 31)", which is executed 30 million times. None of the other two tests include such a boundary check. However, this is actually not the whole truth, since both the BitArray and the bool array do boundary checks internally.

Although i didn't test, i expect that eliminating the boundary checks will make the UInt32 and BoolArray tests perform similarly, but that would not be a good proposition for a public API.

来源：https://stackoverflow.com/questions/16471759/is-bitarray-faster-in-c-sharp-for-getting-a-bit-value-than-a-simple-conjuction-w

标签

performance

bit-shift

logical-operators

bitarray