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Re: Immediate doubles (up to 2^256) and rationals coming to Guile 3

From: Ludovic Courtès
Subject: Re: Immediate doubles (up to 2^256) and rationals coming to Guile 3
Date: Tue, 11 Jun 2019 10:39:38 +0200
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/26.2 (gnu/linux)


Mark H Weaver <address@hidden> skribis:

> Ludovic Courtès <address@hidden> writes:


>> IIUC, your plan is to have a different tagging on 32-bit platforms,
>> without fixflos, right?  I’m curious to see how much complexity would
>> entail from that.
> Yes, although I'm avoiding the term "fixflos" because IEEE doubles are
> also fixed width, and thus the term "fixflos" wouldn't adequately
> distinguish them from IEEE doubles.


> Anyway, I agree that it's inconvenient to have different tags on
> different targets, and I've been working to minimize the differences.
> At present, I'm currently implementing an alternative strategy where
> pairs are tagged in their pointers instead of in their CARs, which
> enables us to separate the heap tags and immediate tags into two
> independent spaces.

At first this sounds rather radical :-), but maybe it’s preferable this

> In this new approach, the heap tags are left unchanged, and the only
> tags that vary with target word size are the fixints, fixrats, iflos,
> and pair pointers.  All other tags will be uniform across targets,
> including the non-number immediates.  Here's the new version:
> ;; /* with iflos:   xxx:  iflo (000 < xxx < 110)
> ;;    (64-bit)     0111:  fixrat
> ;;                 1111:  fixnum
> ;;                 0110:  pair
> ;;                  000:  tagged heap object (thob)
> ;;             tttt1110:  other immediate
> ;;
> ;; without iflos:     1:  fixnum
> ;;    (32-bit)      010:  fixrat
> ;;                  100:  pair
> ;;                  000:  tagged heap object (thob)
> ;;             tttt1110:  other immediate
> This new approach brings its own complications, mainly two:
> (1) It breaks the long-standing assumptions in Guile that all
>     non-immediates have a tag in their first word and that pointers are
>     always untagged.  In my preliminary patch, I introduce a new concept
>     called a "tagged heap object" or "thob", and most existing checks
>     for SCM_NIMP or !SCM_IMP must be changed to use SCM_THOB_P.

Though an immediate, like a fixnum or an iflo, is still something
different from a tagged heap object like a pair, right?  So I would
expect SCM_THOB_P to be a different test, not a drop-in replacement for
SCM_NIMP, is that correct?

> (2) Our existing VM instructions almost invariably specify offsets with
>     a granularity of whole words.  To support tagged pair pointers with
>     good performance, I think we need a few new instructions that
>     specify byte offsets, to avoid the expensive extra step of removing
>     the tag before accessing the CAR or CDR of a pair.

So instead of a pointer dereference, SCM_CAR becomes mask + dereference,

I think we disable GC “interior pointer” scanning.  With this scheme, an
SCM for a pair would actually point in the middle of a pair; could this
be an issue for GC?

Thank you!


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