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Re: Proposal: block-based vector allocator

From: Stefan Monnier
Subject: Re: Proposal: block-based vector allocator
Date: Fri, 01 Jun 2012 13:40:38 -0400
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/24.1.50 (gnu/linux)

>>> -  return (p == m->start&&  m->type == MEM_TYPE_VECTORLIKE);
>>> +  if (m->type == MEM_TYPE_VECTORLIKE)
>>> +    {
>>> +      if (m->end - m->start == VECTOR_BLOCK_BYTES)
>> Please just use a new MEM_TYPE, so we don't need to worry about the
>> possibility of having a non-vector-block which just happens to have the
>> same size as a vector-block.

> I don't understand this.  Any block-allocated vector P0 has the size
> less than or equal to VECTOR_BLOCK_BYTES.  So, mem_find (P0) will
> always return mem_node M0 where M0->end - M0->start ==
> VECTOR_BLOCK_BYTES (IOW, such a mem_node always corresponds to
> a vector block).  If P1's size is greater than VECTOR_BLOCK_BYTES,
> including the case when it's equal to sizeof (vector_block), mem_find
> (P1) will always return it's own corresponding node M1 where P1 is
> equal to M1->start and M1-> end - M1->start is never equal to

I'm not saying the code is wrong.  I'm saying that its correctness
should be made more explicit by using a different MEM_TYPE for
vector blocks.  This way the correctness argument is trivial rather than
relying on some reasoning about which kind of vector allocation
is used in which circumstance.


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