Re: Segment registers support for i386

["RÃmy Saissy" <remy dot saissy at gmail dot com>]

> Ross Ridge wrote:
> > Remy Saissy wrote:

> > What I understand is that there is two kind of managment for attribute :

> Attributes are handled in various different ways depending on what the
> attribute does.  To handle your case correctly, you'ld have to change how
> the tree to RTL conversion generates RTL addresses expressions whenever
> a pointer with the "far" attribute is dereferenced.  This is probably
> going to be a lot work.

I've looked for a target specific callback to modify but I've found
nothing, even in the gcc internals info pages. Do you mean I would
have to modify some code outside of the i386 directory ? Or maybe to
add such a callback if it doesn't exist ;)
In which file does the tree to RTL conversion code is located ?

> > Therefore, I can consider the following relationship:
> >  (mem:SI (plus:SI (unspec:SI [(reg:HI fs)] SEGREF) (reg:SI var)))
> >       |      |                    |                    |
> >      \/     \/                   \/                   \/
> >      int *                 __attribute__((far("fs")))                      p;

> No, that's not what the RTL expression represents.  Declarations aren't
> represented in RTL.  The example RTL expression I gave is just an
> expression, not a full RTL instruction.  It's something that could be
> used as the memory operand of an instruction.  The RTL expression I gave
> would correspond to a C expression (not a statement) like this:

> 	*(int * __atribute__((far("fs")))) var

Ok, I thanks for the precisions :)

> > does (reg:HI fs) care about the type of the parameter fs ?

> See the GCC Internals documentation.  In my example, since I don't know
> what the actual hard register number you assigned to the FS segment
> register, I just put "fs" in the place where the actual register number
> would appear.  Similarily, the "var" in  "(reg:SI var)" represents
> the number of the pseudo-register GCC would allocate for an automatic
> variable named "var".

Does it mean that an RTL expression which use reg: force gcc to use a
particular pseudo
register ? Is there something I should care about when I choose the
number of this pseudo
register ?

> > how does gcc recognize such an expression ?

> Since this expression is a memory operand, it's recognized by the
> GO_IF_LEGITIMATE_ADDRESS() macro.  In the i386 port, that's implemented
> by legitimate_address_p() in "i386.c".

Ok thank you very much.

> 					Ross Ridge


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RÃmy Saissy            JabberID: remy.saissy@jabber.fr
                               Web:     http://remysaissy.free.fr
"L'homme qui a le plus vÃcu n'est pas celui qui a comptà le plus d'annÃes,
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J.-J. Rousseau, Emile.