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

Gavin Ray ray.gavin97@gmail.com
Sun Dec 11 13:44:15 GMT 2022


Whoops, the last line should be pages_span(storage, ...)

On Sun, Dec 11, 2022 at 8:38 AM Gavin Ray <ray.gavin97@gmail.com> wrote:

> @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.
>>
>> >
>> >
>>
>


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