Re: [RFC/RFA] gdb extension for Harvard architectures

[Michael Snyder <msnyder at cygnus dot com>]

Andrew Cagney wrote:
> 
> > Andrew Cagney <ac131313@cygnus.com> writes:
> >
> >> Would it be better if the cast operator, by default, preserved the
> >> address space of the pointer being cast?
> >
> >
> > That would get a bit hairy.  If I've got a value of the type:
> >
> >         (@code int * @code * @code * @code)
> >
> > --- that is, "a pointer in code space to a pointer in code space to a
> > pointer in code space to an int in code space" --- and cast it to
> >
> >         (int **)
> >
> > (note that I've dropped a layer of pointers here), how far down do we
> > go?  Does that become a `@code int * @code * @code'?  Or just a `int
> > * @code *'?  It's a bit weird.
> 
> You don't.
> 
> > I kind of think that casts should just work the normal way.  People
> > working on machines with separate address spaces have to think a
> > little harder --- I don't think we can really conceal that.
> 
> Remember we're talking about poerations that potentially cast a code
> pointer to a data or I/O pointer so this can't be described as normal.
> It is definitly way outside the bounds of the C language spec.
> 
> Any way, consider the intent of someone entering a sequence like:
> 
>         (gdb) x/w foo
>         0x0
>         (gdb) x/i foo
>         nop

Andrew, right now the "x" command has no knowledge about
separate code and data spaces, so both "x/w foo" and "x/i foo" 
will refer to the same address.  Whether that address comes
from code or data space depends entirely on the data type of foo.
If foo is a data-like object (eg. an int or an int pointer)
it will come from data space.  If foo is a function, it will come
from code space.  The purpose of the @code and @data modifiers
is to override this default.

>         (gdb) print/x *(int*)foo
>         0xdeadbeef

An (int *) is by default a data pointer.
A function pointer is by default a code pointer.

> vs
> (gdb) print/x *(int*)foo
>         0x0
> 
> and where should this go:
> 
>         (gdb) set *(int*)foo = 0xdeadbeef

This will go to data space.  Any "normal" data type will default to data 
space (as it already does).  The @code and @data modifiers are for when 
you want to alter the default behavior.

> This mysterious address switching strikes me as wierd.

You are right -- it's wierd.  ;-)