Re: IA32: printing FP register variables

[Michael Meissner <meissner@cygnus.com>]

On Thu, Jul 08, 1999 at 10:59:00PM -0600, Jeffrey A Law wrote:
> 
>   In message <000d01bec9c2$06f4fdb0$3404010a@metrowerks.com>you write:
>   > I would suggest that we may use a "negative" ST value.  The debugger can
>   > always know the depth of the stack from reading the status registers, so
>   > saying that something was in ST(7) could be interpreted as the top-most
>   > stack item, ST(6) as one below that, and so on.  As long as the relative
>   > position of items on the stack didn't change (this var is always 2 from the
>   > top), this should be OK.
> Unfortunately, I believe the relative positions of items does change.
> 
> Jim's brought up an interesting problem.
> 
> I believe dwarf2 can handle this in a straightforward manner -- it has the
> capability to say variable X is in location Y between points A & B and in
> location Z between points C & D.
> 
> This is more difficult to do with stabs, but possible since Cygnus defined a
> set of extensions to describe the same basic concepts.
> 
> The more difficult problem is getting information about the state of the
> FP regstack  into dwarf2out.c & dbxout.c in a reasonably clean manner.

This is actually a symptom of a much deeper problem that I would like to see
addressed in the compiler at some point, namely tracking variables being
reloaded, etc.  For example, if the compiler determines that a variable is on
the stack, but needs to load up a variable into a register to work on it
(reload in gcc speak), we currently don't track this, so if you put a
breakpoint after the load has been done, but before it is used, you won't be
able to change the value to affect the running of the program.

Also when I first did the LRS changes, I thought that you could do LRS debug
information in any debug format, including crippled ones like COFF, if you were
willing to create a new block for every state change (ie, variable moving from
stack to register).

-- 
Michael Meissner, Cygnus Solutions
PMB 198, 174 Littleton Road #3, Westford, Massachusetts 01886
email: meissner@cygnus.com	phone: 978-486-9304	fax: 978-692-4482