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, 65 insertions, 0 deletions

diff --git a/gdb/dwarf2/abbrev-cache.c b/gdb/dwarf2/abbrev-cache.c
new file mode 100644
index 0000000..2168f92
--- /dev/null
+++ b/gdb/dwarf2/abbrev-cache.c
@@ -0,0 +1,65 @@
+/* DWARF 2 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/>.  */
+
+#include "defs.h"
+#include "dwarf2/read.h"
+#include "dwarf2/abbrev-cache.h"
+
+/* Hash function for an abbrev table.  */
+
+hashval_t
+abbrev_cache::hash_table (const void *item)
+{
+  const struct abbrev_table *table = (const struct abbrev_table *) item;
+  return to_underlying (table->sect_off);
+}
+
+/* Comparison function for abbrev table.  */
+
+int
+abbrev_cache::eq_table (const void *lhs, const void *rhs)
+{
+  const struct abbrev_table *l_table = (const struct abbrev_table *) lhs;
+  const search_key *key = (const search_key *) rhs;
+  return (l_table->section == key->section
+	  && l_table->sect_off == key->offset);
+}
+
+abbrev_cache::abbrev_cache ()
+  : m_tables (htab_create_alloc (20, hash_table, eq_table,
+				 htab_delete_entry<abbrev_table>,
+				 xcalloc, xfree))
+{
+}
+
+void
+abbrev_cache::add (abbrev_table_up table)
+{
+  /* We allow this as a convenience to the caller.  */
+  if (table == nullptr)
+    return;
+
+  search_key key = { table->section, table->sect_off };
+  void **slot = htab_find_slot_with_hash (m_tables.get (), &key,
+					  to_underlying (table->sect_off),
+					  INSERT);
+  /* If this one already existed, then it should have been reused.  */
+  gdb_assert (*slot == nullptr);
+  *slot = (void *) table.release ();
+}