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From: Goswin von Brederlow <goswin-v-b@web.de>
To: Pascal Cuoq <Pascal.Cuoq@cea.fr>
Cc: caml-list@yquem.inria.fr
Subject: Re: [Caml-list] Alignment of data
References: <20100127161719.C6A10BC37@yquem.inria.fr>
	<F65E4E9D-6D00-4F37-8494-806E942E9028@cea.fr>
Date: Wed, 27 Jan 2010 22:01:51 +0100
In-Reply-To: <F65E4E9D-6D00-4F37-8494-806E942E9028@cea.fr> (Pascal Cuoq's
	message of "Wed, 27 Jan 2010 17:38:13 +0100")
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X-Spam: no; 0.00; alignment:01 cuoq:01 cuoq:01 alloc:01 mlsize:01 alignment:01 byte:01 condemns:01 compactions:01 arrays:01 gcc:01 byte:01 inserting:01 compaction:01 mfg:98 

Pascal Cuoq <Pascal.Cuoq@cea.fr> writes:

> Goswin von Brederlow <goswin-v-b@web.de> wrote:
>
>
>     You need to write a new function
>
>     CAMLextern value caml_alloc_double_array (mlsize_t),
>
>     or similar that ensures alignment on 8 byte for double even for 32bit
>     systems.
>
>     You should also check the CAMLextern value caml_copy_double (double);
>     that it does the same.
>
>
> If you decide to go this route, which this message
> neither endorses not condemns, you also need to
>
> A1/ allocate the doubles directly in the major heap, and
> A2/ deactivate compactions
>
> or
>
> B/ modify the garbage-collector.
>
> Pascal

Doubles are tagged with Double_tag and arrays of doubles with
Double_array_tag. So the GCC knows where doubles are.

Would it be hard to patch the allocation to leave a 4 byte gap in the
minor heap when needed to align doubles and patch the compation to do
the same?

The 4 bytes would mean inserting an Atom(0) during allocation and
compaction. Not the nicest way to do this but should be simple to patch
in.

MfG
        Goswin