Why the performance difference with the following code loops.

  • Thread starter Thread starter Dave Harris
  • Start date Start date
D

Dave Harris

In theory all three loops should operate at the same speed.

Loop1 is the fastest by quite a bit,
Loop2 is the slowest
Loop3 is somewhat faster than Loop2.

This is built in production with the optimizer on 32 and 64 bit code. If
I force the functions to not inline then the speed is identical, so best I
can figure is that the JIT somehow makes different inlining decisions. This
is more of a curiosity than anything else, I wouldn't base coding decisions
on this finding, but still it would be interesting to know what's up.


using System;
using System.Diagnostics;

namespace ConsoleApplication1
{

public class M
{
public static bool IsZero(double d1)
{
return (-.000001 < d1 && d1 < .000001);
}

public static bool IsZero2(double d1)
{
return (d1 > -.000001 && d1 < .000001);
}

public static bool IsZero3(double d1)
{
return (d1 > -.000001 && .000001 > d1);
}


static public void Main()
{
double sum;

{
Stopwatch st = new Stopwatch();
st.Start();
Console.WriteLine("Ignore - make sure any initialization needed in this
class is done");
}

{
sum = 0;
double x = UInt32.MaxValue;

Stopwatch st = new Stopwatch();
st.Start();

for (uint j = 0; j < UInt32.MaxValue; j++)
{
x = x - 1.0;
if (IsZero(x))
{
sum += 1;
}
}

st.Stop();
Console.WriteLine(st.ElapsedMilliseconds);
Console.WriteLine(sum);
}

{
sum = 0;
double x = UInt32.MaxValue;

Stopwatch st = new Stopwatch();
st.Start();

for (uint j = 0; j < UInt32.MaxValue; j++)
{
x = x - 1.0;
if (IsZero2(x))
{
sum += 1;
}
}

st.Stop();
Console.WriteLine(st.ElapsedMilliseconds);
Console.WriteLine(sum);
}

{
sum = 0;
double x = UInt32.MaxValue;

Stopwatch st = new Stopwatch();
st.Start();

for (uint j = 0; j < UInt32.MaxValue; j++)
{
x = x - 1.0;
if (IsZero3(x))
{
sum += 1;
}
}

st.Stop();
Console.WriteLine(st.ElapsedMilliseconds);
Console.WriteLine(sum);
}



}
}
}
 
Dave Harris said:
In theory all three loops should operate at the same speed.

Loop1 is the fastest by quite a bit,
Loop2 is the slowest
Loop3 is somewhat faster than Loop2.

This is built in production with the optimizer on 32 and 64 bit code.
If
I force the functions to not inline then the speed is identical, so best I
can figure is that the JIT somehow makes different inlining decisions.
This
is more of a curiosity than anything else, I wouldn't base coding
decisions
on this finding, but still it would be interesting to know what's up.


using System;
using System.Diagnostics;

namespace ConsoleApplication1
{

public class M
{
public static bool IsZero(double d1)
{
return (-.000001 < d1 && d1 < .000001);
}

public static bool IsZero2(double d1)
{
return (d1 > -.000001 && d1 < .000001);
}

public static bool IsZero3(double d1)
{
return (d1 > -.000001 && .000001 > d1);
}


static public void Main()
{
double sum;

{
Stopwatch st = new Stopwatch();
st.Start();
Console.WriteLine("Ignore - make sure any initialization needed in this
class is done");
}

{
sum = 0;
double x = UInt32.MaxValue;

Stopwatch st = new Stopwatch();
st.Start();

for (uint j = 0; j < UInt32.MaxValue; j++)
{
x = x - 1.0;
if (IsZero(x))
{
sum += 1;
}
}

st.Stop();
Console.WriteLine(st.ElapsedMilliseconds);
Console.WriteLine(sum);
}

{
sum = 0;
double x = UInt32.MaxValue;

Stopwatch st = new Stopwatch();
st.Start();

for (uint j = 0; j < UInt32.MaxValue; j++)
{
x = x - 1.0;
if (IsZero2(x))
{
sum += 1;
}
}

st.Stop();
Console.WriteLine(st.ElapsedMilliseconds);
Console.WriteLine(sum);
}

{
sum = 0;
double x = UInt32.MaxValue;

Stopwatch st = new Stopwatch();
st.Start();

for (uint j = 0; j < UInt32.MaxValue; j++)
{
x = x - 1.0;
if (IsZero3(x))
{
sum += 1;
}
}

st.Stop();
Console.WriteLine(st.ElapsedMilliseconds);
Console.WriteLine(sum);
}



}
}
}
Click to expand...


The first loop is fastest in part, because the first test of the two is
always false, therefore the second test is not done.
 
Family Tree Mike said:
The first loop is fastest in part, because the first test of the two is
always false, therefore the second test is not done.
Click to expand...


Sorry, ignore that. I read it wrong... Switch case one's logic arround
though and I think it should be faster.
 
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