Guillaume07 Guillaume07 - 1 month ago 15
C# Question

JIT & loop optimization

using System;

namespace ConsoleApplication1
{
class TestMath
{
static void Main()
{
double res = 0.0;

for(int i =0;i<1000000;++i)
res += System.Math.Sqrt(2.0);

Console.WriteLine(res);

Console.ReadKey();
}
}
}


By benchmarking this code against the c++ version, I discover than performance are 10 times slower than c++ version. I have no problem with that , but that lead me to the following question :

It seems (after a few search) that JIT compiler can't optimize this code as c++ compiler can do, namely just call sqrt once and apply *1000000 on it.

Is there a way to force JIT to do it ?

Answer

I repro, I clock the C++ version at 1.2 msec, the C# version at 12.2 msec. The reason is readily visible if you take a look at the machine code the C++ code generator and optimizer emits. It rewrites the loop like this (using the C# equivalent):

double temp = Math.Sqrt(2.0);
for (int i = 0; i < 1000000; ++i) {
    res += temp;
}

That's a combination of two optimizations, called "invariant code motion" and "loop hoisting". In other words, the C++ compiler knows enough about the sqrt() function to know that its return value is not affected by the surrounding code so can be moved at will. And that it is then worth-while to move that code outside of the loop and create an extra local variable to store the result. And that calculating sqrt() is slower than adding. Sounds obvious but that's a rule that has to built into the optimizer and has to be considered, one of many, many rules.

And yes, the jitter optimizer misses that one. It is guilty of not being able to spent the same amount of time as the C++ optimizer, it operates under heavy time constraints. Because if it takes too long then the program takes too much time getting started.

Tongue in cheek: a C# programmer needs to be a bit smarter than the code generator and recognize these optimization opportunities himself. This is a fairly obvious one. Well, now that you know about it anyway :)

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