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Re: [std-proposals] Relocation in C++

From: Edward Catmur <ecatmur_at_[hidden]>
Date: Thu, 22 Dec 2022 19:02:34 +0100
On Thu, 22 Dec 2022 at 18:15, Sébastien Bini <sebastien.bini_at_[hidden]>

> On Thu, Dec 22, 2022 at 5:36 PM Edward Catmur <ecatmur_at_[hidden]>
> wrote:
>> On Thu, 22 Dec 2022 at 16:15, Sébastien Bini <sebastien.bini_at_[hidden]>
>> wrote:
>>> > If there is no guarantee that `reloc param;` would immediately destroy
>>> a value
>>> > parameter, would the (ugly?) implementation-defined fallback for
>>> caller-destroy
>>> > be that - if a move assignment operator is defined - immediately the
>>> object is
>>> > move-assigned to a temporary variable and that temporary variable is
>>> destroyed
>>> > in turn. And after the function call, the moved-out-object is
>>> destroyed by the
>>> > caller?
>>> See below.
>>> On Thu, Dec 22, 2022 at 11:38 AM Edward Catmur <ecatmur_at_[hidden]>
>>> wrote:
>>>> On Thu, 22 Dec 2022 at 11:11, Sébastien Bini <sebastien.bini_at_[hidden]>
>>>> wrote:
>>>>> That makes me wonder what would happen in the following code (I don't
>>>>> recall we ever took a definitive decision on that point):
>>>>> void do_something(std::lock_guard<std::mutex> guard)
>>>>> {
>>>>> if (!some_test())
>>>>> {
>>>>> reloc guard; /* attempt to release the lock early
>>>>> as the log function doesn't need it */
>>>>> log("thread " << std::this_thread::get_id() << " failed");
>>>>> return;
>>>>> }
>>>>> bar();
>>>>> }
>>>>> If the ABI is caller-destroy then `guard` destructor will forcibly be
>>>>> called at function exit.
>>>>> What would happen if `guard` were a non-function-parameter local
>>>>> variable, or the ABI callee-destroy? In other terms, should `reloc` offer
>>>>> any guarantee on when the destructor is called?
>>>>> If we want to keep `reloc` consistent in all situations, then `reloc
>>>>> x;` should never call the destructor of x (which will be destroyed normally
>>>>> at its end of scope). If this approach is taken then everyone is dragged
>>>>> down because of that ABI issue which only some have and that may be
>>>>> resolved in the future.
>>>>> This is a missed opportunity in my opinion. `reloc x;` should, when
>>>>> possible, call the destructor right away. That would allow developers to
>>>>> preemptively call the destructor of an object, without wrapping the object
>>>>> in an optional (or use unique_lock in our case). The language will keep
>>>>> track of the destruction state for us (especially is used in conditional
>>>>> branches), and this would no longer be a burden for the developer (which
>>>>> means less bugs).
>>>>> I believe that `reloc src`:
>>>>> - if `src` is a local object and not a function parameter, then
>>>>> `src` must be left in a destructed state (either because it was passed to a
>>>>> relocation constructor or by a direct call to its destructor) at the end of
>>>>> the expression evaluation.
>>>>> - otherwise (`src` is a function parameter passed by value) then
>>>>> when `src` is destroyed is implementation-defined. Typically, as soon as
>>>>> the ABI permits.
>>>>> In the last case, if a `reloc src;` statement is used and the ABI does
>>>>> not allow to call the destructor right-away, then compilers can still emit
>>>>> a warning.
>>>> I think this is too dangerous. If `src` is movable, then it's OK to
>>>> move-construct a temporary and defer destruction of the moved-from object
>>>> (compilers can warn on this if they feel like it). But if it is
>>>> relocate-only, and the ABI is caller-destroy on that parameter, `reloc src`
>>>> must be ill-formed.
>>> Yes, I have in mind to force callee-destroy ABI for functions that take
>>> relocate-only value parameters.
>>>> But we want that to work, so if compilers want to allow relocate-only
>>>> parameters to be caller-destroy, that must be a compiler extension, with
>>>> the default for such parameters being callee-destroy. There should probably
>>>> be an example showing this:
>>>> struct A { A() = default; A(A&&); ~A(); };
>>>> struct B { B() = default; B(B); ~B(); };
>>>> void f(A a, B b) {
>>>> reloc a; // may move and destroy at end
>>>> reloc b; // must destroy immediately
>>>> }
>>> I get what you mean, but B being relocate-only, `f` will get
>>> callee-destroy ABI so in your case `reloc a` won't defer the destructor
>>> call.
>> Possibly, but it would also be reasonable to have callee-destroy ABI
>> fine-grained on a per parameter basis, making behavior more consistent if a
>> relocate-only parameter is added or removed. I think we should leave that
>> to the implementation.
> I don't see why one would want it to be on a parameter basis, but I am
> okay with leaving it implementation-defined. The requirement then becomes
> that a function must be in charge of the lifetime of its relocate-only
> value parameters. Implementations are free to take any ABI decisions.
> Okay, things are getting confused for me so let's write things down.
>>> `reloc src` will turn `src` into a prvalue (a temporary).
>> Unless `src` is a reference. In that case, it will be turned into a
>> temporary lvalue or xvalue, depending on the type of reference.
> Yes.
> If the temporary is materialized, then it will be as follows:
>>> 1. If `src` is not a value parameter, or the ABI is callee-destroy,
>>> then the temporary is initialized using the relocation, move or copy
>>> constructor. If the last two are picked, then the destructor of the source
>>> object is called ;
>>> 2. Otherwise, the temporary is initialized using the move or copy
>>> constructor, and the destructor of the source object is deferred until the
>>> function exits.
>>> That being said, we don't know if the `reloc src;` statement should
>>> require the temporary materialization. Given today's rules, that would be
>>> yes. But it's really not necessary should we be in the first case: the
>>> destructor can simply be called.
>>> If no materialisation is needed, then we can write `reloc guard;` (with
>>> guard of type `std::lock_guard`), and it will simply destroy the guard,
>>> even so lock_guard has no copy, move or reloc constructor.
>>> I am in favor of not requiring the temporary materialization in that
>>> case. It will still be required in the second case (`src` is a value
>>> parameter and ABI is not callee-destroy).
>> Yes, definitely. There is the question of how to accomplish that: do we
>> special-case discarded-value relocation expressions, or do we allow (maybe
>> require) elision of relocate+destroy into destroy? The latter feels more
>> elegant, but it might mean that destruction is deferred until the end of
>> the full-expression, and also might mean that `reloc src` is ill-formed for
>> completely non-relocatable types (no copy, move or relocating constructor).
>> Whether that last is a problem or not I'm not sure; it might actually be a
>> benefit. (For example, it would simplify the rule for a function taking
>> `lock_guard` by value.)
> The second approach seems better. But what do you mean with "destruction
> is deferred until the end of the full-expression"? in `reloc src;` I expect
> the destructor to be called when the code reaches the next instruction. How
> is that an issue? Unless you have more complex expressions in mind?

