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Re: [std-proposals] Callee-cleanup, was Re: Relocation in C++

From: Gašper Ažman <gasper.azman_at_[hidden]>
Date: Tue, 1 Feb 2022 19:48:30 +0000
Sorry, no links, just some impressions from telecons.

On Tue, Feb 1, 2022, 19:46 Arthur O'Dwyer <arthur.j.odwyer_at_[hidden]> wrote:

> On Tue, Feb 1, 2022 at 12:37 PM Gašper Ažman via Std-Proposals <
> std-proposals_at_[hidden]> wrote:
>> For anyone considering callee-cleanup to be a desirable direction, please
>> know that the general thought is more towards caller-cleanup and that
>> callee-cleanup was a mistake.
>> There are disagreements on the issue, but I don't think any feature that
>> would end up mandating either of those will pass the committee. We're
>> having similar issues with contracts (albeit there, we're having issues
>> only on callee-cleanup arches).
> Got any links for those who want to find out more? I'm ignorant of any
> recent discussion.
> My impression has always been that callee-cleanup is annoying because only
> Microsoft does it; all "real" ABIs are caller-cleanup, and then there's
> this one funky ABI that destroys parameters in the "wrong" order and
> complicates the Standard.
> *But*, every time cleanup order pops into the zeitgeist, it's someone
> claiming that *callee*-cleanup (a.k.a. [[clang::trivial_abi]]) is
> strictly better for codegen, because it lets you eliminate more aliasing,
> turn more things into tail-calls (because now the caller's job is done at
> the `call` instruction — no cleanup to be done afterward), and so on.
> https://www.youtube.com/watch?v=rHIkrotSwcc&t=1303s (Chandler Carruth,
> 2019) for example.
> So I'm surprised to hear that *callee*-cleanup is the "mistaken" one.
> I absolutely agree (if needed) that no vendor is ever going to change
> their calling convention from caller to callee nor from callee to caller,
> so a proposal *mandating* either of those (at least for existing ordinary
> types, as opposed to some brand-new species of type that has never existed
> before) would be dead on arrival.
> –Arthur

Received on 2022-02-01 19:48:41