C# Method Call Depth

Method calls can be nested. This impacts performance: the more nesting, the more depth to the stack. A method call will require stack space unless it is inlined. By flattening the call stack, we reduce stack usage—and improve performance.

This C# benchmark compares the performance of method call depths.

Test method call depths

First, in compiler theory, the term "activation record" refers to a section of memory on the stack that contains data about a method call. As more nested methods are invoked, the stack grows downwards, consuming more and more memory. When a method terminates, its memory is forgotten from the stack and the pointer is readjusted.

This program contains eight important methods: A1, B1, C1, D1; and A2, B2, C2, D2. When A1 is called, it calls B1 and then B1 calls C1, etc. On the other hand, when A2 is called, it simply calls B2, C2, and D2 sequentially. The A2 method should consume less stack space.

Program that compares method call depths [C#]

using System;
using System.Diagnostics;

class Program
{
    const int _max = 100000000;
    static void Main()
    {
	A1();
	A2();

	var s1 = Stopwatch.StartNew();
	for (int i = 0; i < _max; i++)
	{
	    A1();
	}
	s1.Stop();
	var s2 = Stopwatch.StartNew();
	for (int i = 0; i < _max; i++)
	{
	    A2();
	}
	s2.Stop();
	Console.WriteLine(((double)(s1.Elapsed.TotalMilliseconds * 1000 * 1000) /
	    _max).ToString("0.00 ns"));
	Console.WriteLine(((double)(s2.Elapsed.TotalMilliseconds * 1000 * 1000) /
	    _max).ToString("0.00 ns"));
	Console.Read();
    }

    static int _i = 1;

    static void A1()
    {
	B1();
    }

    static void B1()
    {
	if (_i == 0)
	    throw new Exception();
	C1();
    }

    static void C1()
    {
	if (_i == 0)
	    throw new Exception();
	D1();
    }

    static void D1()
    {
	if (_i == 0)
	    throw new Exception();
    }

    static void A2()
    {
	B2();
	C2();
	D2();
    }

    static void B2()
    {
	if (_i == 0)
	    throw new Exception();
    }

    static void C2()
    {
	if (_i == 0)
	    throw new Exception();
    }

    static void D2()
    {
	if (_i == 0)
	    throw new Exception();
    }
}

Output

3.54 ns
0.96 ns

Results. The results of this program show that when A1->B1->C1->D1 are called in a nested sequence, the program is much slower than when A2->B2 + C2 + D2 are called in a more sequential order. The Exception throws in the program are to confuse the inliner in the JIT.

Summary

It is possible to improve the performance of method calls in the C# language by reducing the depth of the call stack. In other words, changing deeply nested method invocations into more sequential invocations can yield performance benefits. This is likely not terribly useful except on performance critical parts of important programs.