std.allocator: false pointers

[Steven Schveighoffer via Digitalmars-d]

On Fri, 02 May 2014 14:00:18 -0400, Andrei Alexandrescu  
<SeeWebsiteForEmail at erdani.org> wrote:

> On 5/2/14, 10:33 AM, Steven Schveighoffer wrote:
>> On Fri, 02 May 2014 13:26:41 -0400, Steven Schveighoffer
>> <schveiguy at yahoo.com> wrote:
>>
>>> Why not keep the 3 states, but just treat unmarked blocks as free?
>>> Then the next time you go through tracing, change the bit to free if
>>> it was already marked.
>>
>> Sorry, if it was already *unmarked* (or marked as garbage).
>
> Yah, understood. Unfortunately I just realized that would require either  
> to keep the bits together or to scan two memory areas when trying to  
> allocate, both of which have disadvantages. Well, I guess I'll go with  
> the post-tracing pass. -- Andrei

What is the problem with keeping the bits together?

-Steve