From: Marius Vollmer <mvo@zagadka.de>
Cc: guile-devel@gnu.org
Subject: Re: guile 1.8 and x86_64
Date: 09 May 2006 00:52:10 +0300 [thread overview]
Message-ID: <87k68wjib9.fsf@minimini.mvo.home> (raw)
In-Reply-To: <1147084130.23680.94.camel@localhost.localdomain>
Andy Wingo <wingo@pobox.com> writes:
> Hi,
>
> On Mon, 2006-05-08 at 00:29 +0300, Marius Vollmer wrote:
> > 137c137
> > < #define CELL_P(x) (SCM_ITAG3 (x) == scm_tc3_cons)
> > ---
> > > #define CELL_P(x) ((SCM_UNPACK(x) & (sizeof(scm_t_cell)-1)) == scm_tc3_cons)
>
> I don't really understand how the region can give you unaligned
> pointers, but I do confirm that this patch allows me to build guile.
Excellent!
Here is what is going on: the CELL_P predicate is used during the
conservative scanning of the GC to decide whether a random word can
possibly be a non-immediate SCM value. Non-immediate values are the
ones that point into the heap. The type tag for such a non-immediate
value is "lower three bits zero". On 32-bit architectures, a cell is
8 bytes, which means that a non-immediate value is always aligned to a
cell. On 64-bit machines, a cell is 16 bytes, and that means that a
word with "lower three bits zero" can still be invalid because it
points into the middle of a cell.
(We have similar check already for double-cells, which are 16 bytes on
32-bit machines.)
--
GPG: D5D4E405 - 2F9B BCCC 8527 692A 04E3 331E FAF8 226A D5D4 E405
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next prev parent reply other threads:[~2006-05-08 21:52 UTC|newest]
Thread overview: 16+ messages / expand[flat|nested] mbox.gz Atom feed top
2006-04-07 19:40 guile 1.8 and x86_64 Quanah Gibson-Mount
2006-04-10 8:14 ` Ludovic Courtès
2006-04-10 22:55 ` Neil Jerram
2006-04-10 23:03 ` Quanah Gibson-Mount
2006-04-11 20:37 ` Neil Jerram
2006-04-11 20:52 ` Quanah Gibson-Mount
2006-05-05 14:05 ` Miroslav Lichvar
2006-05-06 11:12 ` Miroslav Lichvar
2006-05-07 21:29 ` Marius Vollmer
2006-05-08 10:28 ` Andy Wingo
2006-05-08 21:52 ` Marius Vollmer [this message]
2006-05-09 6:49 ` Neil Jerram
2006-05-09 13:23 ` Andy Wingo
2006-05-09 22:31 ` Default stack limit Marius Vollmer
2006-05-10 14:31 ` Andy Wingo
2006-05-10 23:58 ` Kevin Ryde
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