[PATCH 00/14] Share DWARF partial symtabs between objfiles

[Tom Tromey]

A long-term goal for multi-inferior debugging has been to "split" an
objfile, so that the bulk of the data can be shared across inferiors.
Although a lot of progress has been made on this front, it has turned
out to be surprisingly difficult to fully implement.

"Recently" (a year or two ago) I realized that we could get most of
the benefits of this split by sharing partial symbol tables.  This is
true because reading partial symbols is the slowest operation that
users see -- in most cases, expanding a full symtab is reasonably
quick.

Implementing this also turned out to be tricky; but unlike the
situation with types and symbols, it was possible to do incrementally.

This series implements this idea for DWARF.  It separates
dwarf2_per_objfile into shareable and unshareable parts; then, when
possible, the object is attached to the BFD.

You can see the difference when timing "add-inferior -exec ./gdb",
after "gdb ./gdb":

Before: Command execution time: 1.667661 (cpu), 1.687607 (wall)
After : Command execution time: 0.150011 (cpu), 0.151292 (wall)

In this series I did not rename dwarf2_per_objfile.  I'd prefer to do
this after landing this series; it seemed like a large, mostly
cosmetic, and yet hard-to-rebase patch.  Also, I wanted to consult
with everyone else about what we ought to call it.

I also didn't attempt to implement this sharing for CTF.  I don't
really know much about CTF; but it seems like it ought to be possible.
(Though I wonder if CTF wouldn't benefit more from simply bypassing
partial symbols entirely.)

Regression tested on x86-64 Fedora 28.  I also did various tests by
hand, and tested it by hand, under valgrind, on a gdb built with
-fdebug-types-section.

Like I mentioned earlier, this is a tricky series, so it would benefit
from careful review.  The main danger is that, if I missed a spot, we
could end up in a situation where gdb will crash in a multi-inferior
scenario.

Tom