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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 14:21:55 +0200 |
User-agent: |
Gnus/5.13 (Gnus v5.13) Emacs/26.2 (gnu/linux) |
Hi,
Mark H Weaver <address@hidden> skribis:
> Ludovic Courtès <address@hidden> writes:
>> 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?
>
> That's right. It's not possible to create a drop-in replacement for
> SCM_NIMP, because it is being used to answer two different questions
> which used to be effectively equivalent, but no longer are:
>
> (1) Is X a pointer to a heap object with a heap tag in the first word?
> (2) Is X a reference to a heap object?
>
> Test (1) needs to be done before checking the heap tag, to implement
> type predicates for heap objects. Test (2) is needed in relatively few
> places, e.g. to decide whether to register disappearing links when
> adding an entry to a weak hash table.
>
> Actually, in my current branch I've removed the SCM_IMP and SCM_NIMP
> macros outright, because it seems to me they are likely to be misused.
>
> SCM_THOB_P implements test (1) and SCM_HEAP_OBJECT_P implements test (2).
I see.
> There's no masking involved. Rather, it is subtracted from the pointer,
> which allows the tag to be fused with the field offset. For example, on
> x86-64, whereas CAR and CDR were previously:
>
> 1c0: 48 8b 07 mov (%rdi),%rax ;old car
> and:
> 1d0: 48 8b 47 08 mov 0x8(%rdi),%rax ;old cdr
>
> Now they become:
>
> 1e0: 48 8b 47 fa mov -0x6(%rdi),%rax ;new car
> and:
> 1f0: 48 8b 47 02 mov 0x2(%rdi),%rax ;new cdr
Looks reasonable. :-)
> Fortunately, BDW-GC provides GC_REGISTER_DISPLACEMENT, which allows us
> to register a small offset K, such that BDW-GC should recognize pointers
> that point K bytes into a heap block. We've been using this in both 2.0
> and 2.2 from scm_init_struct (), and in 'master' it's done in
> scm_storage_prehistory ().
>
> This new approach entails registering one additional displacement.
Cool; if it’s just one displacement, that’s OK.
> What do you think?
It all looks perfectly reasonable to me!
Thanks for explaining,
Ludo’.
Re: Immediate doubles (up to 2^256) and rationals coming to Guile 3, Mark H Weaver, 2019/06/07