Wrong code generation for Unsafe.AsRef<T>, Read<T> when T is a structure #64802
Description
Description
Looks like JIT doesn't properly evaluate the size of a structure when emitting Unsafe.AsRef for structures less than 4 bytes consisting of a single primitive type field. I have structure, storing ushort in it and sometimes get AccessViolationException when trying to read it from the memory.
I created a gist in sharplab.io to illustrate the case.
Reproduction Steps
[StructLayout(LayoutKind.Sequential, Pack = 1)]
public struct ShortStruct
{
public short myValue;
}
public class Program {
public static unsafe T Get<T>(void* a) {
return Unsafe.AsRef<T>(a);
}
public unsafe static void Main() {
int a = 1;
var x = Get<ShortStruct>(&a);
Console.WriteLine(x);
}
}
Expected behavior
I expect 2 bytes will be read and the generated code be equivalent to
Program.Get[[System.Int16, System.Private.CoreLib]](Void*)
L0000: movsx rax, word ptr [rcx]
L0004: ret
Actual behavior
It actually tries to read 4 bytes
Program.Get[[ShortStruct, _]](Void*)
L0000: mov eax, [rcx]
L0002: ret
Regression?
Yes, if I switch the target to .NET Framework (x64) in that sharplab.io gist - everything is generating correctly
Program.Get[[System.Int16, mscorlib]](Void*)
L0000: mov rax, rcx
L0003: movsx rax, word [rax]
L0007: ret
Program.Get[[ShortStruct, _]](Void*)
L0000: mov rax, rcx
L0003: movsx rax, word [rax]
L0007: ret
Known Workarounds
No response
Configuration
At least .NET 6.0 (x64) is affected
Other information
No response