[PATCH] PR libstdc++/87135 Rehash only when necessary (LWG2156)

["François Dumont" <frs.dumont@gmail.com>]

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All changes limited to hashtable_c++0x.cc file.

_Prime_rehash_policy::_M_next_bkt now really does what its comment was 
declaring that is to say:
   // Return a prime no smaller than n.

_Prime_rehash_policy::_M_need_rehash rehash only when _M_next_size is 
exceeded, not only when it is reach.

     PR libstdc++/87135
     * src/c++11/hashtable_c++0x.cc:
     (_Prime_rehash_policy::_M_next_bkt): Return a prime no smaller than
     requested size, but not necessarily greater.
     (_Prime_rehash_policy::_M_need_rehash): Rehash only if target size is
     strictly greater than next resize threshold.
     * testsuite/23_containers/unordered_map/modifiers/reserve.cc: Adapt 
test
     to validate that there is no rehash as long as number of insertion is
     lower or equal to the reserved number of elements.

unordered_map tests successful, ok to commit once all other tests 
completed ?

François

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diff --git a/libstdc++-v3/src/c++11/hashtable_c++0x.cc b/libstdc++-v3/src/c++11/hashtable_c++0x.cc
index a776a8506fe..ec6031b3f5b 100644
--- a/libstdc++-v3/src/c++11/hashtable_c++0x.cc
+++ b/libstdc++-v3/src/c++11/hashtable_c++0x.cc
@@ -46,10 +46,10 @@ namespace __detail
   {
     // Optimize lookups involving the first elements of __prime_list.
     // (useful to speed-up, eg, constructors)
-    static const unsigned char __fast_bkt[13]
-      = { 2, 2, 3, 5, 5, 7, 7, 11, 11, 11, 11, 13, 13 };
+    static const unsigned char __fast_bkt[]
+      = { 2, 2, 2, 3, 5, 5, 7, 7, 11, 11, 11, 11, 13, 13 };
 
-    if (__n <= 12)
+    if (__n < sizeof(__fast_bkt) / sizeof(unsigned char))
       {
 	_M_next_resize =
 	  __builtin_ceil(__fast_bkt[__n] * (long double)_M_max_load_factor);
@@ -65,9 +65,8 @@ namespace __detail
     // iterator that can be dereferenced to get the last prime.
     constexpr auto __last_prime = __prime_list + __n_primes - 1;
 
-    // Look for 'n + 1' to make sure returned value will be greater than n.
     const unsigned long* __next_bkt =
-      std::lower_bound(__prime_list + 6, __last_prime, __n + 1);
+      std::lower_bound(__prime_list + 6, __last_prime, __n);
 
     if (__next_bkt == __last_prime)
       // Set next resize to the max value so that we never try to rehash again
@@ -95,7 +94,7 @@ namespace __detail
   _M_need_rehash(std::size_t __n_bkt, std::size_t __n_elt,
 		 std::size_t __n_ins) const
   {
-    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/testsuite/23_containers/unordered_map/modifiers/reserve.cc b/libstdc++-v3/testsuite/23_containers/unordered_map/modifiers/reserve.cc
index e9cf7fd6f67..7f34325df87 100644
--- a/libstdc++-v3/testsuite/23_containers/unordered_map/modifiers/reserve.cc
+++ b/libstdc++-v3/testsuite/23_containers/unordered_map/modifiers/reserve.cc
@@ -18,23 +18,46 @@
 // <http://www.gnu.org/licenses/>.
 
 #include <unordered_map>
+
 #include <testsuite_hooks.h>
 
 void test01()
 {
-  const int N = 1000;
-
   typedef std::unordered_map<int, int> Map;
   Map m;
-  m.reserve(N);
 
-  std::size_t bkts = m.bucket_count();
-  for (int i = 0; i != N; ++i)
+  // Make sure max load factor is 1 so that reserved elements is directly
+  // the bucket count.
+  m.max_load_factor(1);
+
+  int i = -1;
+  for (;;)
     {
-      m.insert(std::make_pair(i, i));
-      // As long as we insert less than the reserved number of elements we
-      // shouldn't experiment any rehash.
+      m.reserve(m.bucket_count());
+
+      std::size_t bkts = m.bucket_count();
+
+      m.reserve(bkts);
       VERIFY( m.bucket_count() == bkts );
+
+      for (++i; i < bkts; ++i)
+	{
+	  m.insert(std::make_pair(i, i));
+
+	  // As long as we insert less than the reserved number of elements we
+	  // shouldn't experiment any rehash.
+	  VERIFY( m.bucket_count() == bkts );
+
+	  VERIFY( m.load_factor() <= m.max_load_factor() );
+	}
+
+      // One more element should rehash.
+      m.insert(std::make_pair(i, i));
+      VERIFY( m.bucket_count() != bkts );
+      VERIFY( m.load_factor() <= m.max_load_factor() );
+
+      if (i > 1024)
+	break;
     }
 }