Re: [PATCH] c++: Micro-optimize most_specialized_partial_spec

[Jason Merrill via Gcc-patches]

On 8/31/22 17:15, Patrick Palka wrote:

This introduces an early exit test to most_specialized_partial_spec for
the common case where we have no partial specializations, which allows
us to avoid some unnecessary work.  In passing, clean the function up a
bit.

Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look OK for
trunk?

OK.

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.
---
  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 fe7e809fc2d..497a18ef728 100644
--- a/gcc/cp/pt.cc
+++ b/gcc/cp/pt.cc
@@ -187,7 +187,6 @@ 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_arg_types (tree, tree, tree, tsubst_flags_t, tree);
  static tree tsubst_function_type (tree, tree, tsubst_flags_t, tree);
@@ -25756,15 +25755,7 @@ 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);
@@ -25788,13 +25779,18 @@ 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);
@@ -25806,7 +25802,8 @@ 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);

@@ -25829,10 +25826,8 @@ 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 the 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);
@@ -25843,13 +25838,11 @@ 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
@@ -25868,9 +25861,9 @@ 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;
@@ -25889,7 +25882,7 @@ 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)),