Re: 64bit on 32bit

[Doug B. <dou...@ya...>]

John:

--- John Tromp <Joh...@cw...> wrote:

> dear Judy developers,
> 
> first of all a big thanks to your wonderful efforts
> that have put high performance memory efficient trees within
> easy reach.

Thanks.

> 
> i have used Judy to great effect in computing the
> Number of legal Go positions, as you can see at
> 
>    http://www.cwi.nl/~tromp/go/legal.html

A little beyond me, I never played go.

> 
> My keys and values are both naturally 64 bit integers, so
> JudyL is a perfect fit.

I take it for 64 bit machine only.

> However, I'd love to run the same code on some 32-bit servers.
> With a long long declaration, you can get 64 bit values on such
> machines, even though pointers remain 32 bits.
> Is it possible to make a version of Judy that runs on 32 bit machines
> while providing the 64 bit long long interface?

Realistically, No.  But if you use JudyHS(), the keys are an arbitrary
number of bytes in length (so 64 bit keys would be 8 bytes).  The
values can be a pointer to an "array" of 8 bytes.  However, in a 64 bit
machine that would be a waste of a little memory (because the value and
pointer are 8 bytes).  It may be a bit tricky getting code to work on
both big and little endian machines.  (see below).

I suggest using JudyHS, if you do not need the "neighbor" search (I.E.
JudyLNext() and JudyLCount()) capability.

> 
> I think it should be possible in principle, although perhaps a huge
> undertaking, as Judy probably makes the assumption that pointer size
> equals long size. I wouldn't mind even if this version wastes
> space by using 64 bit pointers with the upper 32 bits all 0.

I wrote JudyHS() for people that want larger than 32 bit keys. 
Performance (memory and speed) is still very good.  But you open up
pandora's box when casting a "long" to a "char array" when the machine
can be either big or little endian.  As I recall, JudyHS solves the
problem by always moving the "long" to a byte array one byte at a time
and vise-versa. Modern machines do this amazingly quickly.

I hope this helps.

Doug Baskins

> 
> regards,
> 
> %!PS                       %  -John Tromp (http://www.cwi.nl/~tromp/)
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Doug Baskins <dou...@ya...>