[tuples/LTO] RFC: houghts on auto-generating GS_* data structures

[Diego Novillo <dnovillo at google dot com>]

I chatted briefly with Ken today about the reader/writer being
implemented for LTO.  One of the difficulties in keeping the
reader/writer up-to-date is the evolving IL data structures.  Whenever
these change, the reader must be adapted to the new format.

So, while the reader/writer is fairly straightforward and mechanical,
it's a pain to have to keep changing it as new IL elements are added, or
existing ones are modified.

The idea, then, is to have the IL data structures defined in
http://gcc.gnu.org/wiki/tuples to be in a .h file auto-generated from
some template.

I'm slightly tempted to consider that this would be too much
abstraction, but we do change the IL from time to time and according to
Ken, changing the reader/writer is somewhat of a pain.

Given that we already have other autogenerated .h/.c files, I guess we
could use some of the existing functionality.

But, first, I'd like to know what folks think about this.  Would it be
generally useful for us to have the IL data structures auto-generated
this way?  I can see the benefits in the reader/writer.  But also, we
are going to have to re-implement the reader/writer when we move GIMPLE
out of the tree data structures.  OTOH, we will probably change them,
add new codes and having them autogenerated may have other advantages.

Thoughts?  Thanks.