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

[Andrew Pinski <pinskia@gmail.com>]

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