Why does C# execute Math.Sqrt() more slowly than VB.NET?
Background
While running benchmark tests this morning, my colleagues and I discovered some strange things concerning performance of C# code vs. VB.NET code.
We started out comparing C# vs. Delphi Prism calculating prime numbers, and found that Prism was about 30% faster. I figured CodeGear optimized code more when generating IL (the exe was about twice as big as C#'s and had all sorts of different IL in it.)
I decided to write a test in VB.NET as well, assuming that Microsoft's compilers would end up writing essentially the same IL for each language. However, the result there was more shocking: the code ran more than three times slower on C# than VB with the same operation!
The generated IL was different, but not extremely so, and I'm not good enough at reading it to understand the differences.
Benchmarks
I've included the code for each below. On my machine, VB finds 348513 primes in about 6.36 seconds. C# finds the same number of primes in 21.76 seconds.
Computer Specs and Notes
- Intel Core 2 Quad 6600 @ 2.4Ghz
Every machine I've tested on there is a noticeable difference in the benchmark results between C# and VB.NET.
Both of the console applications were compiled in Release mode, but otherwise no project settings were changed from the defaults generated by Visual Studio 2008.
VB.NET code
Imports System.Diagnostics
Module Module1
Private temp As List(Of Int32)
Private sw As Stopwatch
Private totalSeconds As Double
Sub Main()
serialCalc()
End Sub
Private Sub serialCalc()
temp = New List(Of Int32)()
sw = Stopwatch.StartNew()
For i As Int32 = 2 To 5000000
testIfPrimeSerial(i)
Next
sw.Stop()
totalSeconds = sw.Elapsed.TotalSeconds
Console.WriteLine(String.Format("{0} seconds elapsed.", totalSeconds))
Console.WriteLine(String.Format("{0} primes found.", temp.Count))
Console.ReadKey()
End Sub
Private Sub testIfPrimeSerial(ByVal suspectPrime As Int32)
For i As Int32 = 2 To Math.Sqrt(suspectPrime)
If (suspectPrime Mod i = 0) Then
Exit Sub
End If
Next
temp.Add(suspectPrime)
End Sub
End Module
C# Code
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Diagnostics;
namespace FindPrimesCSharp {
class Program {
List<Int32> temp = new List<Int32>();
Stopwatch sw;
double totalSeconds;
static void Main(string[] args) {
new Program().serialCalc();
}
private void serialCalc() {
temp = new List<Int32>();
sw = Stopwatch.StartNew();
for (Int32 i = 2; i <= 5000000; i++) {
testIfPrimeSerial(i);
}
sw.Stop();
totalSeconds = sw.Elapsed.TotalSeconds;
Console.WriteLine(string.Format("{0} seconds elapsed.", totalSeconds));
Console.WriteLine(string.Format("{0} primes found.", temp.Count));
Console.ReadKey();
}
private void testIfPrimeSerial(Int32 suspectPrime) {
for (Int32 i = 2; i <= Math.Sqrt(suspectPrime); i++) {
if (suspectPrime % i == 0)
return;
}
temp.Add(suspectPrime);
}
}
}
Why is C#'s execution of Math.Sqrt() slower than VB.NET?
The C# implementation is recalculating Math.Sqrt(suspectPrime) each time through the loop, while VB only calculates it at the beginning of the loop. This is just due to the nature of the control structure. In C#, for is just a fancy while loop, while in VB it's a separate construct.
Using this loop will even up the score:
Int32 sqrt = (int)Math.Sqrt(suspectPrime)
for (Int32 i = 2; i <= sqrt; i++) {
if (suspectPrime % i == 0)
return;
}
I agree with the statement that the C# code is computing sqrt on every iteration and here is the proof straight out of Reflector:
VB version:
private static void testIfPrimeSerial(int suspectPrime)
{
int VB$t_i4$L0 = (int) Math.Round(Math.Sqrt((double) suspectPrime));
for (int i = 2; i <= VB$t_i4$L0; i++)
{
if ((suspectPrime % i) == 0)
{
return;
}
}
temp.Add(suspectPrime);
}
C# version:
private void testIfPrimeSerial(int suspectPrime)
{
for (int i = 2; i <= Math.Sqrt((double) suspectPrime); i++)
{
if ((suspectPrime % i) == 0)
{
return;
}
}
this.temp.Add(suspectPrime);
}
Which kinda points to VB generating code that performs better even if the developer is naive enough to have the call to sqrt in the loop definition.
Here is the decompiled IL from the for loops. If you compare the two you will see VB.Net only does the Math.Sqrt(...) onces while C# checks it on each pass. To fix this you would need to do something like var sqrt = (int)Math.Sqrt(suspectPrime); as other have suggested.
... VB ...
.method private static void CheckPrime(int32 suspectPrime) cil managed
{
// Code size 34 (0x22)
.maxstack 2
.locals init ([0] int32 i,
[1] int32 VB$t_i4$L0)
IL_0000: ldc.i4.2
IL_0001: ldarg.0
IL_0002: conv.r8
IL_0003: call float64 [mscorlib]System.Math::Sqrt(float64)
IL_0008: call float64 [mscorlib]System.Math::Round(float64)
IL_000d: conv.ovf.i4
IL_000e: stloc.1
IL_000f: stloc.0
IL_0010: br.s IL_001d
IL_0012: ldarg.0
IL_0013: ldloc.0
IL_0014: rem
IL_0015: ldc.i4.0
IL_0016: bne.un.s IL_0019
IL_0018: ret
IL_0019: ldloc.0
IL_001a: ldc.i4.1
IL_001b: add.ovf
IL_001c: stloc.0
IL_001d: ldloc.0
IL_001e: ldloc.1
IL_001f: ble.s IL_0012
IL_0021: ret
} // end of method Module1::testIfPrimeSerial
... C# ...
.method private hidebysig static void CheckPrime(int32 suspectPrime) cil managed
{
// Code size 26 (0x1a)
.maxstack 2
.locals init ([0] int32 i)
IL_0000: ldc.i4.2
IL_0001: stloc.0
IL_0002: br.s IL_000e
IL_0004: ldarg.0
IL_0005: ldloc.0
IL_0006: rem
IL_0007: brtrue.s IL_000a
IL_0009: ret
IL_000a: ldloc.0
IL_000b: ldc.i4.1
IL_000c: add
IL_000d: stloc.0
IL_000e: ldloc.0
IL_000f: conv.r8
IL_0010: ldarg.0
IL_0011: conv.r8
IL_0012: call float64 [mscorlib]System.Math::Sqrt(float64)
IL_0017: ble.s IL_0004
IL_0019: ret
} // end of method Program::testIfPrimeSerial
Off on a tangent, if you're up and running with VS2010, you can take advantage of PLINQ and make C# (probably VB.Net as well) faster.
Change that for loop to...
var range = Enumerable.Range(2, 5000000);
range.AsParallel()
.ForAll(i => testIfPrimeSerial(i));
I went from 7.4 -> 4.6 seconds on my machine. Moving it to release mode shaves a little more time on top of that.
The difference is in the loop; your C# code is computing the square root on every iteration. Changing that one line from:
for (Int32 i = 2; i <= Math.Sqrt(suspectPrime); i++) {
to:
var lim = Math.Sqrt(suspectPrime);
for (Int32 i = 2; i <= lim; i++) {
dropped the time on my machine from 26 seconds to 7.something.