c++: optimize push_to_top_level [PR64500]

Profiling showed that the loop to save away IDENTIFIER_BINDINGs from open
binding levels was taking 5% of total compilation time in the PR116285
testcase.  This turned out to be because we were unnecessarily trying to do
this for namespaces, whose bindings are found through
DECL_NAMESPACE_BINDINGS, not IDENTIFIER_BINDING.

As a result we would frequently loop through everything in std::, checking
whether it needs to be stored, and never storing anything.

This change actually appears to speed up compilation for the PR116285
testcase by ~20%.

The replaced comments referred either to long-replaced handling of classes
and templates, or to wanting b to point to :: when the loop exits.

	PR c++/64500
	PR c++/116285

gcc/cp/ChangeLog:

	* name-lookup.cc (push_to_top_level): Don't try to store_bindings
	for namespace levels.

1 files changed, 12 insertions, 8 deletions

diff --git a/gcc/cp/name-lookup.cc b/gcc/cp/name-lookup.cc
index d3aa47b..df033ed 100644
--- a/gcc/cp/name-lookup.cc
+++ b/gcc/cp/name-lookup.cc
@@ -8675,6 +8675,9 @@ store_class_bindings (vec<cp_class_binding, va_gc> *names,
 
 static GTY((deletable)) struct saved_scope *free_saved_scope;
 
+/* Temporarily make the current scope the global namespace, saving away
+   the current scope for pop_from_top_level.  */
+
 void
 push_to_top_level (void)
 {
@@ -8716,18 +8719,19 @@ push_to_top_level (void)
     store_class_bindings (previous_class_level->class_shadowed,
 			  &s->old_bindings);
 
-  /* Have to include the global scope, because class-scope decls
-     aren't listed anywhere useful.  */
+  /* Save and clear any IDENTIFIER_BINDING from local scopes.  */
   for (; b; b = b->level_chain)
     {
       tree t;
 
-      /* Template IDs are inserted into the global level. If they were
-	 inserted into namespace level, finish_file wouldn't find them
-	 when doing pending instantiations. Therefore, don't stop at
-	 namespace level, but continue until :: .  */
-      if (global_scope_p (b))
-	break;
+      /* We don't need to consider namespace scopes, they don't affect
+	 IDENTIFIER_BINDING.  */
+      if (b->kind == sk_namespace)
+	{
+	  /* Jump straight to '::'.  */
+	  b = NAMESPACE_LEVEL (global_namespace);
+	  break;
+	}
 
       store_bindings (b->names, &s->old_bindings);
       /* We also need to check class_shadowed to save class-level type