Redundant write barriers around byref-like structs #111575
Description
Reported in #111127 (comment) by @NinoFloris
public class Program
{
object _obj = new();
Test M0() => new Test() { Obj = _obj, Obj1 = _obj, Obj2 = _obj };
void M1(ref Test ret) => ret = new Test() { Obj = _obj, Obj1 = _obj, Obj2 = _obj };
void M2(out Test ret) => ret = new Test() { Obj = _obj, Obj1 = _obj, Obj2 = _obj };
public ref struct Test
{
public object Obj;
public object Obj1;
public object Obj2;
}
}Currently all three M0, M1, M2 emit redundant write barriers, although, they're not needed due to "ByRefLike" modifier on Test struct (meaning it can live only on stack):
; Assembly listing for method Program:M0():Program+Test:this (FullOpts)
G_M000_IG01:
push rsi
push rbx
mov rbx, rdx
G_M000_IG02:
mov rsi, gword ptr [rcx+0x08]
mov rdx, rsi
mov rcx, rbx
call CORINFO_HELP_CHECKED_ASSIGN_REF
lea rcx, bword ptr [rbx+0x08]
mov rdx, rsi
call CORINFO_HELP_CHECKED_ASSIGN_REF
lea rcx, bword ptr [rbx+0x10]
mov rdx, rsi
call CORINFO_HELP_CHECKED_ASSIGN_REF
mov rax, rbx
G_M000_IG03:
pop rbx
pop rsi
ret
; Total bytes of code 50
; Assembly listing for method Program:M1(byref):this (FullOpts)
G_M000_IG01:
push rsi
push rbx
mov rbx, rdx
G_M000_IG02:
mov rsi, gword ptr [rcx+0x08]
mov rdx, rsi
mov rcx, rbx
call CORINFO_HELP_CHECKED_ASSIGN_REF
lea rcx, bword ptr [rbx+0x08]
mov rdx, rsi
call CORINFO_HELP_CHECKED_ASSIGN_REF
lea rcx, bword ptr [rbx+0x10]
mov rdx, rsi
call CORINFO_HELP_CHECKED_ASSIGN_REF
nop
G_M000_IG03:
pop rbx
pop rsi
ret
; Total bytes of code 48
; Assembly listing for method Program:M2(byref):this (FullOpts)
G_M000_IG01:
push rsi
push rbx
mov rbx, rdx
G_M000_IG02:
mov rsi, gword ptr [rcx+0x08]
mov rdx, rsi
mov rcx, rbx
call CORINFO_HELP_CHECKED_ASSIGN_REF
lea rcx, bword ptr [rbx+0x08]
mov rdx, rsi
call CORINFO_HELP_CHECKED_ASSIGN_REF
lea rcx, bword ptr [rbx+0x10]
mov rdx, rsi
call CORINFO_HELP_CHECKED_ASSIGN_REF
nop
G_M000_IG03:
pop rbx
pop rsi
ret
; Total bytes of code 48Expected:
no CORINFO_HELP_CHECKED_ASSIGN_REF calls in the codegen.