Devirtualize calls dominated by a type assertion

[NinoFloris]

Currently devirtualization kicks in after a cast to a type for which the method is sealed. Additionally if any type assertions are available (e.g. the a.GetType() == typeof(Exact) branch) the cast itself gets elided too.

However both seem to be required for the code to devirtualize. Instead, I would like to see the following work as well:

using System;
using System.Runtime.CompilerServices;
class C {
    public int M(A a) 
    {
        if (a.GetType() == typeof(B))
            return a.Do();
            
        return 2;
    }
}

abstract class A
{
     public abstract int Do();   
}

sealed class B : A
{
    public override int Do() => 1;
}

In this case a.Do(); does not get devirtualized, as we are missing the explicit cast to B.

[omariom]

Ideally should devirtualize for if (a is B) too, as B is sealed.

[EgorBo]

@NinoFloris TBH, for such things we just rely on Dynamic PGO to do the right thing. Devirtualization and Inlining are early phases, while assertions are a late thing.

This is the codegen for real Tier1 of your function:

; Assembly listing for method C:M(A):int:this (Tier1)
G_M14267_IG01:  ;; offset=0x0000
G_M14267_IG02:  ;; offset=0x0000
       mov      rax, 0x7FF9559EE6E0      ; B
       cmp      qword ptr [rdx], rax
       jne      SHORT G_M14267_IG04
       mov      eax, 1
G_M14267_IG03:  ;; offset=0x0014
       ret      
G_M14267_IG04:  ;; offset=0x0015
       mov      eax, 2
G_M14267_IG05:  ;; offset=0x001A
       ret      
; Total bytes of code 27

[JulieLeeMSFT]

@NinoFloris TBH, for such things we just rely on Dynamic PGO to do the right thing. Devirtualization and Inlining are early phases, while assertions are a late thing.

@NinoFloris, is your question answered by EgorBo's comment? If you have any further questions or if there are outstanding concerns, please let us know. Otherwise, we will proceed to close this issue.

[dotnet-policy-service]

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