[LTO] Flattening memory expressions?

[Diego Novillo <dnovillo at redhat dot com>]

When I originally thought of flattening the data structures for
representing GIMPLE, I had not really contemplated the idea of totally
lowering the memory expressions represented by ARRAY_REF and COMPONENT_REF.

However, as Kenny pointed out today, it may be useful for us to
completely simplify these expressions to remove their recursive
structure.  Something like:

ARRAY_REF : ID [ VAL ]

COMPONENT_REF : ID . ID
                | INDIRECT_REF . ID

INDIRECT_REF : * ID

We would probably need INDIRECT_REFs in COMPONENT_REF nodes to avoid
lots of memory copying.

The rationale is that by removing the recursiveness of these
expressions, it would make it easier to emit the LTO bytecodes without
resorting to a stack machine.

These memory expressions are the only ones that I think would need to be
simplifed.  All the other GIMPLE expressions are already flat enough for
LTO purposes.

This flattening, however, could bring debug code generation issues.  I'm
not sure.

Please don't get too fixated on the above pseudo-code.  I have not
thought about this through in detail.  I just want to bring this up,
particularly for folks doing data dependency.  Perhaps we need different
 expressions altogether, we don't need to stick to the current scheme if
it's not sufficiently useful.

Thoughts?