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Hashtable Small size optimization


I started considering PR libstdc++/68303.

First thing was to write a dedicated performance test case, it is the unordered_small_size.cc I'd like to add with this patch.

The first runs show a major difference between tr1 and std implementations, tr1 being much better:

std::tr1::unordered_set<int> without hash code cached: 1st insert       9r    9u    1s  14725920mem    0pf std::tr1::unordered_set<int> with hash code cached: 1st insert       8r    9u    0s  14719680mem    0pf std::unordered_set<int> without hash code cached: 1st insert      17r   17u    0s  16640080mem    0pf std::unordered_set<int> with hash code cached: 1st insert      14r   14u    0s  16638656mem    0pf

I had a look in gdb to find out why and the answer was quite obvious. For 20 insertions tr1 implementation bucket count goes through [11, 23] whereas for std it is [2, 5, 11, 23], so 2 more expensive rehash.

As unordered containers are dedicated to rather important number of elements I propose to review the rehash policy with this patch so that std also starts at 11 on the 1st insertion. After the patch figures are:

std::tr1::unordered_set<int> without hash code cached: 1st insert       9r    9u    0s  14725920mem    0pf std::tr1::unordered_set<int> with hash code cached: 1st insert       8r    8u    0s  14719680mem    0pf std::unordered_set<int> without hash code cached: 1st insert      15r   15u    0s  16640128mem    0pf std::unordered_set<int> with hash code cached: 1st insert      12r   12u    0s  16638688mem    0pf

Moreover I noticed that performance tests are built with -O2, is that intentional ? The std implementation uses more abstractions than tr1, looks like building with -O3 optimizes away most of those abstractions making tr1 and std implementation much closer:

std::tr1::unordered_set<int> without hash code cached: 1st insert       2r    1u    1s  14725952mem    0pf std::tr1::unordered_set<int> with hash code cached: 1st insert       2r    1u    0s  14719536mem    0pf std::unordered_set<int> without hash code cached: 1st insert       2r    2u    0s  16640064mem    0pf std::unordered_set<int> with hash code cached: 1st insert       2r    2u    0s  16638608mem    0pf

Note that this patch also rework the alternative rehash policy based on powers of 2 so that it also starts with a larger number of bucket (16) and respects LWG2156.

Last I had to wider the memory column so that alignment is preserved even when memory diff is negative.

Tested under Linux x86_64.

Ok to commit ?

François


diff --git a/libstdc++-v3/include/bits/hashtable_policy.h b/libstdc++-v3/include/bits/hashtable_policy.h
index 66fbfbe5f21..70d3c5c8194 100644
--- a/libstdc++-v3/include/bits/hashtable_policy.h
+++ b/libstdc++-v3/include/bits/hashtable_policy.h
@@ -536,6 +536,13 @@ namespace __detail
     std::size_t
     _M_next_bkt(std::size_t __n) noexcept
     {
+      if (__n < 0x11)
+	{
+	  _M_next_resize =
+	    __builtin_floor(0x10 * (long double)_M_max_load_factor);
+	  return 0x10;
+	}
+
       const auto __max_width = std::min<size_t>(sizeof(size_t), 8);
       const auto __max_bkt = size_t(1) << (__max_width * __CHAR_BIT__ - 1);
       std::size_t __res = __clp2(__n);
@@ -553,7 +560,7 @@ namespace __detail
 	_M_next_resize = std::size_t(-1);
       else
 	_M_next_resize
-	  = __builtin_ceil(__res * (long double)_M_max_load_factor);
+	  = __builtin_floor(__res * (long double)_M_max_load_factor);
 
