[lto] function to DECL associations for WPA repackaging

["Ollie Wild" <aaw at google dot com>]

Doug,

Yesterday, we spoke briefly about the need to efficiently determine
the DECL's required by each function.  Here's a more detailed
overview.  During the WPA phase of WHOPR, we will be reading in a
collection of object files, performing analysis on call-graph/summary
data, and outputting repackaged object files.  This is described more
fully in the "Repackaging" section at
http://gcc.gnu.org/wiki/whopr/wpa.  During a first implementation, the
output files are likely to mirror the input files with inlinable
function bodies appended.  As our WPA implementation gets more
sophisticated, we will likely want to support more complex repackaging
schemes based on call-graph partitioning.

In either case, we need a way to efficiently determine which DECL's
must accompany a given set of function bodies.  DECL references are
currently stored as integral indexes within the serialized function
bodies.  These need to be reproduced elsewhere.  Right now, I'm
thinking the call-graph nodes are the best place for this.  Kenny
might have some suggestions on this front.

In the end, WPA should look something like this:

1) Read in call-graph nodes, DECL's, and summary data from the input files.
2) Do some analysis.  Partition the functions described in the call-graph nodes.
3) Scan each partition's call graph to determine which DECL's are needed.
4) Write each partition's function bodies, DECL's, and call-graph
nodes to an output file.

If you could work on the DECL association logic, that would be greatly
appreciated.

Ollie