Unsafe.As doesn't update type information in JIT

[EgorBo]

Noticed in #84370 (comment)

public class ClassA
{
    public virtual int GetVal() => 1;
}

public sealed class ClassB : ClassA
{
    public override int GetVal() => 42;
}


void GetValUnsafe(ClassA tc) => Unsafe.As<ClassB>(tc).GetVal();

Current codegen:

; Method GetValUnsafe(ClassA):this
       sub      rsp, 40
       mov      rcx, rdx
       mov      rax, qword ptr [rdx]
       mov      rax, qword ptr [rax+48H]
       call     [rax+20H]ClassA:GetVal():int:this
       nop      
       add      rsp, 40
       ret      
; Total bytes of code: 23

Expected codegen:

; Method GetValUnsafe(ClassB):this
       cmp      byte  ptr [rdx], dl
       ret      
; Total bytes of code: 3

The idea that Unsafe.As JIT intrinsic should spill its input to a local and set class for that local based on signature.

runtime/src/coreclr/jit/importercalls.cpp

Lines 3977 to 3985 in 042a350

	case NI_SRCS_UNSAFE_As:
	{
	assert((sig->sigInst.methInstCount == 1) || (sig->sigInst.methInstCount == 2));
	// ldarg.0
	// ret
	return impPopStack().val;
	}

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Issue Details

---

Noticed in #84370 (comment)

public class ClassA
{
    public virtual int GetVal() => 1;
}

public sealed class ClassB : ClassA
{
    public override int GetVal() => 42;
}


void GetValUnsafe(ClassA tc) => Unsafe.As<ClassB>(tc).GetVal();

Current codegen:

; Method GetValUnsafe(ClassA):this
       sub      rsp, 40
       mov      rcx, rdx
       mov      rax, qword ptr [rdx]
       mov      rax, qword ptr [rax+48H]
       call     [rax+20H]ClassA:GetVal():int:this
       nop      
       add      rsp, 40
       ret      
; Total bytes of code: 23

Expected codegen:

; Method GetValUnsafe(ClassB):this
       cmp      byte  ptr [rdx], dl
       ret      
; Total bytes of code: 3

The idea that Unsafe.As JIT intrinsic should spill its input to a local and set class for that local based on signature.

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