Re: RFC: Variables in blocks of registers

[Daniel Jacobowitz <drow@mvista.com>]

On Sat, Feb 01, 2003 at 10:48:32AM -0500, Andrew Cagney wrote:
> >On my i386-unknown-freebsd4.7 system, various tests in
> >gdb.base/store.exp fail.  The reason is related to the problem
> >described in tdep/214; register variables that don't fit in a single
> >variable.  GDB assumes that such variables are stored in consecutive
> >registers (according to its own register numbering scheme), which
> >defenitely isn't what GCC uses on the i386.
> >
> >I'm looking into the suggestion Daniel made in tdep/214; teaching GDB
> >about the order in which GCC allocates registers.  There are several
> >caveats though:
> >
> >* While GCC allocates its registers in a particular order right now,
> >  and always allocates blocks of consecutive registers, there is no
> >  guarantee that it will continue to do so.
> >
> >* I have no idea what other compilers do.  If GDB's register numbering
> >  was chosen to match for example the System V compiler, teaching GDB
> >  GCC's register ordering will cause regressions on system that use
> >  it.  We might play tricks with gcc_compiled of course.
> >
> >Since AFAIK GDB's internal register ordering is still not decoupled
> >from the remote interface, I propose to add a new multi-arch function
> >"next_regnum" which returns the next register to look in based on the
> >register number passed to it as an argument.
> >
> >Comments?
> 
> dwarf2 makes it possible to scatter a value across both memory and 
> registers.  It's been proposed that the `struct value' be augmented with 
> something like `struct location' that knows how to find any sub 
> component of a value.

However, right now GCC doesn't generate this.  Probably because it
would kill us.  If I have any mental energy left after location lists,
I may implement support for DW_OP_piece.

Michael, I think the new multi-arch function is a good idea as long as
it is a fallback from explicit debug info support, when we have such. 
I also think it needs a better name; but I'm not quite sure what.  Hmm,
that could be mitigated by adequate commenting.

-- 
Daniel Jacobowitz
MontaVista Software                         Debian GNU/Linux Developer