Leak-detection of references to scoped class instances

[mipri]

On Monday, 25 November 2019 at 12:08:54 UTC, Per Nordlöw wrote:
> On Monday, 25 November 2019 at 08:22:02 UTC, Jacob Carlborg 
> wrote:
>> On Sunday, 24 November 2019 at 21:49:19 UTC, Per Nordlöw wrote:
>>> I guess we need a builtin language qualifier for scoped 
>>> classes for that to work, right?
>>
>> We have that:
>>
>> scope a = new Object;
>>
>> —
>> /Jacob Carlborg
>
> Ahh, nice.
>
> Does/Will DMD/GDC/LDC (in release builds) make the allocation 
> of `Object` happen on the stack?

class C {
     int x;
     this(int n) { x = n; }
}

int heap() {
     auto x = new C(2); // vgc: `new` causes a GC allocation
     return x.x;
}
/+
int example.heap():
         sub     rsp, 8
         mov     edi, OFFSET FLAT:example.C.__Class
         call    _d_newclass
         mov     DWORD PTR [rax+16], 2
         mov     eax, 2
         add     rsp, 8
         ret
+/

int stack() {
     scope x = new C(2); // (no vgc output)
     return x.x;
}
/+
int example.stack():
         sub     rsp, 40
         mov     rdi, rsp
         mov     QWORD PTR [rsp], OFFSET FLAT:example.C.__vtbl
         mov     QWORD PTR [rsp+8], 0
         mov     DWORD PTR [rsp+16], 2
         call    _d_callfinalizer
         mov     eax, 2
         add     rsp, 40
         ret
+/

gdc assembly. ldc eliminates the object entirely.