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

From: Lénárd Szolnoki <cpp_at_[hidden]>
Date: Thu, 01 Sep 2022 07:36:41 +0100

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.


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
>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
>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
>So consider this a +1 from me for adding something like this to the
>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.
>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.
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>>> On Sun, Aug 21, 2022 at 4:38 PM, Phil Bouchard via Std-Proposals
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Received on 2022-09-01 06:36:49