JIT: Devirtualization: "this" should always have exact/final class.

[EgorBo]

Contributes to #7541

using System;

public class C : IDisposable
{
    public void Dispose() {}

    public void CallDispose(IDisposable d) => d.Dispose();
    
    public void Test() => CallDispose(this);
}

Current codegen for Test:

C.Test()
    L0000: mov r11, 0x7ffc31027028
    L000a: mov rax, [0x7ffc31027028]
    L0011: jmp rax

Expected codegen for Test:

C.Test()
    L0000: ret

From jitdump:

impDevirtualizeCall: Trying to devirtualize interface call:
    class for 'this' is Program (attrib 20000000)
    base method is System.IDisposable::Dispose
    base class is interface
    devirt to Program::Dispose -- final method
               [000003] --C-G-------              *  CALLV stub void   System.IDisposable.Dispose
               [000000] ------------ this in rcx  \--*  LCL_VAR   ref    V00 arg0         
    Class not final or exact

In this case it should not matter if C is sealed or not - this should mean the class is exact and Dispose method should be devirtualized (inlined as a no-op).

[jkotas]

What about:

public class C : IDisposable
{
    public void Dispose() {}

    public void CallDispose(IDisposable d) => d.Dispose();
    
    public void Test() => CallDispose(this);
}

public class D : C, IDisposable
{
    public void Dispose()
    {
        Console.WriteLine("I am here");
    }
}

new D().Test(); has to print "I am here" in this case.

[EgorBo]

Oops, indeed 😪