Array append proposal

[Steven Schveighoffer]

"Sergey Gromov" wrote
> Fri, 10 Oct 2008 11:11:40 -0400,
> Steven Schveighoffer wrote:
>> First, store the allocated length of the array on the heap along with the
>> array data.  On a 32-bit system, the size will always be 4 bytes, but 
>> there
>> can be padding bytes added to keep the alignment for the first element in
>> the array correct.  The maximum padding, I am not sure about.  I remember
>> people saying that for certain operations, 16 byte alignment is required?
>> Or is it just 8?  So for instance an array of doubles, there would be an
>> 8-byte field storing the length in the first 4 bytes, and padding in the
>> second 4 bytes.
>>
>> Perhaps someone with more knowledge of alignment requirements can 
>> comment.
>> I'd also appreciate some insight on how this would look on a 64 bit 
>> system.
>>
>> The final detail is how to tell whether the element before an array is 
>> the
>> length or not.  I propose to use the most significant bit in the array
>> length field.  This prevents having arrays of > 2GB, but that's probably
>> impossible anyways ;)
>>
>> If this bit is set, then the array can be appended to in place if its 
>> length
>> equals the length stored in the heap (and of course, the memory block
>> contains enough space).  If not, then a new array is allocated, and the 
>> data
>> copied, the length stored in the heap is left alone.
>
> How do you implement a slice?  It points in the middle of another array,
> data around that point is arbitrary.  There is no way you can check if
> you are at the beginning of an array or not.

If the first element of the slice is not the first element of the allocated 
array, then it does not have its 'at the beginning' bit set in its local 
length.

-Steve