c++: redundant targ coercion for var/alias tmpls

When stepping through the variable/alias template specialization code
paths, I noticed we perform template argument coercion twice: first from
lookup_template_variable / instantiate_alias_template and again from
tsubst_decl (during instantiate_template).  It'd be nice to avoid this
redundant second coercion.

It turns out this coercion in tsubst_decl could be safely elided whenever
fully specializing a primary variable/alias template, because we can rely
on lookup_template_variable / instantiate_alias_template to already have
coerced the arguments.

The only other situation to consider seems to be when fully specializing
a partial variable template specialization (from instantiate_template),
in which case the passed 'args' are the (already coerced) arguments
relative to the partial template, and the resulting 'argvec' are the
(uncoerced) arguments relative to the primary template, so coercion is
still necessary.  But computing 'argvec' here is only really necessary
in order to look up (and insert into) the specializations table.  And
instantiate_template already performs a lookup, so if we just made it
register the resulting specialization too then we could avoid having to
compute 'argvec' when called from instantiate_template, which in turns
means we could avoid the coercion altogether.  This patch implements
this approach.

gcc/cp/ChangeLog:

	* pt.cc (tsubst_function_decl): Add defaulted 'use_spec_table'
	flag parameter.  Don't look up or insert into the specializations
	table if 'use_spec_table' is false.
	(tsubst_decl): Add defaulted 'use_spec_table' flag parameter.
	Check for error_mark_node.
	<case FUNCTION_DECL>: Pass 'use_spec_table' to
	tsubst_function_decl.
	<case TYPE/VAR_DECL>: Don't call coerce_template_parms.
	Don't look up or insert into the specializations table if
	'use_spec_table' is false.  Exit earlier if the substituted
	type is erroneous and we're not complaining, and do so for
	alias specializations as well.
	(instantiate_template): Pass false as 'use_spec_table'
	to tsubst_decl.  Call register_specialization afterwards.

1 files changed, 65 insertions, 50 deletions

diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
index 255d18b..d882e9d 100644
--- a/gcc/cp/pt.cc
+++ b/gcc/cp/pt.cc
@@ -204,7 +204,7 @@ static bool invalid_nontype_parm_type_p (tree, tsubst_flags_t);
 static bool dependent_template_arg_p (tree);
 static bool dependent_type_p_r (tree);
 static tree tsubst_copy	(tree, tree, tsubst_flags_t, tree);
-static tree tsubst_decl (tree, tree, tsubst_flags_t);
+static tree tsubst_decl (tree, tree, tsubst_flags_t, bool = true);
 static tree tsubst_scope (tree, tree, tsubst_flags_t, tree);
 static void perform_instantiation_time_access_checks (tree, tree);
 static tree listify (tree);
@@ -14304,7 +14304,7 @@ maybe_rebuild_function_decl_type (tree decl)
 
 static tree
 tsubst_function_decl (tree t, tree args, tsubst_flags_t complain,
-		      tree lambda_fntype)
+		      tree lambda_fntype, bool use_spec_table = true)
 {
   tree gen_tmpl = NULL_TREE, argvec = NULL_TREE;
   hashval_t hash = 0;
@@ -14345,22 +14345,27 @@ tsubst_function_decl (tree t, tree args, tsubst_flags_t complain,
 
       /* Calculate the complete set of arguments used to
 	 specialize R.  */
-      argvec = tsubst_template_args (DECL_TI_ARGS
-				     (DECL_TEMPLATE_RESULT
-				      (DECL_TI_TEMPLATE (t))),
-				     args, complain, in_decl);
-      if (argvec == error_mark_node)
-	return error_mark_node;
-
-      /* Check to see if we already have this specialization.  */
-      if (!lambda_fntype)
+      if (use_spec_table)
 	{
-	  hash = spec_hasher::hash (gen_tmpl, argvec);
-	  if (tree spec = retrieve_specialization (gen_tmpl, argvec, hash))
-	    /* The spec for these args might be a partial instantiation of the
-	       template, but here what we want is the FUNCTION_DECL.  */
-	    return STRIP_TEMPLATE (spec);
+	  argvec = tsubst_template_args (DECL_TI_ARGS
+					 (DECL_TEMPLATE_RESULT
+					  (DECL_TI_TEMPLATE (t))),
+					 args, complain, in_decl);
+	  if (argvec == error_mark_node)
+	    return error_mark_node;
+
+	  /* Check to see if we already have this specialization.  */
+	  if (!lambda_fntype)
+	    {
+	      hash = spec_hasher::hash (gen_tmpl, argvec);
+	      if (tree spec = retrieve_specialization (gen_tmpl, argvec, hash))
+		/* The spec for these args might be a partial instantiation of the
+		   template, but here what we want is the FUNCTION_DECL.  */
+		return STRIP_TEMPLATE (spec);
+	    }
 	}
+      else
+	argvec = args;
     }
   else
     {
@@ -14527,12 +14532,15 @@ tsubst_function_decl (tree t, tree args, tsubst_flags_t complain,
 	= build_template_info (gen_tmpl, argvec);
       SET_DECL_IMPLICIT_INSTANTIATION (r);
 