       return __res;
     }
@@ -571,7 +578,7 @@ namespace __detail
     _M_need_rehash(std::size_t __n_bkt, std::size_t __n_elt,
 		   std::size_t __n_ins) noexcept
     {
-      if (__n_elt + __n_ins >= _M_next_resize)
+      if (__n_elt + __n_ins > _M_next_resize)
 	{
 	  long double __min_bkts = (__n_elt + __n_ins)
 					/ (long double)_M_max_load_factor;
diff --git a/libstdc++-v3/src/c++11/hashtable_c++0x.cc b/libstdc++-v3/src/c++11/hashtable_c++0x.cc
index b9b11ff4385..6df9775b954 100644
--- a/libstdc++-v3/src/c++11/hashtable_c++0x.cc
+++ b/libstdc++-v3/src/c++11/hashtable_c++0x.cc
@@ -46,14 +46,11 @@ namespace __detail
   {
     // Optimize lookups involving the first elements of __prime_list.
     // (useful to speed-up, eg, constructors)
-    static const unsigned char __fast_bkt[]
-      = { 2, 2, 2, 3, 5, 5, 7, 7, 11, 11, 11, 11, 13, 13 };
-
-    if (__n < sizeof(__fast_bkt))
+    if (__n < 12)
       {
 	_M_next_resize =
-	  __builtin_floor(__fast_bkt[__n] * (long double)_M_max_load_factor);
-	return __fast_bkt[__n];
+	  __builtin_floor(11 * (long double)_M_max_load_factor);
+	return 11;
       }
 
     // Number of primes (without sentinel).
@@ -66,7 +63,7 @@ namespace __detail
     constexpr auto __last_prime = __prime_list + __n_primes - 1;
 
     const unsigned long* __next_bkt =
-      std::lower_bound(__prime_list + 6, __last_prime, __n);
+      std::lower_bound(__prime_list + 5, __last_prime, __n);
 
