乘法比.NET中的比较更快？

Question

Good morning, afternoon or night,

Up until today, I thought comparison was one of the basic processor instructions, and so that it was one of the fastest operations one can do in a computer... On the other hand I know multiplication in sometimes trickier and involves lots of bits operations. However, I was a little shocked to look at the results of the following code:

Stopwatch Test = new Stopwatch();

int     a = 0;
int     i = 0, j = 0, l = 0;
double  c = 0, d = 0;

for (i = 0; i < 32; i++)
{
    Test.Start();

    for (j = Int32.MaxValue, l = 1; j != 0; j = -j + ((j < 0) ? -1 : 1), l = -l)
    {
        a = l * j;
    }

    Test.Stop();

    Console.WriteLine("Product: {0}", Test.Elapsed.TotalMilliseconds);
    c += Test.Elapsed.TotalMilliseconds;

    Test.Reset();
    Test.Start();

    for (j = Int32.MaxValue, l = 1; j != 0; j = -j + ((j < 0) ? -1 : 1), l = -l)
    {
        a = (j < 0) ? -j : j;
    }

    Test.Stop();

    Console.WriteLine("Comparison: {0}", Test.Elapsed.TotalMilliseconds);
    d += Test.Elapsed.TotalMilliseconds;

    Test.Reset();
    }

    Console.WriteLine("Product: {0}", c / 32);
    Console.WriteLine("Comparison: {0}", d / 32);
    Console.ReadKey();
}

Result:

Product: 8558.6
Comparison: 9799.7

Quick explanation : j is an ancillary alternate variable which goes like (...), 11, -10, 9, -8, 7, (...) until it reaches zero, l is a variable which stores j 's sign, and a is the test variable, which I want always to be equal to the modulus of j . The goal of the test was to check whether it is faster to set a to this value using multiplication or the conditional operator.

Can anyone please comment on these results?

Thank you very much.

Answer 1

Your second test it's not a mere comparison, but an if statement.

That's probably translated in a JUMP/BRANCH instruction in CPU , involving branch prediction (with possible blocks of the pipeline) and then is likely slower than a simple multiplication (even if not so much).

Answer 2

It can often be very difficult to make such assertions about an optimising compiler. They do lots of tricks that make simple cases different from real code. That said, you aren't just doing a comparison, you're doing a compare/assign in a very tight loop. The thread you're working on may have to pause many times at the branch; the multiplication can assign as many times as it likes, as long as the last assignment is still last, so many multiplies can be going on at once.

As is the general rule, make your code clear and ignore minor timing issues unless they become a problem. If you do have a speed problem a good tracing/timing tool will guide you much better than knowing if one operation is faster than other in a specific case.

Answer 3

I guess one comment I would make is that you are doing a lot more in the second operation:

a = (j < 0) ? -j : j; 

Not only are you doing a comparison, but also effectivly a "if..else.." with the ? operator and a negation of j.

Answer 4

您应该尝试运行此测试大约1000次并使用avrage来比较您现在从未在后台进行的CLR