ctf: speed up the dwarf2ctf duplicate detector

ctf: speed up the dwarf2ctf duplicate detector

dwarf2ctf is very slow.  By far its most expensive pass is the duplicate
detector.  Of necessity this has to scan every object file in the kernel
to identify shared types, which is pretty expensive even when the cache
is hot, but it's not doing this particularly efficiently and can be sped
up quite a lot.

As of commit f0d0ebd0b4, the duplicate detector's job was cut in two:
the first pass identifies all non-structure types and non-opaque
structures used by more than one module, and the second "alias fixup"
pass repeatedly unifies opaque and non-opaque structures with the same
name in different translation units, sharing their definitions.  Both of
these passes generate or modify type IDs, so both need access to the
DWARF DIE of the types in question, since the type ID is derived
recursively from the DIE: but the second pass need not look at the DWARF
of any translation units that do not contain structures that might be
unified.  However, the two are currently written in the same way, using
process_file() to traverse all the kernel's DWARF, even though the alias
fixup pass does almost nothing with that DWARF, and has less and less to
do on each iteration.

The sheer amount of wasted time here is remarkable.  We traverse the
DWARF once for primary duplicate detection, once for CTF emission, but
often four or five or even seven or eight times for the alias fixup pass
(the precise count depends upon the relationships between types and the
order in which the DWARF files are traversed).

So improve things by tracking all types that the alias fixup pass is
interested in (structure types that are not anonymous inner structures
nor opaque nor used only as the types of array indices) and stash them
away during the first duplicate detection pass in a new temporary
singly-linked list, detect_duplicates_state.named_structs.  We remember
the filename and DWARF DIE offset (so we can look the type up again) and
the type ID (because we just worked it out, so recomputing it would be a
waste of time).  Then, rather than doing a process_file() for the alias
fixup pass, traverse the linked list, opening DWARF files as needed to
mark things as shared (but no more often than that: marking non-opaque
types needs the DIE so we can traverse into all its subtypes and mark
them shared too, but marking opaque types needs no DIE at all).

This has a significant effect on dwarf2ctf speed, slashing 25% off its
runtime in my tests, reducing the duplicate detector's share of the
runtime from about 80s to about 24s.

The dominant time consumer is now CTF emission rather than the
duplicate detector.

Orabug: 25815306
Signed-off-by: Nick Alcock <nick.alcock@oracle.com>
Reviewed-by: tomas.jedlicka@oracle.com