-      tree new_r
-	= register_specialization (r, gen_tmpl, argvec, false, hash);
-      if (new_r != r)
-	/* We instantiated this while substituting into
-	   the type earlier (template/friend54.C).  */
-	return new_r;
+      if (use_spec_table)
+	{
+	  tree new_r
+	    = register_specialization (r, gen_tmpl, argvec, false, hash);
+	  if (new_r != r)
+	    /* We instantiated this while substituting into
+	       the type earlier (template/friend54.C).  */
+	    return new_r;
+	}
 
       /* We're not supposed to instantiate default arguments
 	 until they are called, for a template.  But, for a
@@ -14855,10 +14863,14 @@ enclosing_instantiation_of (tree tctx)
 
 /* Substitute the ARGS into the T, which is a _DECL.  Return the
    result of the substitution.  Issue error and warning messages under
-   control of COMPLAIN.  */
+   control of COMPLAIN.  The flag USE_SPEC_TABLE controls if we look up
+   and insert into the specializations table or if we can assume it's
+   the caller's responsibility; this is used by instantiate_template
+   to avoid doing some redundant work.  */
 
 static tree
-tsubst_decl (tree t, tree args, tsubst_flags_t complain)
+tsubst_decl (tree t, tree args, tsubst_flags_t complain,
+	     bool use_spec_table /* = true */)
 {
 #define RETURN(EXP) do { r = (EXP); goto out; } while(0)
   location_t saved_loc;
@@ -14866,6 +14878,9 @@ tsubst_decl (tree t, tree args, tsubst_flags_t complain)
   tree in_decl = t;
   hashval_t hash = 0;
 
+  if (t == error_mark_node)
+    return error_mark_node;
+
   /* Set the filename and linenumber to improve error-reporting.  */
   saved_loc = input_location;
   input_location = DECL_SOURCE_LOCATION (t);
@@ -14879,7 +14894,8 @@ tsubst_decl (tree t, tree args, tsubst_flags_t complain)
       break;
 
     case FUNCTION_DECL:
-      r = tsubst_function_decl (t, args, complain, /*lambda*/NULL_TREE);
+      r = tsubst_function_decl (t, args, complain, /*lambda*/NULL_TREE,
+				use_spec_table);
       break;
 
     case PARM_DECL:
@@ -15228,21 +15244,17 @@ tsubst_decl (tree t, tree args, tsubst_flags_t complain)
 	    if (!spec)
 	      {
 		tmpl = DECL_TI_TEMPLATE (t);
-		gen_tmpl = most_general_template (tmpl);
-		argvec = tsubst (DECL_TI_ARGS (t), args, complain, in_decl);
-		if (argvec != error_mark_node
-		    && PRIMARY_TEMPLATE_P (gen_tmpl)
-		    && TMPL_ARGS_DEPTH (args) >= TMPL_ARGS_DEPTH (argvec))
-		  /* We're fully specializing a template declaration, so
-		     we need to coerce the innermost arguments corresponding to
-		     the template.  */
-		  argvec = (coerce_template_parms
-			    (DECL_TEMPLATE_PARMS (gen_tmpl),
-			     argvec, tmpl, complain));
-		if (argvec == error_mark_node)
-		  RETURN (error_mark_node);
-		hash = spec_hasher::hash (gen_tmpl, argvec);
-		spec = retrieve_specialization (gen_tmpl, argvec, hash);
+		if (use_spec_table)
+		  {
+		    argvec = tsubst (DECL_TI_ARGS (t), args, complain, in_decl);
+		    if (argvec == error_mark_node)
+		      RETURN (error_mark_node);
+		    gen_tmpl = most_general_template (tmpl);
+		    hash = spec_hasher::hash (gen_tmpl, argvec);
+		    spec = retrieve_specialization (gen_tmpl, argvec, hash);
+		  }
+		else
+		  argvec = args;
 	      }
 	  }
 	else
@@ -15287,20 +15299,20 @@ tsubst_decl (tree t, tree args, tsubst_flags_t complain)
 	    type = tsubst (type, args, tcomplain, in_decl);
 	    /* Substituting the type might have recursively instantiated this
 	       same alias (c++/86171).  */
-	    if (gen_tmpl && DECL_ALIAS_TEMPLATE_P (gen_tmpl)
+	    if (use_spec_table && gen_tmpl && DECL_ALIAS_TEMPLATE_P (gen_tmpl)
 		&& (spec = retrieve_specialization (gen_tmpl, argvec, hash)))
 	      {
 		r = spec;
 		break;
 	      }
 	  }
+	if (type == error_mark_node && !(complain & tf_error))
+	  RETURN (error_mark_node);
 	r = copy_decl (t);
 	if (VAR_P (r))
 	  {
 	    DECL_INITIALIZED_P (r) = 0;
 	    DECL_TEMPLATE_INSTANTIATED (r) = 0;
-	    if (type == error_mark_node)
-	      RETURN (error_mark_node);
 	    if (TREE_CODE (type) == FUNCTION_TYPE)
 	      {
 		/* It may seem that this case cannot occur, since:
@@ -15404,7 +15416,7 @@ tsubst_decl (tree t, tree args, tsubst_flags_t complain)
 
