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

[Christoffer Lernö via llvm-dev]

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. 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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