     if (__next_bkt == __last_prime)
       // Set next resize to the max value so that we never try to rehash again
diff --git a/libstdc++-v3/testsuite/performance/23_containers/insert_erase/unordered_small_size.cc b/libstdc++-v3/testsuite/performance/23_containers/insert_erase/unordered_small_size.cc
new file mode 100644
index 00000000000..a2f44c07082
--- /dev/null
+++ b/libstdc++-v3/testsuite/performance/23_containers/insert_erase/unordered_small_size.cc
@@ -0,0 +1,269 @@
+// { dg-do run { target c++11 } }
+
+// Copyright (C) 2018 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/>.
+
+#include <sstream>
+#include <vector>
+#include <tr1/unordered_set>
+#include <unordered_set>
+#include <testsuite_performance.h>
+namespace
+{
+  const int nb = 20;
+  const int nb_insts = 10000;
+
+  // Bench using an unordered_set<int>. Hash functor for int is quite
+  // predictable so it helps bench very specific use cases.
+  template<typename _ContType>
+    void bench(const char* desc)
+    {
+      using namespace __gnu_test;
+
+      time_counter time;
+      resource_counter resource;
+
+      std::vector<_ContType> insts(nb_insts);
+      for (int j = 0; j != nb_insts; ++j)
+	insts.emplace_back();
+
+      start_counters(time, resource);
+
+      for (auto& us : insts)
+	for (int i = 0; i != nb; ++i)
+	  us.insert(i);
+
+      stop_counters(time, resource);
+
+      std::ostringstream ostr;
+      ostr << desc << " 1st insert";
+      report_performance(__FILE__, ostr.str().c_str(), time, resource);
+
+      start_counters(time, resource);
+
+      // Here is the worst erase use case when hashtable implementation was
+      // something like vector<forward_list<>>. Erasing from the end was very
+      // costly because we need to return the iterator following the erased
+      // one, as the hashtable is getting emptier at each step there are
+      // more and more empty bucket to loop through to reach the end of the
+      // container and find out that it was in fact the last element.
+      for (auto& us : insts)
+	for (int i = nb - 1; i >= 0; --i)
+	  {
+	    auto it = us.find(i);
+	    if (it != us.end())
+	      us.erase(it);
+	  }
+
+      stop_counters(time, resource);
+
+      ostr.str("");
+      ostr << desc << " erase it  ";
+      report_performance(__FILE__, ostr.str().c_str(), time, resource);
+
+      start_counters(time, resource);
+
+      // This is a worst insertion use case for the current implementation as
+      // we insert an element at the beginning of the hashtable and then we
+      // insert starting at the end so that each time we need to seek up to the
+      // first bucket to find the first non-empty one.
+      for (auto& us : insts)
+	{
+	  us.insert(0);
+	  for (int i = nb - 1; i >= 0; --i)
+	    us.insert(i);
+	}
+
+      stop_counters(time, resource);
+      ostr.str("");
+      ostr << desc << " 2nd insert";
+      report_performance(__FILE__, ostr.str().c_str(), time, resource);
+
+      start_counters(time, resource);
+
+      for (auto& us : insts)
+	for (int j = nb - 1; j >= 0; --j)
+	  us.erase(j);
+
+      stop_counters(time, resource);
+      ostr.str("");
+      ostr << desc << " erase key ";
+      report_performance(__FILE__, ostr.str().c_str(), time, resource);
+    }
+
+  // Bench using unordered_set<string> that show how important it is to cache
+  // hash code as computing string hash code is quite expensive compared to
+  // computing it for int.
+  template<typename _ContType>
+    void bench_str(const char* desc)
+    {
+      using namespace __gnu_test;
+
+      time_counter time;
+      resource_counter resource;
+
+      // First generate once strings that are going to be used throughout the
+      // bench:
+      std::ostringstream ostr;
+      std::vector<std::string> strs;
+      {
+	strs.reserve(nb);
+	for (int i = 0; i != nb; ++i)
+	  {
+	    ostr.str("");
+	    ostr << "string #" << i;
+	    strs.push_back(ostr.str());
+	  }
+      }
+
+      std::vector<_ContType> insts(nb_insts);
+      for (int j = 0; j != nb_insts; ++j)
+	insts.emplace_back();
+
+      start_counters(time, resource);
+
+      for (auto& us : insts)
+	for (int i = 0; i != nb; ++i)
+	  us.insert(strs[i]);
+
+      stop_counters(time, resource);
+
+      ostr.str("");
+      ostr << desc << " 1st insert";
+      report_performance(__FILE__, ostr.str().c_str(), time, resource);
+
+      start_counters(time, resource);
+
+      for (auto& us : insts)
+	for (int j = nb - 1; j >= 0; --j)
+	  {
+	    auto it = us.find(strs[j]);
