re PR libstdc++/50257 ([C++0x] unordered_map slow initialization due to huge __prime_list)

2011-09-06  Paolo Carlini  <paolo.carlini@oracle.com>

	PR libstdc++/50257
	* include/bits/hashtable_policy.h (_Prime_rehash_policy::
   	_M_next_bkt): Optimize for small argument.

From-SVN: r178581

libstdc++-v3/ChangeLog

+2011-09-06  Paolo Carlini  <paolo.carlini@oracle.com>
+
+	PR libstdc++/50257
+	* include/bits/hashtable_policy.h (_Prime_rehash_policy::
+   	_M_next_bkt): Optimize for small argument.
+
 2011-09-02  François Dumont  <fdumont@gcc.gnu.org>
 
 	* testsuite/util/testsuite_allocator.h (tracker_allocator_counter::

libstdc++-v3/include/bits/hashtable_policy.h

@@ -427,8 +427,15 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
   _Prime_rehash_policy::
   _M_next_bkt(std::size_t __n) const
   {
-    const unsigned long __p = *std::lower_bound(__prime_list, __prime_list
-						+ _S_n_primes, __n);
+    // Optimize lookups involving the first elements of __prime_list.
+    // (useful to speed-up, eg, constructors)
+    static const unsigned char __fastbkt[12]
+      = { 2, 2, 2, 3, 5, 5, 7, 7, 11, 11, 11, 11 };
+
+    const unsigned long __p
+      = __n <= 11 ? __fastbkt[__n]
+                  : *std::lower_bound(__prime_list + 5,
+				      __prime_list + _S_n_primes, __n);
     _M_next_resize =
       static_cast<std::size_t>(__builtin_floor(__p * _M_max_load_factor));
     return __p;