Create a correctly-sized demangled names hashtable

If we have a minsym count, we know the demangled names hashtable will
be at least that big.  So use that count to size it, so we don't
have to resize/rehash it as much.

This is a 6% improvement in minsym loading time.

2019-11-22  Christian Biesinger  <cbiesinger@google.com>

	* symtab.c (create_demangled_names_hash): Use per_bfd->
	minimal_symbol_count for computing the initial size, if greater
	than our default size.

Change-Id: I1f074d38e1d90af58705ec852f90c84cc034cd2e

1 files changed, 12 insertions, 2 deletions

diff --git a/gdb/symtab.c b/gdb/symtab.c
index 0064800..6affdef 100644
--- a/gdb/symtab.c
+++ b/gdb/symtab.c
@@ -770,10 +770,20 @@ create_demangled_names_hash (struct objfile_per_bfd_storage *per_bfd)
   /* Choose 256 as the starting size of the hash table, somewhat arbitrarily.
      The hash table code will round this up to the next prime number.
      Choosing a much larger table size wastes memory, and saves only about
-     1% in symbol reading.  */
+     1% in symbol reading.  However, if the minsym count is already
+     initialized (e.g. because symbol name setting was deferred to
+     a background thread) we can initialize the hashtable with a count
+     based on that, because we will almost certainly have at least that
+     many entries.  If we have a nonzero number but less than 256,
+     we still stay with 256 to have some space for psymbols, etc.  */
+
+  /* htab will expand the table when it is 3/4th full, so we account for that
+     here.  +2 to round up.  */
+  int minsym_based_count = (per_bfd->minimal_symbol_count + 2) / 3 * 4;
+  int count = std::max (per_bfd->minimal_symbol_count, minsym_based_count);
 
   per_bfd->demangled_names_hash.reset (htab_create_alloc
-    (256, hash_demangled_name_entry, eq_demangled_name_entry,
+    (count, hash_demangled_name_entry, eq_demangled_name_entry,
      free_demangled_name_entry, xcalloc, xfree));
 }