Help with DMD internals

[Steven Schveighoffer]

On 5/21/18 6:37 PM, Manu wrote:
> On 21 May 2018 at 15:29, Steven Schveighoffer via Digitalmars-d
> <digitalmars-d at puremagic.com> wrote:
>> On 5/21/18 6:18 PM, Manu wrote:
>>>
>>> On 21 May 2018 at 06:10, Steven Schveighoffer via Digitalmars-d
>>> <digitalmars-d at puremagic.com> wrote:
>>>>
>>>> On 5/20/18 9:49 PM, Manu wrote:
>>>>>
>>>>>
>>>>> On 20 May 2018 at 17:14, Walter Bright via Digitalmars-d
>>
>>
>>>>>> Yes, and only for delete.
>>>>
>>>>
>>>>
>>>> Why? This doesn't make a lot of sense, since delete is freeing the
>>>> memory,
>>>> it shouldn't matter what state the memory is left in. I would argue that
>>>> should only be done in debug mode, and actually, I wonder if some other
>>>> kind
>>>> of sentinel memory pattern should be written instead.
>>>>
>>>>>
>>>>> destroy() also seems to do it.
>>>>>
>>>>
>>>> Yes, because destroy leaves the memory available for reuse. This is
>>>> intentional.
>>>
>>>
>>> It's uninitialised memory though, Wouldn't the same argument you made
>>> above also apply?
>>
>>
>> Uninitialized, but allocated and usable. The difference between this and
>> delete is that delete is going to unallocate that memory. The next time it's
>> allocated, it will be overwritten with an init pattern for the new type.
>>
>> Basically, in D when you have access to memory, it should be in a valid
>> state.
> 
> Why is it reasonable to expect that the buffer after `destroy()`
> should be 'valid'?
> Who's to say that the init state is 'valid'?

It's valid in that it's not garbage, and that it's not full of dangling 
pointers.

> 
> Surely it's not reasonable to just start using a class after calling
> destroy(), as if it's a newly constructed class?

No, of course not. For classes, actually the vtable is zeroed out, so 
any usage gets a segfault. You'd have to fix that before using again. 
But potentially, yes, you can use it if you call a ctor on it.

Fun fact: originally destroy (clear at the time) was going to call the 
default ctor on a class instance so it could be used again.

> Surely we should expect that dead buffer is emplaced with a new construction?
> 
>> This is a valid usage in D:
>>
>> struct S
>> {
>>     this(...) {...}
>> }
>>
>> auto s = S(a, b, c);
>> destroy(s);
>>
>> // s is now a fully initialized S ready to be constructed
>> s.__ctor(a, b, c);
> 
> And you've demonstrated exactly what I'm saying... there's no need for
> destroy() to return initialised memory here. It could return
> 0xfeeefeee memory or something (for debugging purposes).

No, the ctor assumes all values are in init state. If that's not the 
case, then the code will crash or corrupt memory.

-Steve