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:	-