Ahh alright, thanks Jonathan & Andrew, appreciate the replies @Jonathan > ... "Without a proper implementation of start_lifetime_as (which GCC doesn't provide yet)"
I mailed the author of that proposal yesterday after learning about it (it's very useful!) and they told me as much Had written them to ask about a naive implementation I tried to see if I understood it correctly: - P2590R2: "Explicit lifetime management" (start_lifetime_as) implementation (github.com) <https://gist.github.com/GavinRay97/b69cb6ebab6a0e13d2cfe74a6ed7cdc6> > > > > *"Your code copies bytes around. The compiler *might* optimise that away > but I'mnot sure. The idea of start_lifetime_as is that it would compile > down to noinstructions - it would always be a no-op at runtime. My > understanding is thatit cannot be implemented by the user in C++, the > implementation would have to bea "magic" function using compiler > intrinsics."* About the memmove thing, based on a Godbolt link I found from Google -- I think it's relying on UB to initialize storage + lifetime? - Compiler Explorer (godbolt.org) <https://godbolt.org/z/VuRtNC> But I don't pretend to understand the technicalities of the above, ha. @Andrew Would you be willing to attempt to explain the linked issue to someone not familiar with the details of C++'s object storage + lifetime model? Seems like an interesting thing but I don't have the technical background/context to understand the full discussion happening there. On Sat, Dec 10, 2022 at 1:44 PM Andrew Pinski <pins...@gmail.com> 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. > > Thanks, > Andrew Pinski >
