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

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

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On Sun, 11 Dec 2022, 09:12 Richard Biener, <richard.guenther@gmail.com>
wrote:

> On Sat, Dec 10, 2022 at 7:45 PM Andrew Pinski via Gcc <gcc@gcc.gnu.org>
> 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.
>
> I wouldn't call them "proposals" - basically the C++ language providing
> holes into the TBAA system is a misdesign, it will be incredibly difficult
> to implement this "hole" without sacrifying optimization which means
> people will complain endlessly why std::start_lifetime_as isn't a way
> to circumvent TBAA without losing optimization.
>

People already make holes in the type system, this just lets them do it
without UB. If it's not as fast as their UB, that's ok IMHO.

>

But I don't see what start_lifetime_as has to do with the original problem
anyway. The placement new expression will start lifetimes:

page* pages = new (storage) page[NUM_PAGES];

There's no need to mess with the type system here.