C# - AggressiveInlining: MethodImpl Attribute

AggressiveInlining. In C# programs, the JIT compiler logically determines which methods to inline. But sometimes we may know better than it does.

With AggressiveInlining, we give the compiler a hint. We tell it that the method should be inlined. This does not guarantee inlining, but it makes is more likely.

Attribute

Example. This example benchmarks a method with no attribute, and with AggressiveInlining. The method body contains several lines of useless code.

Info This makes the method large in bytes, so the JIT compiler may decide not to inline it.

Benchmark

And We apply the MethodImplOptions.AggressiveInlining option to Method2. This is an enum.

Result We see that with no options, the method calls required seven nanoseconds each.

But With inlining specified (with AggressiveInlining), the calls required less than one nanosecond each.

using System;
using System.Diagnostics;
using System.Runtime.CompilerServices;

class Program
{
    const int _max = 10000000;
    static void Main()
    {
        // ... Compile the methods.
        Method1();
        Method2();
        int sum = 0;

        var s1 = Stopwatch.StartNew();
        for (int i = 0; i < _max; i++)
        {
            sum += Method1();
        }
        s1.Stop();
        var s2 = Stopwatch.StartNew();
        for (int i = 0; i < _max; i++)
        {
            sum += Method2();
        }
        s2.Stop();
        Console.WriteLine(((double)(s1.Elapsed.TotalMilliseconds * 1000000) / _max).ToString("0.00 ns"));
        Console.WriteLine(((double)(s2.Elapsed.TotalMilliseconds * 1000000) / _max).ToString("0.00 ns"));
    }

    static int Method1()
    {
        // ... No inlining suggestion.
        return "one".Length + "two".Length + "three".Length +
            "four".Length + "five".Length + "six".Length +
            "seven".Length + "eight".Length + "nine".Length +
            "ten".Length;
    }

    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    static int Method2()
    {
        // ... Aggressive inlining.
        return "one".Length + "two".Length + "three".Length +
            "four".Length + "five".Length + "six".Length +
            "seven".Length + "eight".Length + "nine".Length +
            "ten".Length;
    }
}7.34 ns    No options
0.32 ns    MethodImplOptions.AggressiveInlining

Discussion. For critical algorithms, using a micro-optimization on hot code is often worthwhile. But this is the uncommon case—most code needs little optimization.

When to use AggressiveInlining. When should we apply this option? I have found that larger methods, such as the one in the example, benefit from this sort of inlining.

And If a method is called only once in an enclosing method, it may help to inline it even if it is large.

However If a large method is called in many places in a program, inlining it will reduce locality of reference and may ruin performance.

Method calls have a cost, but also a benefit. They require an activation record, but also reduce instruction count. Our challenge is to find the best balance between these considerations.