c++: Reuse identical ATOMIC_CONSTRs during normalization

Profiling revealed that sat_hasher::equal accounts for nearly 40% of
compile time in some cmcstl2 tests.

This patch eliminates this bottleneck by caching the ATOMIC_CONSTRs
returned by normalize_atom.  This in turn allows us to replace the
expensive atomic_constraints_identical_p check in sat_hasher::equal
with cheap pointer equality, with no loss in cache hit rate.

With this patch, compile time for the cmcstl2 test
test/algorithm/set_symmetric_difference4.cpp drops from 19s to 11s with
an --enable-checking=release compiler.

gcc/cp/ChangeLog:

	* constraint.cc (atom_cache): Define this deletable hash_table.
	(normalize_atom): Use it to cache ATOMIC_CONSTRs when not
	generating diagnostics.
	(sat_hasher::hash): Use htab_hash_pointer instead of
	hash_atomic_constraint.
	(sat_hasher::equal): Test for pointer equality instead of
	atomic_constraints_identical_p.
	* cp-tree.h (struct atom_hasher): Moved and renamed from ...
	* logic.cc (struct constraint_hash): ... here.
	(clause::m_set): Adjust accordingly.

1 files changed, 1 insertions, 16 deletions

diff --git a/gcc/cp/logic.cc b/gcc/cp/logic.cc
index 194b743..6701488 100644
--- a/gcc/cp/logic.cc
+++ b/gcc/cp/logic.cc
@@ -47,21 +47,6 @@ along with GCC; see the file COPYING3.  If not see
 #include "toplev.h"
 #include "type-utils.h"
 
-/* Hash functions for atomic constrains.  */
-
-struct constraint_hash : default_hash_traits<tree>
-{
-  static hashval_t hash (tree t)
-  {
-    return hash_atomic_constraint (t);
-  }
-
-  static bool equal (tree t1, tree t2)
-  {
-    return atomic_constraints_identical_p (t1, t2);
-  }
-};
-
 /* A conjunctive or disjunctive clause.
 
    Each clause maintains an iterator that refers to the current
@@ -219,7 +204,7 @@ struct clause
   }
 
   std::list<tree> m_terms; /* The list of terms.  */
-  hash_set<tree, false, constraint_hash> m_set; /* The set of atomic constraints.  */
+  hash_set<tree, false, atom_hasher> m_set; /* The set of atomic constraints.  */
   iterator m_current; /* The current term.  */
 };