On Mon, Oct 18, 2010 at 11:31 AM, Jecel Assumpcao Jr.
<jecel at merlintec.com>wrote:

> We have had several discussions about the tags in Squeak. xxxxxx1 is a
> SmallInteger and xxxxxx00 is an object pointer, but xxxxxx10 is not
> generally used. In one of the previous discussions somebody did mention
> a very obscure case in which this is actually used, but I can't find
> this email right now and don't remember the details. I do remember that
> at the time it seemed to me that it wouldn't be hard to change this to
> free up the 10 tag for some other use.
>
> The most interesting use, in my opinion, would be to have unboxed
> floating point numbers. A very common reaction here to this is that we
> will all be moving to 64 bits anyway so it is not worth the trouble to
> make this change.  I might have agreed a few years ago, but today it
> looks like we will have to deal with 32 bit Squeak for quite a while
> yet. And with 4.2 introducing some important image format changes
> anyway, this might be the right time to revisit this issue.
>
> Boxed floats have the disadvantage of requiring an object allocation for
> every operation. Their great advantage is being 64 bits, even in 32 bit
> images. The unboxed floats would have to be 30 bits, not even 32 bits.
> So the fraction size goes from 53 bits to only 21. And while they would
> mostly speed things up, the current tag tests would have to go from two
> alternatives to three and that would slow them down. The extra tests
> would be in what is already a slow path anyway, so it wouldn't hurt as
> much as it would initially seem.
>
> Does anybody have any data related to this or any opinions?
>

I think immediate characters are much more generally useful, especially
considering unicode.  The current implementation of only codes 0 to 255
being == is error-prone.  The problem 30 bit floats have is that they're not
a useful subset of Float.  A 60-bit immediate float is a different beast
altogether.  You really need the number of bits to be sihnificanty greater
than the size of the mantissa.  With 30 bits it is still 23 bits short.

2¢
Eliot

>
> -- Jecel
>
>
>
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