Documenting CLR JIT Strategy

Question

I'd like to know what scope and sequence the CLR applies to JIT compilation.

For example, if my application calls only a single method of a given class, do the unused methods of that class get JIT compiled needlessly? And if yes, are they all JIT compiled before executing the one method I needed, or are they lazily compiled after the fact?

And what about branches in a method? Does the CLR allow half of the code in a method to be compiled, while allowing a separate branch in the same method to remain uncompiled until needed?

It seems over time I've found articles that provide a glimpse for some of these details, but right now I'm not finding anything that gives a consolidated, readable summary of how and when the CLR chooses to JIT a section of code. Any suggested books or links?

It would be best if any such guide would break down such JIT decision-logic by .net version.

Brian Rasmussen · Accepted Answer · 2013-02-12T20:53:37.117

The way JIT works in .NET is that before a method has been jitted the method table entry points to a small stub that will JIT the method when invoked. After that the method table is updated to reference the location of the JIT compiled code.

Given that only methods that are invoked are JIT compiled there's no JIT overhead for methods that are not invoked.

The JIT compiler will compile an entire method when needed. If it is a release build code may be optimized away but otherwise the method is compiled in full.

You can inspect the method tables using WinDbg/SOS. Consider the following:

class SomeType
{
    public void Called()
    {
        Console.WriteLine("called");
    }

    public void NotCalled()
    {
        Console.WriteLine("not called");
    }
}

Assume that we create an instance of SomeType, call Called and then inspect the method table for SomeType. On x86 you'll see something like this:

0:000> !dumpmt -md 00a7381c
EEClass:         00a712d0
Module:          00a72e94
Name:            ConsoleApplication1.SomeType
mdToken:         02000002
File:            c:\temp\ConsoleApplication1\ConsoleApplication1\bin\Debug\ConsoleApplication1.exe
BaseSize:        0xc
ComponentSize:   0x0
Slots in VTable: 7
Number of IFaces in IFaceMap: 0
--------------------------------------
MethodDesc Table
   Entry MethodDe    JIT Name
72ca4960 729a6728 PreJIT System.Object.ToString()
72c98790 729a6730 PreJIT System.Object.Equals(System.Object)
72c98360 729a6750 PreJIT System.Object.GetHashCode()
72c916f0 729a6764 PreJIT System.Object.Finalize()
00df00d8 00a73814    JIT ConsoleApplication1.SomeType..ctor()
00df0110 00a737fc    JIT ConsoleApplication1.SomeType.Called()
00a7c031 00a73808   NONE ConsoleApplication1.SomeType.NotCalled()

Notice that Called is JIT compiled, but as we have not invoked NotCalled yet, it has not been JIT compiled.

Also, notice that the methods from object have all been PreJIT compiled.

Keep in mind that in release build short methods may be inlined in which case they are not called as methods, but simply included as part of the generated code for the call site.

Documenting CLR JIT Strategy

1 Answers1

Linked