PR 54075 Restore 4.6 growth factor

[François Dumont <frs dot dumont at gmail dot com>]

Hi

    Here is the patch to restore the 4.6 growth factor of 2. I prefer 
to validate the restored behavior by adding a performance test. Without 
the patch the result was:

unordered_set.cc             unordered_set 10000000 insertions  403r  
329u   73s 402825280mem    0pf

after the patch:

unordered_set.cc             unordered_set 10000000 insertions  112r   
86u   25s 402825104mem    0pf

It validates the 3x times performance hint.

Tested under Linux x86_64.

2012-07-28  François Dumont  <fdumont@gcc.gnu.org>

    PR libstdc++/54075
    * include/bits/hashtable_policy.h
    (_Prime_rehash_policy::_M_next_bkt): Add a growth factor set to 2
    to boost growth in the number of buckets.
    * testsuite/performance/23_containers/insert/unordered_set.cc: New.

Even if it is not a Standard conformity issue I think we can apply it to 
the 4.7 branch too.

François

Index: include/bits/hashtable_policy.h
===================================================================
--- include/bits/hashtable_policy.h	(revision 189893)
+++ include/bits/hashtable_policy.h	(working copy)
@@ -395,6 +395,8 @@
 
     enum { _S_n_primes = sizeof(unsigned long) != 8 ? 256 : 256 + 48 };
 
+    static const std::size_t _S_growth_factor = 2;
+
     float                _M_max_load_factor;
     mutable std::size_t  _M_prev_resize;
     mutable std::size_t  _M_next_resize;
@@ -415,28 +417,27 @@
     static const unsigned char __fast_bkt[12]
       = { 2, 2, 2, 3, 5, 5, 7, 7, 11, 11, 11, 11 };
 
-    if (__n <= 11)
+    const std::size_t __grown_n = __n * _S_growth_factor;
+    if (__grown_n <= 11)
       {
 	_M_prev_resize = 0;
 	_M_next_resize
-	  = __builtin_ceil(__fast_bkt[__n] * (long double)_M_max_load_factor);
-	return __fast_bkt[__n];
+	  = __builtin_ceil(__fast_bkt[__grown_n]
+			   * (long double)_M_max_load_factor);
+	return __fast_bkt[__grown_n];
       }
 
-    const unsigned long* __p
-      = std::lower_bound(__prime_list + 5, __prime_list + _S_n_primes, __n);
+    const unsigned long* __next_bkt
+      = std::lower_bound(__prime_list + 5, __prime_list + _S_n_primes,
+			 __grown_n);
+    const unsigned long* __prev_bkt
+      = std::lower_bound(__prime_list + 1, __next_bkt, __n / _S_growth_factor);
 
-    // Shrink will take place only if the number of elements is small enough
-    // so that the prime number 2 steps before __p is large enough to still
-    // conform to the max load factor:
     _M_prev_resize
-      = __builtin_floor(*(__p - 2) * (long double)_M_max_load_factor);
-
-    // Let's guaranty that a minimal grow step of 11 is used
-    if (*__p - __n < 11)
-      __p = std::lower_bound(__p, __prime_list + _S_n_primes, __n + 11);
-    _M_next_resize = __builtin_ceil(*__p * (long double)_M_max_load_factor);
-    return *__p;
+      = __builtin_floor(*(__prev_bkt - 1) * (long double)_M_max_load_factor);
+    _M_next_resize
+      = __builtin_ceil(*__next_bkt * (long double)_M_max_load_factor);
+    return *__next_bkt;
   }
 
   // Return the smallest prime p such that alpha p >= n, where alpha
Index: testsuite/performance/23_containers/insert/unordered_set.cc
===================================================================
--- testsuite/performance/23_containers/insert/unordered_set.cc	(revision 0)
+++ testsuite/performance/23_containers/insert/unordered_set.cc	(revision 0)
@@ -0,0 +1,42 @@
+// Copyright (C) 2012 Free Software Foundation, Inc.
+//
+// This file is part of the GNU ISO C++ Library.  This library 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, or (at your option)
+// any later version.
+
+// This library 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 library; see the file COPYING3.  If not see
+// <http://www.gnu.org/licenses/>.
+
+// { dg-options "-std=c++11" }
+
+#include <unordered_set>
+#include <testsuite_performance.h>
+
+int main()
+{
+  using namespace __gnu_test;
+
+  time_counter time;
+  resource_counter resource;
+
+  const int sz = 10000000;
+
+  std::unordered_set<int> s;
+  start_counters(time, resource);
+
+  for (int i = 0; i != sz ; ++i)
+    s.insert(i);
+
+  stop_counters(time, resource);
+  report_performance(__FILE__, "unordered_set 10000000 insertions",
+		     time, resource);
+  return 0;
+}