RFC: Variables in blocks of registers

[Andrew Cagney]

> 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.

Andrew