Introduce DWARF abbrev cache

The replacement for the DWARF psymbol reader works in a somewhat
different way.  The current reader reads and stores all the DIEs that
might be interesting.  Then, if it is missing a DIE, it re-scans the
CU and reads them all.  This approach is used for both intra- and
inter-CU references.

I instrumented the partial DIE hash to see how frequently it was used:

    [  0] -> 1538165
    [  1] ->    4912
    [  2] ->   96102
    [  3] ->     175
    [  4] ->     244

That is, most DIEs are never used, and some are looked up twice -- but
this is just an artifact of the implementation of
partial_die_info::fixup, which may do two lookups.

Based on this, the new implementation doesn't try to store any DIEs,
but instead just re-scans them on demand.  In order to do this,
though, it is convenient to have a cache of DWARF abbrevs.  This way,
if a second CU is needed to resolve an inter-CU reference, the abbrevs
for that CU need only be computed a single time.

1 files changed, 58 insertions, 0 deletions

diff --git a/gdb/dwarf2/abbrev-cache.h b/gdb/dwarf2/abbrev-cache.h
new file mode 100644
index 0000000..7af28cd
--- /dev/null
+++ b/gdb/dwarf2/abbrev-cache.h
@@ -0,0 +1,58 @@
+/* DWARF abbrev table cache
+
+   Copyright (C) 2022 Free Software Foundation, Inc.
+
+   This file is part of GDB.
+
+   This program is free software; you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
+
+#ifndef GDB_DWARF2_ABBREV_CACHE_H
+#define GDB_DWARF2_ABBREV_CACHE_H
+
+#include "dwarf2/abbrev.h"
+#include <unordered_set>
+#include "gdbtypes.h"
+
+class abbrev_cache
+{
+public:
+  abbrev_cache ();
+  DISABLE_COPY_AND_ASSIGN (abbrev_cache);
+
+  abbrev_table *find (struct dwarf2_section_info *section, sect_offset offset)
+  {
+    search_key key = { section, offset };
+
+    return (abbrev_table *) htab_find_with_hash (m_tables.get (), &key,
+						 to_underlying (offset));
+  }
+
+  void add (abbrev_table_up table);
+
+private:
+
+  static hashval_t hash_table (const void *item);
+  static int eq_table (const void *lhs, const void *rhs);
+
+  struct search_key
+  {
+    struct dwarf2_section_info *section;
+    sect_offset offset;
+  };
+
+  /* Hash table of abbrev tables.  */
+  htab_up m_tables;
+};
+
+#endif /* GDB_DWARF2_ABBREV_CACHE_H */