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

[Richard Biener <richard.guenther@gmail.com>]

On Sun, Dec 11, 2022 at 1:02 PM Jonathan Wakely <jwakely.gcc@gmail.com> wrote:
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> On Sun, 11 Dec 2022, 09:12 Richard Biener, <richard.guenther@gmail.com> wrote:
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>> On Sat, Dec 10, 2022 at 7:45 PM Andrew Pinski via Gcc <gcc@gcc.gnu.org> wrote:
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>> > On Sat, Dec 10, 2022 at 10:36 AM Jonathan Wakely via Gcc
>> > <gcc@gcc.gnu.org> wrote:
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>> > > 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:
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>> > > > 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.
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>> 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.
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> 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.
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> But I don't see what start_lifetime_as has to do with the original problem anyway. The placement new expression will start lifetimes:
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> page* pages = new (storage) page[NUM_PAGES];
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> 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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