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

[Jonathan Wakely <jwakely.gcc@gmail.com>]

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.