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

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

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@Richard

That's some intense code, I appreciate the code-samples and explanation,
thank you =)

@Jonathan

Maybe there was some misunderstanding?
I didn't make the connection either but I also don't know that much about
C++

It seems like that expression is valid then? Good to know =)

As a random aside if I may -- what is the difference between placement-new
of pointers in
std::byte storage, and making a std::span over the storage area?

std::byte storage[PAGE_SIZE * NUM_PAGES];

// A)
page* pages = new (storage) page[NUM_PAGES];
// B)
std::span<page, NUM_PAGES> pages_span(pages, NUM_PAGES);


On Sun, Dec 11, 2022 at 8:31 AM Richard Biener <richard.guenther@gmail.com>
wrote:

> On Sun, Dec 11, 2022 at 1:02 PM Jonathan Wakely <jwakely.gcc@gmail.com>
> wrote:
> >
> >
> >
> > 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.
>
> That's true, and that should work, not sure what the problem should be
> here.
>
> Richard.
>
> >
> >
>