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Re: [std-proposals] Proposal: std::obj_from_dmbr

From: Michael Scire <sciresm_at_[hidden]>
Date: Wed, 31 Aug 2022 23:46:38 -0700
My proposal paper has a use case that can't be solved via base members (for
intrusive lists, where an object can be in more than one list at once).

On Wed, Aug 31, 2022 at 11:36 PM Lénárd Szolnoki via Std-Proposals <
std-proposals_at_[hidden]> wrote:

> Hi,
> I also like the proposal, but keep in mind that you can already do this if
> your subobject is a base member instead of a non-static data member of the
> complete object. I don't know if this would be applicable for your use case
> though, this proposal does add some flexibility that is not there with base
> members.
> Cheers,
> Lénárd
> On 1 September 2022 05:35:13 BST, Lewis Baker via Std-Proposals <
> std-proposals_at_[hidden]> wrote:
>> I also ran into another use-case that I have for P2300-related
>> optimisations that I'd like to implement.
>> In my case, I am exploring whether it's possible to avoid child
>> operation_state objects from needing to store the receiver
>> if the child operation_state object is a sub-object of a parent
>> operation_state object and the receiver simply contains
>> a pointer to the parent operation_state (a very common situation).
>> In this case, we can instead just compute the receiver object on-demand
>> by calculating the address of the parent
>> operation_state object from the address of the child operation_state
>> object and can avoid needing to store the receiver
>> containing the pointer to the parent object.
>> This may seem a small win on an individual operation level, but if you
>> have complex sender-compositions where
>> there might be many-levels-deep nesting of operation-states all allocated
>> inline in a single top-level operation-state
>> then this can quickly add up to a saving of many pointer-sized receivers
>> (plus any additional padding necessary).
>> Further, when a leaf operation wants to look up some context on the
>> receiver (e.g. get_stop_token(r)) which then
>> forwards this query up the chain of parent receivers, this often results
>> in pointer-chasing.
>> e.g. one receiver contains a pointer to parent operation-state then loads
>> next receiver from that operation-state
>> which then needs to load the next receiver from the next operation-state
>> etc.
>> This means calling get_stop_token() on a leaf receiver when the
>> stop-token is provided 5 levels up will require
>> 5 sequential pointer-dereferences to obtain the stop-token.
>> If instead, we are able to calculate the address of the parent
>> operation-state as an offset from the child operation
>> state then getting the address of the top-level operation-state
>> containing the stop-token just becomes subtracting
>> the relative offset of each child-operation in its parent and applying
>> this successively to get the address of the
>> top-level operation-state. The compiler can then easily constant-fold
>> these offsets to reduce it to a single offset
>> from the leaf operation-state.
>> Applying this optimisation has the potential to both reduce the size of
>> operation-state objects, increasing cache
>> locality, reducing memory pressure, etc, but also to improve performance
>> of receiver queries by avoiding the
>> pointer-chasing stack-walk behaviour that composed operations currently
>> need to do.
>> For an example see https://godbolt.org/z/nKnYnfv4E
>> This optimisation relies on the ability to calculate the address of the
>> parent-object given the address of the sub-object,
>> however, which is not currently possible to do without invoking undefined
>> behaviour.
>> So consider this a +1 from me for adding something like this to the
>> standard.
>> Please also feel free to add this as additional motivation for the paper.
>> Let me know if there is anything I can do to help move this forward.
>> Cheers,
>> Lewis.
>> On Mon, Aug 22, 2022 at 9:07 AM Michael Scire via Std-Proposals <
>> std-proposals_at_[hidden]> wrote:
>>> I e-mailed std::proposals about this last year
>>> (std::parent_of_member/std::containing_object_of_member), and got as far as
>>> writing a proposal paper, though I got too busy to submit it as a
>>> P-numbered proposal/etc.
>>> See here:
>>> https://github.com/SciresM/containing_object_of_member/blob/master/containing_object_of_member.pdf
>>> I am still interested in submitting the above, and it's the same
>>> fundamental operation you're requesting here.
>>> On Sun, Aug 21, 2022 at 4:23 PM Phil Bouchard via Std-Proposals <
>>> std-proposals_at_[hidden]> wrote:
>>>> On 8/21/22 19:10, Walt Karas wrote:
>>>> Yes I'd bet big this technique is an often reinvented wheel.
>>>> Why do you prefer two static casts involving void * over a reinterpret
>>>> cast? Also, your C style cast is really a reinterpret cast. I think the
>>>> reason for having reinterpret cast is that it's a (greppable) warning that
>>>> something is dangerous. And this technique is fairly dangerous, mistakes
>>>> can elude static checking.
>>>> Yeah I just got the habit of not using any reinterpret_casts from the
>>>> Boost community standards, so it's just another way of writing them down.
>>>> And I wasn't sure how alignments and little to big endian machines
>>>> would convert them.
>>>> Sent from Yahoo Mail on Android
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>>>> On Sun, Aug 21, 2022 at 4:38 PM, Phil Bouchard via Std-Proposals
>>>> <std-proposals_at_[hidden]> <std-proposals_at_[hidden]>
>>>> wrote:
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Received on 2022-09-01 06:46:53