Yes, such as `reloc guard, f();` - note the comma. I think here `f()`
should be protected by the guard.

I don't see the benefits of making it ill-formed for immovable types, while
> I clearly see some benefits of allowing it. A function taking a lock_guard
> by value needs not do anything different than before, since lock_guard has
> no relocation constructor. If the ABI allows it, then `reloc guard;` will
> call the destructor, otherwise it's ill-formed.

OK. I want to avoid making discarded-value expressions magic, but maybe we
can accomplish that by having the relocation occur (or potentially occur)
at temporary materialization, as you've suggested.

The other thing I want is for well-formedness of code (here `reloc guard;`)
to not be platform-dependent. So `reloc guard;` is always valid, but may
prevent linking a calling TU compiled by an old compiler or under an old

Having said that, things flow naturally for `reloc src;` :
>>> - if we are in the first case and materialisation is elided, then we
>>> simply call the destructor of src ;
>>> - if we are in the first case and materialisation is needed, then we
>>> materialize the temporary and destruct it. It has the same effect as
>>> destroying `src` directly, except that it's suboptimized and ill-formed for
>>> immovable objects ;
>>> - if we are in the second case, then we materialize the temporary
>>> and destruct it. The source object destruction is deferred until the
>>> function ends as it is the only thing we can do. Compilers should emit a
>>> warning in that case IMO.
>>> Hence, in my code snippet where I relocate a lock_guard value parameter
>>> (reloc guard;), the program is either ill-formed if the ABI is not
>>> callee-destroy or materialization is required (because lock_guard has no
>>> move or copy constructor to create a temporary), or else calls the
>>> destructor of the guard. And in:
>>> void do_something(std::mutex& m)
>>> {
>>> std::lock_guard guard{m};
>>> if (!some_test())
>>> {
>>> reloc guard;
>>> log("thread " << std::this_thread_get_id() << "failed");
>>> return;
>>> }
>>> bar();
>>> }
>>> `reloc guard;` would either be ill-formed if materialisation is
>>> required, or will simply call the destructor of the guard.
>> Yes; I'm leaning towards making it ill-formed, under the premise that
>> materialization is required but elided.

Received on 2022-12-22 18:02:46