Re: are double cells safe to use?

[Mark H Weaver]

Roland Orre <address@hidden> writes:

> I consider using double cells for efficient implementation of binary trees.
>
> Is this safe? (i.e. no pre-assumptions made somewhere what these are used for)
>
> For prototyping I intended to add
> double-cons
> set-cbr!
> set-ccr!
> cbr
> ccr
>
> to the scheme level. Is this safe?

No, this won't work.  First of all, double cells cannot be used to store
four SCM values, because the first word of a double cell has to contain
the type tag.  Pairs avoid the type tag by the clever hack: the low bit
of every SCM value is 0, so if the first word of a heap object has 0 in
the low bit, that heap object is assumed to be a pair.  But this means
that every other heap object must start with a word whose low bit is 1.

So the best you could do with a double cell would be to store three SCM
objects, which is no better space efficiency than you already get with a
vector of size 3 (in the master branch, which will become Guile 2.2).

Another problem with creating yet another new fundamental data type is
that in order to use it efficiently, we'd need to create more VM
instructions to access it.  That means more opcodes from our limited
8-bit opcode space, and more code in the VM, which is bad because
ideally a VM should be compact for good cache behavior.

I think you should just use records, or maybe vectors, for this.

     Mark