Re: [Adobe-source-devel] any_regular_t and capacity

[Sean P. <sea...@ma...>]

I don't think anyone has looked into this - you could do this in the  
case the new type is smaller - if you want to go back up then you  
would also need to store the size and that would break ABI  
compaticility (you'd need a version_2 any_regular_t...).

I think reserve() would be tricky because of the small object  
optimization and ABI compatibility. Most of the cases I dealt with  
were small objects (dictionary, adobe::vector, string_t, basic  
types...). Being a small object is one advantage adobe::vector has  
over std::vector.

Sean


On Jan 20, 2010, at 3:15 PM, Florin Trofin wrote:

> Hello,
>
> I know any_regular_t has in-object allocation for types smaller than  
> 8 bytes (64 bits). Larger types are allocated on the heap. Consider  
> this scenario:
>
> BigObject obj; // 56 bytes->heap
> any_regular_t value(obj);
> [...]
> SmallerObject obj2; // 24 bytes
> value.assign(obj2); // destroys current object and re-allocates
>
> I looked at how any_regular_t::assign is implemented and it tries to  
> preserve its capacity if the new type is the same as the existing  
> one. If not, it destroys the guts of the current object and re- 
> allocates.
>
> Since heap allocations are expensive and a major bottleneck for  
> scalability I was wondering if anybody considered worth implementing  
> an optimization that checks if the current capacity is enough when  
> re-assigning. A reserve() and clear() methods would be a nice bonus.
>
> Best regards,
>
> Florin
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