[gcc r14-276] c++: micro-optimize most_specialized_partial_spec

[gcc r14-276] c++: micro-optimize most_specialized_partial_spec

[Patrick Palka]

https://gcc.gnu.org/g:50d866038a910ceb9075b97295a12d77a8d09a3a

commit r14-276-g50d866038a910ceb9075b97295a12d77a8d09a3a
Author: Patrick Palka <ppalka@redhat.com>
Date:   Wed Apr 26 15:43:26 2023 -0400

    c++: micro-optimize most_specialized_partial_spec
    
    This introduces an early exit test to most_specialized_partial_spec for
    templates which have no partial specializations, saving some unnecessary
    work during class template instantiation in the common case.  In passing,
    modernize the code a bit.
    
    gcc/cp/ChangeLog:
    
            * pt.cc (most_specialized_partial_spec): Exit early when
            DECL_TEMPLATE_SPECIALIZATIONS is empty.  Move local variable
            declarations closer to their first use.  Remove redundant
            flag_concepts test.  Remove redundant forward declaration.

Diff:
---
 gcc/cp/pt.cc | 45 +++++++++++++++++++--------------------------
 1 file changed, 19 insertions(+), 26 deletions(-)

diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
index a668825b5f9..678cb7930e3 100644
--- a/gcc/cp/pt.cc
+++ b/gcc/cp/pt.cc
@@ -183,7 +183,6 @@ static int unify_pack_expansion (tree, tree, tree,
 static tree copy_template_args (tree);
 static tree tsubst_template_parms (tree, tree, tsubst_flags_t);
 static void tsubst_each_template_parm_constraints (tree, tree, tsubst_flags_t);
-tree most_specialized_partial_spec (tree, tsubst_flags_t);
 static tree tsubst_aggr_type (tree, tree, tsubst_flags_t, tree, int);
 static tree tsubst_aggr_type_1 (tree, tree, tsubst_flags_t, tree, int);
 static tree tsubst_arg_types (tree, tree, tree, tsubst_flags_t, tree);
@@ -26002,15 +26001,7 @@ most_general_template (tree decl)
 tree
 most_specialized_partial_spec (tree target, tsubst_flags_t complain)
 {
-  tree list = NULL_TREE;
-  tree t;
-  tree champ;
-  int fate;
-  bool ambiguous_p;
-  tree outer_args = NULL_TREE;
-  tree tmpl, args;
-
-  tree decl;
+  tree tmpl, args, decl;
   if (TYPE_P (target))
     {
       tree tinfo = CLASSTYPE_TEMPLATE_INFO (target);
@@ -26034,13 +26025,18 @@ most_specialized_partial_spec (tree target, tsubst_flags_t complain)
   else
     gcc_unreachable ();
 
+  tree main_tmpl = most_general_template (tmpl);
+  tree specs = DECL_TEMPLATE_SPECIALIZATIONS (main_tmpl);
+  if (!specs)
+    /* There are no partial specializations of this template.  */
+    return NULL_TREE;
+
   push_access_scope_guard pas (decl);
   deferring_access_check_sentinel acs (dk_no_deferred);
 
-  tree main_tmpl = most_general_template (tmpl);
-
   /* For determining which partial specialization to use, only the
      innermost args are interesting.  */
+  tree outer_args = NULL_TREE;
   if (TMPL_ARGS_HAVE_MULTIPLE_LEVELS (args))
     {
       outer_args = strip_innermost_template_args (args, 1);
@@ -26052,7 +26048,8 @@ most_specialized_partial_spec (tree target, tsubst_flags_t complain)
      fully resolve everything.  */
   processing_template_decl_sentinel ptds;
 
-  for (t = DECL_TEMPLATE_SPECIALIZATIONS (main_tmpl); t; t = TREE_CHAIN (t))
+  tree list = NULL_TREE;
+  for (tree t = specs; t; t = TREE_CHAIN (t))
     {
       const tree ospec_tmpl = TREE_VALUE (t);
 
@@ -26075,10 +26072,8 @@ most_specialized_partial_spec (tree target, tsubst_flags_t complain)
 	  if (outer_args)
 	    spec_args = add_to_template_args (outer_args, spec_args);
 
-          /* Keep the candidate only if the constraints are satisfied,
-             or if we're not compiling with concepts.  */
-          if (!flag_concepts
-	      || constraints_satisfied_p (ospec_tmpl, spec_args))
+	  /* Keep the candidate only if its constraints are satisfied.  */
+	  if (constraints_satisfied_p (ospec_tmpl, spec_args))
             {
 	      list = tree_cons (spec_args, ospec_tmpl, list);
               TREE_TYPE (list) = TREE_TYPE (t);
@@ -26089,13 +26084,11 @@ most_specialized_partial_spec (tree target, tsubst_flags_t complain)
   if (! list)
     return NULL_TREE;
 
-  ambiguous_p = false;
-  t = list;
-  champ = t;
-  t = TREE_CHAIN (t);
-  for (; t; t = TREE_CHAIN (t))
+  tree champ = list;
+  bool ambiguous_p = false;
+  for (tree t = TREE_CHAIN (list); t; t = TREE_CHAIN (t))
     {
-      fate = more_specialized_partial_spec (tmpl, champ, t);
+      int fate = more_specialized_partial_spec (tmpl, champ, t);
       if (fate == 1)
 	;
       else
@@ -26114,9 +26107,9 @@ most_specialized_partial_spec (tree target, tsubst_flags_t complain)
     }
 
   if (!ambiguous_p)
-    for (t = list; t && t != champ; t = TREE_CHAIN (t))
+    for (tree t = list; t && t != champ; t = TREE_CHAIN (t))
       {
-	fate = more_specialized_partial_spec (tmpl, champ, t);
+	int fate = more_specialized_partial_spec (tmpl, champ, t);
 	if (fate != 1)
 	  {
 	    ambiguous_p = true;
@@ -26135,7 +26128,7 @@ most_specialized_partial_spec (tree target, tsubst_flags_t complain)
       else
 	error ("ambiguous template instantiation for %q#D", target);
       str = ngettext ("candidate is:", "candidates are:", list_length (list));
-      for (t = list; t; t = TREE_CHAIN (t))
+      for (tree t = list; t; t = TREE_CHAIN (t))
         {
 	  tree subst = build_tree_list (TREE_VALUE (t), TREE_PURPOSE (t));
           inform (DECL_SOURCE_LOCATION (TREE_VALUE (t)),