Re: Bug with GCC's handling of lifetimes of implicit-lifetime types

[Gavin Ray <ray.gavin97@gmail.com>]

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Ahh alright, thanks Jonathan & Andrew, appreciate the replies

@Jonathan
> ... "Without a proper implementation of start_lifetime_as (which GCC
doesn't provide yet)"

I mailed the author of that proposal yesterday after learning about it
(it's very useful!) and they told me as much
Had written them to ask about a naive implementation I tried to see if I
understood it correctly:
- P2590R2: "Explicit lifetime management" (start_lifetime_as)
implementation (github.com)
<https://gist.github.com/GavinRay97/b69cb6ebab6a0e13d2cfe74a6ed7cdc6>


>
>
>
> *"Your code copies bytes around. The compiler *might* optimise that away
> but I'mnot sure. The idea of start_lifetime_as is that it would compile
> down to noinstructions - it would always be a no-op at runtime. My
> understanding is thatit cannot be implemented by the user in C++, the
> implementation would have to bea "magic" function using compiler
> intrinsics."*


About the memmove thing, based on a Godbolt link I found from Google -- I
think it's relying on UB to initialize storage + lifetime?
- Compiler Explorer (godbolt.org) <https://godbolt.org/z/VuRtNC>

But I don't pretend to understand the technicalities of the above, ha.

@Andrew

Would you be willing to attempt to explain the linked issue to someone not
familiar with the details of C++'s object storage + lifetime model?
Seems like an interesting thing but I don't have the technical
background/context to understand the full discussion happening there.






On Sat, Dec 10, 2022 at 1:44 PM Andrew Pinski <pinskia@gmail.com> wrote:

> On Sat, Dec 10, 2022 at 10:36 AM Jonathan Wakely via Gcc
> <gcc@gcc.gnu.org> wrote:
> >
> > On Sat, 10 Dec 2022 at 17:42, Gavin Ray via Gcc <gcc@gcc.gnu.org> wrote:
> > >
> > > This came up when I was asking around about what the proper way was to:
> > >
> > > - Allocate aligned storage for a buffer pool/page cache
> > > - Then create pointers to "Page" structs inside of the storage memory
> area
> > >
> > > I thought something like this might do:
> > >
> > > struct buffer_pool
> > > {
> > >   alignas(PAGE_SIZE) std::byte storage[NUM_PAGES * PAGE_SIZE];
> > >   page* pages = new (storage) page[NUM_PAGES];
> > > }
> > >
> > > Someone told me that this was a valid solution but not to do it,
> because it
> > > wouldn't function properly on GCC
> > > They gave this as a reproduction:
> > >
> > > https://godbolt.org/z/EhzM37Gzh
> > >
> > > I'm not experienced enough with C++ to grok the connection between this
> > > repro and my code,
> >
> > Me neither. I don't think there is any connection, because I don't
> > think the repro shows what they think it shows.
> >
> > > but I figured
> > > I'd post it on the mailing list in case it was useful for others/might
> get
> > > fixed in the future =)
> > >
> > > They said it had to do with "handling of lifetimes of implicit-lifetime
> > > types"
> >
> > I don't think that code is a valid implementation of
> > start_lifetime_as. Without a proper implementation of
> > start_lifetime_as (which GCC doesn't provide yet), GCC does not allow
> > you to read the bytes of a float as an int, and doesn't give you the
> > bytes of 1.0f, it gives you 0.
> >
> > https://godbolt.org/z/dvncY9Pea works for GCC. But this has nothing to
> > do your code above, as far as I can see.
>
> See https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107115#c10 for what
> is going wrong.
> Basically GCC does not have a way to express this in the IR currently
> and there are proposals there on how to do it.
>
> Thanks,
> Andrew Pinski
>