[mercury-users] dynamic linking, Guile

Paul Bone pbone at csse.unimelb.edu.au
Tue Feb 9 13:41:22 AEDT 2010

On Tue, Feb 09, 2010 at 12:58:17PM +1100, Peter Wang wrote:
> On 2010-02-09, Zoltan Somogyi <zs at csse.unimelb.edu.au> wrote:
> > On 09-Feb-2010, Peter Wang <novalazy at gmail.com> wrote:
> > > The changes are not extensive and are not crucial; search for "mercury"
> > > (case insensitive) in the boehm_gc directory.
> > 
> > At least one change from the default is crucial. By default, boehm considers
> > only pointers to the first byte of a heap cell to be a reference to it,
> > but due to our use of tagged pointers, any version used with Mercury programs
> > has to consider any reference to any byte in the first word of heap cell
> > to be a reference to it.
> No, that is handled by calling GC_REGISTER_DISPLACEMENT(i) in
> mercury_wrapper.c.  It is a standard Boehm GC feature, since before
> version 2.2, judging by the change log.
> Peter
> P.S. I also came across the POINTER_MASK and POINTER_SHIFT macros.
> If set, Boehm GC will preprocess candidate pointers using those values.
> I wonder if there is any benefit to using that instead.

It could allow you to use high tag bits in a 64 bit pointer if you can prove
that your heap will never point into certain high addresses.

Current AMD64 chips support a 48 bit address space, meaning that the 16 high
order bits are unused (but must contain the same value is the 47th bit to be
valid addresses.[1] Additionally Linux (and probably most other 64bit OSs) use
a memory map divided in half by the 48th bit where the top half is kernel
memory that userspace can't address anyway, See below.  Therefore there are 16
high order bits available for our use in any pointer on Linux amd64.

0000000000000000 - 00007fffffffffff (=47 bits) user space, different per mm
hole caused by [48:63] sign extension
ffff800000000000 - ffffffffffffffff (=47 bits) kernel space. here be dragons.

    1. http://en.wikipedia.org/wiki/X86-64#Virtual_address_space_details

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