NativeAOT: devirtualize isinst/castclass for monomorphic cases

[EgorBo]

Closes #80828

Luckily, we already have JIT-EE API to get exact list of implementations/subclasses for given type.

interface IMyInterface{} // this interface has a single implementation

class Program : IMyInterface
{
    static void Main()
    {
        Case1(new Program());
        Case2(new Program());
    }


    [MethodImpl(MethodImplOptions.NoInlining)]
    static bool Case1(object o) => o is IMyInterface;

    [MethodImpl(MethodImplOptions.NoInlining)]
    static IMyInterface Case2(object o) => (IMyInterface)o;
}

Previous NativeAOT codegen:

; Method Program:Case1(System.Object):bool
       sub      rsp, 40
       mov      rdx, rcx
       lea      rcx, [(reloc)]
       call     CORINFO_HELP_ISINSTANCEOFINTERFACE
       test     rax, rax
       setne    al
       movzx    rax, al
       add      rsp, 40
       ret      
; Total bytes of code: 33


; Method Program:Case2(System.Object):IMyInterface
       sub      rsp, 40
       mov      rdx, rcx
       lea      rcx, [(reloc)]
       call     CORINFO_HELP_CHKCASTINTERFACE
       nop      
       add      rsp, 40
       ret      
; Total bytes of code: 25

New NativeAOT codegen:

; Method Program:Case1(System.Object):bool
       test     rcx, rcx
       je       SHORT G_M13626_IG05
       lea      rax, [(reloc)]
       cmp      qword ptr [rcx], rax
       je       SHORT G_M13626_IG05
       xor      rcx, rcx
G_M13626_IG05:
       xor      eax, eax
       test     rcx, rcx
       setne    al
       ret      
; Total bytes of code: 28


; Method Program:Case2(System.Object):IMyInterface
       sub      rsp, 40
       mov      rdx, rcx
       mov      rax, rdx
       test     rax, rax
       je       SHORT G_M12806_IG05
       lea      rcx, [(reloc)]
       cmp      qword ptr [rax], rcx
       je       SHORT G_M12806_IG05
       mov      rcx, qword ptr [rsp+20H]
       call     CORINFO_HELP_CHKCASTINTERFACE ;; this is used to throw InvalidCastException only
G_M12806_IG05:
       nop      
       add      rsp, 40
       ret      
; Total bytes of code: 43

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Issue Details

---

Closes #80828

Luckily, we already have JIT-EE API to get exact list of implementations/subclasses for given type.

interface IMyInterface{} // this interface has a single implementation

class Program : IMyInterface
{
    static void Main()
    {
        Case1(new Program());
        Case2(new Program());
    }


    [MethodImpl(MethodImplOptions.NoInlining)]
    static bool Case1(object o) => o is IMyInterface;

    [MethodImpl(MethodImplOptions.NoInlining)]
    static IMyInterface Case2(object o) => (IMyInterface)o;
}

Previous NativeAOT codegen:

; Method Program:Case1(System.Object):bool
       sub      rsp, 40
       mov      rdx, rcx
       lea      rcx, [(reloc)]
       call     CORINFO_HELP_ISINSTANCEOFINTERFACE
       test     rax, rax
       setne    al
       movzx    rax, al
       add      rsp, 40
       ret      
; Total bytes of code: 33


; Method Program:Case2(System.Object):IMyInterface
       sub      rsp, 40
       mov      rdx, rcx
       lea      rcx, [(reloc)]
       call     CORINFO_HELP_CHKCASTINTERFACE
       nop      
       add      rsp, 40
       ret      
; Total bytes of code: 25

New NativeAOT codegen:

; Method Program:Case1(System.Object):bool
       test     rcx, rcx
       je       SHORT G_M13626_IG05
       lea      rax, [(reloc)]
       cmp      qword ptr [rcx], rax
       je       SHORT G_M13626_IG05
       xor      rcx, rcx
G_M13626_IG05:
       xor      eax, eax
       test     rcx, rcx
       setne    al
       ret      
; Total bytes of code: 28


; Method Program:Case2(System.Object):IMyInterface
       sub      rsp, 40
       mov      rdx, rcx
       mov      rax, rdx
       test     rax, rax
       je       SHORT G_M12806_IG05
       lea      rcx, [(reloc)]
       cmp      qword ptr [rax], rcx
       je       SHORT G_M12806_IG05
       mov      rcx, qword ptr [rsp+20H]
       call     CORINFO_HELP_CHKCASTINTERFACE ;; this is used to throw InvalidCastException only
G_M12806_IG05:
       nop      
       add      rsp, 40
       ret      
; Total bytes of code: 43

Author:	EgorBo
Assignees:	-
Labels:	area-CodeGen-coreclr
Milestone:	-

[am11]

Is Case2 regression avoidable?

[AndyAyersMS]

Is Case2 regression avoidable?

The code is bigger but should also be faster, since we will rarely need to call the helper (only if the cast will throw).

Though I'm not quite sure why we're shuffling those registers about like we do...

[MichalStrehovsky]

Curious: if we had hot/cold splitting support in NativeAOT (#77583), would that place the ;; this is used to throw InvalidCastException only in the cold region, or would that be more work?

[jakobbotsch]

Is there any reason this optimization shouldn't kick in for any other potential EE that returns 1 from getExactClasses?

[EgorBo]

@jakobbotsch purely throughput reasons - 2 ifs + non inlined call getExactClasses since I don't think it will be useful outside of NativeAOT anytime soon 🙂

[jakobbotsch]

Seems a bit premature? If I remove the NAOT check and hack SPMI to always return 0 on replay of getExactClasses I see no significant TP differences. IMO, as the general rule (no need to change this now) we should not be sacrificing abstractions unless they have demonstrable impact.

[EgorBo]

no strong opinion on that, will remove as part of some next PR

[EgorBo]

Curious: if we had hot/cold splitting support in NativeAOT (#77583), would that place the ;; this is used to throw InvalidCastException only in the cold region, or would that be more work?

It doesn't look like it's handled in hot/cold splitting now, but at least it's still considered as "cold" in jit