NativeAOT: devirtualize isinst for monomorphic cases

[EgorBo]

public interface IMyInterface {}

public class MyProgram : IMyInterface
{
    static void Main() => IsMyInterface(new MyProgram ());

    [MethodImpl(MethodImplOptions.NoInlining)]
    static bool IsMyInterface(object o) => o is IMyInterface;
}

Since IMyInterface has only one implementation (monomorphic cases aren't rare) we can optimize isinst to basically o is MyProgram and inline it.

Current NativeAOT codegen:

; Method Program:IsMyInterface(System.Object):bool
       4883EC28             sub      rsp, 40
       488BD1               mov      rdx, rcx
       488D0D00000000       lea      rcx, [(reloc 0x40000000004234f8)]
       E800000000           call     CORINFO_HELP_ISINSTANCEOFINTERFACE
       4885C0               test     rax, rax
       0F95C0               setne    al
       0FB6C0               movzx    rax, al      
       4883C428             add      rsp, 40
       C3                   ret      
; Total bytes of code: 33

Expected NativeAOT codegen:

; Method Program:IsMyInterface(System.Object):bool
       4885C9               test     rcx, rcx
       740E                 je       SHORT G_M18292_IG05
       488D0500000000       lea      rax, [(reloc 0x40000000004221d8)]
       483901               cmp      qword ptr [rcx], rax
       7402                 je       SHORT G_M18292_IG05
       33C9                 xor      rcx, rcx
G_M18292_IG05:
       33C0                 xor      eax, eax
       4885C9               test     rcx, rcx
       0F95C0               setne    al
       C3                   ret      
; Total bytes of code: 28

cc @MichalStrehovsky

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Issue Details

---

public interface IMyInterface {}

public class MyProgram : IMyInterface
{
    static void Main() => IsMyInterface(new MyProgram ());

    [MethodImpl(MethodImplOptions.NoInlining)]
    static bool IsMyInterface(object o) => o is IMyInterface;
}

Since IMyInterface has only one implementation (monomorphic cases aren't rare) we can optimize isinst to basically o is MyProgram` and inline the check.

Current NativeAOT codegen:

; Method Program:IsMyInterface(System.Object):bool
       4883EC28             sub      rsp, 40
       488BD1               mov      rdx, rcx
       488D0D00000000       lea      rcx, [(reloc 0x40000000004234f8)]
       E800000000           call     CORINFO_HELP_ISINSTANCEOFINTERFACE
       4885C0               test     rax, rax
       0F95C0               setne    al
       0FB6C0               movzx    rax, al      
       4883C428             add      rsp, 40
       C3                   ret      
; Total bytes of code: 33

Expected NativeAOT codegen:

; Method Program:IsMyInterface(System.Object):bool
       4885C9               test     rcx, rcx
       740E                 je       SHORT G_M18292_IG05
       488D0500000000       lea      rax, [(reloc 0x40000000004221d8)]
       483901               cmp      qword ptr [rcx], rax
       7402                 je       SHORT G_M18292_IG05
       33C9                 xor      rcx, rcx
G_M18292_IG05:
       33C0                 xor      eax, eax
       4885C9               test     rcx, rcx
       0F95C0               setne    al
       C3                   ret      
; Total bytes of code: 28

cc @MichalStrehovsky

Author:	EgorBo
Assignees:	-
Labels:	tenet-performance, area-CodeGen-coreclr, untriaged
Milestone:	-