[llvm-dev] The best way of generating a good representation for an array with header?

[David Blaikie via llvm-dev]

On Tue, Nov 12, 2019 at 12:44 PM Christoffer Lernö via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> Yes, we’re actually viewing the struct at an offset.
>
> So basically it’s a struct like this:
>
> typedef struct {
>   uint32_t size;
>   uint32_t capacity;
>   int array[0];
> } Foo;
>
> The whole thing is malloc:ed with extra bytes at the end, and capacity is
> set to that same number of extra bytes.
>
> What’s then passed around is actually the int pointer at an offset:
> &(foo->array)
>
> Using the that pointer we can obviously in a simple way recover the
> pointer to the struct, but can it be done so that LLVM and DWARF can
> identify the pointer as a pointer to a struct member for a certain struct?
>
> std::vector is as far as I know wrapping a pointer or two.
>
> The advantage of a stretchy buffer is that its length is recoverable even
> if stored as a pointer.


What's the advantage compared to a pointer to the struct, rather than a
pointer to the array? (a pointer to this first element of the array would
still have to be tagged differently from a pointer to an arbitrary int
(either a singular int or an int somewhere in the array) to indicate that
you can backtrack to find the length - so it's not like you get to
generalize all int pointers) - I wouldn't expect (but don't know that much)
that the extra constant offset on array indexing would be particularly
expensive/observable?

But yeah, I think you'd probably have some trouble getting DWARF consumers
to handle the idea that the parameter type to a function is more than the
type itself, or that pointers to that type actually point into the middle
of the object instead of the start.

Not insurmountable, but seems a bit expensive/complicated to try to make
that work - but don't know what your other constraints/data are.


> It’s also incredibly thin, only taking up the same size as a pointer – as
> opposed to std::vector which is likely 2 pointers long.
>
>
> Best Regards,
>
> Christoffer
>
> > Date: Tue, 12 Nov 2019 11:34:42 -0800
> > From: David Blaikie via llvm-dev <llvm-dev at lists.llvm.org>
> >
> > the pointer points to the first element, and you walk backwards from
> there
> > to find the header details about the bounds/etc?
> >
> > In any case - I'd look at something like C++'s std::vector, which is a
> > variable length array, and model your situation similarly. I doubt
> there's
> > anything in particular you'll want to/be able to teach the optimizations
> > about your situation (nothing especially special that they know about
> > std::vector-like things either, that I know of - they maybe can deduce
> > certain things about how the bounds relate, and they certainly can
> optimize
> > a lot of std::vector usage) & debug info would probably look like
> > std::vector, in that it'd be a custom type, etc. Though if my guess above
> > was right about using prefix data to describe the bounds - that might be
> > hard to model in DWARF & you might be better off not being "tricky" like
> > that & modelling this closer to something that you could have written in
> C
> > or C++ more naturally.
> >
> > On Tue, Nov 12, 2019 at 4:14 AM Christoffer Lernö via llvm-dev <
> > llvm-dev at lists.llvm.org> wrote:
> >
> >> I’m considering building in variable arrays by implementing them as a
> >> stretchy buffer, that is a single allocation with header + elements with
> >> the pointer passed around pointing to the first element. (Example:
> >>
> https://www.gamasutra.com/blogs/NiklasGray/20180109/312683/Minimalist_container_library_in_C_part_1.php
> >> )
> >>
> >> Is there a good way to represent this in LLVM? I mean both in terms of
> >> helping the optimizer passes understand how the layout works and to make
> >> sure the debug info looks ok.
> >>
> >>
> >> Best regards,
> >> Christoffer
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>
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