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Re: [std-proposals] Custom call convention per type

From: Jason McKesson <jmckesson_at_[hidden]>
Date: Sun, 7 Aug 2022 14:31:36 -0400
On Sun, Aug 7, 2022 at 10:21 AM Avi Kivity via Std-Proposals
<std-proposals_at_[hidden]> wrote:
> On 29/07/2022 21.42, Arthur O'Dwyer via Std-Proposals wrote:
> On Fri, Jul 29, 2022 at 2:26 PM Marcin Jaczewski <marcinjaczewski86_at_[hidden]> wrote:
>> pt., 29 lip 2022 o 20:20 Arthur O'Dwyer <arthur.j.odwyer_at_[hidden]> napisaƂ(a):
>> >
>> > On Thu, Jul 28, 2022 at 5:26 PM Marcin Jaczewski via Std-Proposals <std-proposals_at_[hidden]> wrote:
>> >>
>> >> Currently there are multiple problems linked to lifetime of function parameters.
>> >> Simply current default behavior does not fit all corner cases.
>> >>
>> >> Example is `std::unique` that generates subpar code because the call
>> >> site is a firewall that prevents the compiler to see at once destructor and move operation.
>> >
>> >
>> > Can you elaborate on this? I admit I've never thought about `std::unique` in this context before; but now that I have spent an hour or so thinking about it, I still don't see how its current behavior can be improved. One might rewrite `std::unique` to use swap instead of move-assignment, but one can't really rewrite it to use relocation because `std::unique` is never responsible for destroying anything. N objects go in, N objects come out (and in the same memory locations, too).
>> > https://p1144.godbolt.org/z/8v1Yj1eGK
>> >
>> > What would you do differently here?
>> >
>> Sorry, I mistyped, I mean type `std::unique_ptr` not algorythm `std::unique`.
> Aha. In that case, is your proposal any different from the existing attribute [[trivial_abi]]?
> https://quuxplusone.github.io/blog/2018/05/02/trivial-abi-101/
> https://libcxx.llvm.org//DesignDocs/UniquePtrTrivialAbi.html
> https://godbolt.org/z/f59T4Tev1
> ([[trivial_abi]] is vendor-specific, not part of the ISO standard; but if your proposal really just boils down to "Hey, WG21 should just standardize [[trivial_abi]]," then you should say that.)
> IMO, that's wrong. The standard leaves the ABI to the implementation. Standardizing [[trivial_abi]] means the standard acknowledges multiple ABIs, and requires the user to micro-manage the ABI of each type if they care about performance.
> The implementations should just switch to a better ABI - callee-destroy, and pass-by-register if it can prove no self-references are involved (e.g. by type-based alias analysis, trivial for std::unique_ptr).
> Yes, switching ABIs is a huge pain, but it has been managed in the past. gcc switched from reference-counted std::string to non-reference-counted std::string with SBO, and there were minor ABI updates when bugs were found in the implementation. The implementations are stuck in a deep hole, but [[trivial_abi]] makes that hole a little deeper.

It's important to understand the sheer *magnitude* of the ABI change
you're talking about compared to that of `std::string` in C++11.

The `std::string` change only affected your code if you used
`std::string` to communicate with code outside of your static
compilation/linking. And the thing is... there actually wasn't all
*that* much such code. That's not to say that there was none, but it
wasn't a huge amount. There are entire libraries that don't deal in
strings at their interface sites, or just don't use `std::string`
directly in interfaces.

Indeed, not using standard library types in interfaces is quite
common, specifically to avoid such ABI problems.

By contrast, the kind of ABI change you're talking about would
effectively break *every function* that happens to use *any* by-value
parameter of a user-defined type. That's a *lot* more code.

Now, I don't much care for "trivial ABI" as a feature. But the idea
that it would *ever* be at all practical to change every ABI on every
platform in such a way is absurd. No matter how much nicer it might
be, it is not going to happen. So we can either continue to see the
problem go unsolved or try to find actual functional solutions.

Received on 2022-08-07 18:31:45