 	    DECL_TEMPLATE_INFO (r) = build_template_info (tmpl, argvec);
 	    SET_DECL_IMPLICIT_INSTANTIATION (r);
-	    if (!error_operand_p (r) || (complain & tf_error))
+	    if (use_spec_table)
 	      register_specialization (r, gen_tmpl, argvec, false, hash);
 	  }
 	else
@@ -21991,7 +22003,6 @@ instantiate_template (tree tmpl, tree orig_args, tsubst_flags_t complain)
   tree targ_ptr = orig_args;
   tree fndecl;
   tree gen_tmpl;
-  tree spec;
   bool access_ok = true;
 
   if (tmpl == error_mark_node)
@@ -22038,9 +22049,8 @@ instantiate_template (tree tmpl, tree orig_args, tsubst_flags_t complain)
 		(DECL_TI_ARGS (DECL_TEMPLATE_RESULT (tmpl)),
 		 targ_ptr));
 
-  /* It would be nice to avoid hashing here and then again in tsubst_decl,
-     but it doesn't seem to be on the hot path.  */
-  spec = retrieve_specialization (gen_tmpl, targ_ptr, 0);
+  hashval_t hash = spec_hasher::hash (gen_tmpl, targ_ptr);
+  tree spec = retrieve_specialization (gen_tmpl, targ_ptr, hash);
 
   gcc_checking_assert (tmpl == gen_tmpl
 		       || ((fndecl
@@ -22108,13 +22118,14 @@ instantiate_template (tree tmpl, tree orig_args, tsubst_flags_t complain)
 	  tree partial_tmpl = TI_TEMPLATE (partial_ti);
 	  tree partial_args = TI_ARGS (partial_ti);
 	  tree partial_pat = DECL_TEMPLATE_RESULT (partial_tmpl);
-	  fndecl = tsubst (partial_pat, partial_args, complain, gen_tmpl);
+	  fndecl = tsubst_decl (partial_pat, partial_args, complain,
+				/*use_spec_table=*/false);
 	}
     }
 
   /* Substitute template parameters to obtain the specialization.  */
   if (fndecl == NULL_TREE)
-    fndecl = tsubst (pattern, targ_ptr, complain, gen_tmpl);
+    fndecl = tsubst_decl (pattern, targ_ptr, complain, /*use_spec_table=*/false);
   if (DECL_CLASS_SCOPE_P (gen_tmpl))
     pop_nested_class ();
   pop_from_top_level ();
@@ -22134,6 +22145,10 @@ instantiate_template (tree tmpl, tree orig_args, tsubst_flags_t complain)
        remember the result of most_specialized_partial_spec for it.  */
     TI_PARTIAL_INFO (DECL_TEMPLATE_INFO (fndecl)) = partial_ti;
 
+  fndecl = register_specialization (fndecl, gen_tmpl, targ_ptr, false, hash);
+  if (fndecl == error_mark_node)
+    return error_mark_node;
+
   set_instantiating_module (fndecl);
 
   /* Now we know the specialization, compute access previously