+	    if (it != us.end())
+	      us.erase(it);
+	  }
+
+      stop_counters(time, resource);
+
+      ostr.str("");
+      ostr << desc << " erase iter";
+      report_performance(__FILE__, ostr.str().c_str(), time, resource);
+
+      start_counters(time, resource);
+
+      for (auto& us : insts)
+	{
+	  us.insert(strs[0]);
+	  for (int i = nb - 1; i >= 0; --i)
+	    us.insert(strs[i]);
+	}
+
+      stop_counters(time, resource);
+      ostr.str("");
+      ostr << desc << " 2nd insert";
+      report_performance(__FILE__, ostr.str().c_str(), time, resource);
+
+      start_counters(time, resource);
+
+      for (auto& us : insts)
+	for (int j = nb - 1; j >= 0; --j)
+	  us.erase(strs[j]);
+
+      stop_counters(time, resource);
+      ostr.str("");
+      ostr << desc << " erase key ";
+      report_performance(__FILE__, ostr.str().c_str(), time, resource);
+    }
+}
+
+template<bool cache>
+  using __uset =
+	      std::__uset_hashtable<int, std::hash<int>, std::equal_to<int>,
+				    std::allocator<int>,
+				    std::__uset_traits<cache>>;
+
+template<bool cache>
+  using __tr1_uset =
+	      std::tr1::__unordered_set<int, std::hash<int>, std::equal_to<int>,
+					std::allocator<int>,
+					cache>;
+
+template<bool cache>
+  using __uset2 =
+	      std::_Hashtable<int, int, std::allocator<int>,
+			      std::__detail::_Identity,
+			      std::equal_to<int>, std::hash<int>,
+			      std::__detail::_Mask_range_hashing,
+			      std::__detail::_Default_ranged_hash,
+			      std::__detail::_Power2_rehash_policy,
+			      std::__uset_traits<cache>>;
+
+template<bool cache>
+  using __str_uset =
+	      std::__uset_hashtable<std::string, std::hash<std::string>,
+				    std::equal_to<std::string>,
+				    std::allocator<std::string>,
+				    std::__uset_traits<cache>>;
+
+template<bool cache>
+  using __tr1_str_uset =
+	      std::tr1::__unordered_set<std::string, std::hash<std::string>,
+					std::equal_to<std::string>,
+					std::allocator<std::string>,
+					cache>;
+
+template<bool cache>
+  using __str_uset2 =
+	      std::_Hashtable<std::string, std::string, std::allocator<std::string>,
+			      std::__detail::_Identity,
+			      std::equal_to<std::string>, std::hash<std::string>,
+			      std::__detail::_Mask_range_hashing,
+			      std::__detail::_Default_ranged_hash,
+			      std::__detail::_Power2_rehash_policy,
+			      std::__uset_traits<cache>>;
+
+int main()
+{
+  bench<__tr1_uset<false>>(
+	"std::tr1::unordered_set<int> without hash code cached:   ");
+  bench<__tr1_uset<true>>(
+	"std::tr1::unordered_set<int> with hash code cached:      ");
+  bench<__uset<false>>(
+	"std::unordered_set<int> without hash code cached:        ");
+  bench<__uset<true>>(
+	"std::unordered_set<int> with hash code cached:           ");
+  bench<std::unordered_set<int>>(
+	"std::unordered_set<int> default cache:                   ");
+  bench<__uset2<false>>(
+	"std::unordered_set2<int> without hash code cached:       ");
+  bench<__uset2<true>>(
+	"std::unordered_set2<int> with hash code cached:          ");
+  bench_str<__tr1_str_uset<false>>(
+	"std::tr1::unordered_set<string> without hash code cached:");
+  bench_str<__tr1_str_uset<true>>(
+	"std::tr1::unordered_set<string> with hash code cached:   ");
+  bench_str<__str_uset<false>>(
+	"std::unordered_set<string> without hash code cached:     ");
+  bench_str<__str_uset<true>>(
+	"std::unordered_set<string> with hash code cached:        ");
+  bench_str<std::unordered_set<std::string>>(
+	"std::unordered_set<string> default cache:                ");
+  bench_str<__str_uset2<false>>(
+	"std::unordered_set2<string> without hash code cached:    ");
+  bench_str<__str_uset2<true>>(
+	"std::unordered_set2<string> with hash code cached:       ");
+  return 0;
+}
diff --git a/libstdc++-v3/testsuite/util/testsuite_performance.h b/libstdc++-v3/testsuite/util/testsuite_performance.h
index 3dfd471221d..7ebf2d5b849 100644
--- a/libstdc++-v3/testsuite/util/testsuite_performance.h
+++ b/libstdc++-v3/testsuite/util/testsuite_performance.h
@@ -224,7 +224,7 @@ namespace __gnu_test
     out << std::setw(4) << t.real_time() << "r" << space;
     out << std::setw(4) << t.user_time() << "u" << space;
     out << std::setw(4) << t.system_time() << "s" << space;
-    out << std::setw(8) << r.allocated_memory() << "mem" << space;
+    out << std::setw(9) << r.allocated_memory() << "mem" << space;
     out << std::setw(4) << r.hard_page_fault() << "pf" << space;
 
     out << std::endl;


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