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

David Blaikie via llvm-dev llvm-dev at lists.llvm.org
Tue Nov 12 17:33:51 PST 2019

On Tue, Nov 12, 2019 at 2:36 PM Christoffer Lernö <christoffer at aegik.com>

> The advantages:
> 1. A pointer to the struct offset can be converted to a pointer without
> any cost.
> 2. A nullpointer to a stretchy buffer can be treated as a zero length
> array. Consequently no actual struct allocation is needed to represent a
> zero length array.

(2) could probably be done as well with the other representation (with the
prefix data representation you'd still have to special case the null test
before going backwards from the pointer to find the size - because you
wouldn't want to go backwards from null and try to read bytes there to find
the size).

> 3. A reference to the array is the same size as to a pointer.

(3) Would be true with either representation I'm picturing. (pointing to
the start of the struct you've described, rather tahn pointing to the
trailing array and walking backwards to find the rest)

> 4. It can be converted to and back from an pointer without losing any
> information about the size & capacity.

Converting back you have to know it's the start of an array, though, right?
(& you could still do that in the other representation - by subtraction,
but yes, wouldn't be free/zero-cost)

> The downsides are what we discuss. But it looks like I have to accept that
> I can only represent it as a pointer with unknown length in DWARF then?

I imagine it'd be difficult to describe the calling convention for passing
this array? Are you going to have instances of this on the stack and passed
by value? or only ever individually dynamically allocated & pointed to from
other places?

If they were individually dynamically allocated & only ever pointed to the
trailing array - the DWARF for that'd be pretty simple - basically just a
special pointer type. The pretty printer for it would know it was allowed
to walk backwards to find some extra things (as would any member functions,
if needed, be implemented in terms of walking the poniter back).

- dave

> Best Regards,
> Christoffer
> On 12 Nov 2019, at 21:49, David Blaikie <dblaikie at gmail.com> wrote:
> 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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