On Thursday, 17 April 2014 at 17:35:17 UTC, bearophile wrote:
> Kagamin:
>
>> Bound checked version of variable size struct: http://dpaste.dzfl.pl/fcd91d6912d3
>
> I think you are missing one of the main points of a variable sized struct, that is to reduce by 1 the number of indirection levels.
>
> Bye,
> bearophile

Yeah, otherwise one could have just used dynamic array field.

On Thursday, 17 April 2014 at 17:35:17 UTC, bearophile wrote:
> Kagamin:
>
>> Bound checked version of variable size struct: http://dpaste.dzfl.pl/fcd91d6912d3
>
> I think you are missing one of the main points of a variable sized struct, that is to reduce by 1 the number of indirection levels.
>
> Bye,
> bearophile

As far as I can see, my implementation has only 1 indirection level.

On Thursday, 17 April 2014 at 17:45:22 UTC, Kagamin wrote:
> On Thursday, 17 April 2014 at 17:35:17 UTC, bearophile wrote:
>> Kagamin:
>>
>>> Bound checked version of variable size struct: http://dpaste.dzfl.pl/fcd91d6912d3
>>
>> I think you are missing one of the main points of a variable sized struct, that is to reduce by 1 the number of indirection levels.
>>
>> Bye,
>> bearophile
>
> As far as I can see, my implementation has only 1 indirection level.

And C-like dynamic structs have 0 indirection.

State* pointer in sokoban example is a perfect 1 indirection.

On Thursday, 17 April 2014 at 18:17:59 UTC, Kagamin wrote:
> State* pointer in sokoban example is a perfect 1 indirection.

It is not related to actual "dynamic struct" thing, which is why I have highlighted the line to look at. Minimal example is this:

struct Dynamic
{
    size_t length;
    int[0] payload;
}

void main()
{
    const length = 42;
    Dynamic* entity = alloca(Dynamic.sizeof + int.sizeof*length);
    entity.length = length;

    // pointer to same stack space so not really an indirection
    entity.payload[5] = 43;
}

How is this different from my example?

On Thursday, 17 April 2014 at 18:29:21 UTC, Kagamin wrote:
> How is this different from my example?

b = new byte[StateImpl.sizeof + CellIndex.sizeof*cellCount];

this line creates heap indirection

So you assert that variable length structs can't be allocated on heap and sokoban example is a wrong example of variable length struct usage?

And how heap indirection is different from stack indirection? It's still indirection.

On Thursday, 17 April 2014 at 17:50:30 UTC, Dicebot wrote:
> On Thursday, 17 April 2014 at 17:45:22 UTC, Kagamin wrote:
>> On Thursday, 17 April 2014 at 17:35:17 UTC, bearophile wrote:
>>> Kagamin:
>>>
>>>> Bound checked version of variable size struct: http://dpaste.dzfl.pl/fcd91d6912d3
>>>
>>> I think you are missing one of the main points of a variable sized struct, that is to reduce by 1 the number of indirection levels.
>>>
>>> Bye,
>>> bearophile
>>
>> As far as I can see, my implementation has only 1 indirection level.
>
> And C-like dynamic structs have 0 indirection.

Well, technically everything on the stack is accessed through an indirection (RBP + offset), but there are more optimisation opportunities if the base and/or offset is known statically. Performance-wise you're gaining nothing over a static array (or any alloca buffer) and potentially losing something compared to normal stack variables / struct members.

On Thursday, 17 April 2014 at 18:40:27 UTC, Kagamin wrote:
> And how heap indirection is different from stack indirection? It's still indirection.

Locality. The stack is (within reason) readily available in cache.