Re: [PATCH] c++: Define built-in for std::tuple_element [PR100157]

[Patrick Palka <ppalka@redhat.com>]

On Wed, 5 Oct 2022, Patrick Palka wrote:

> On Thu, 7 Jul 2022, Jonathan Wakely via Gcc-patches wrote:
> 
> > This adds a new built-in to replace the recursive class template
> > instantiations done by traits such as std::tuple_element and
> > std::variant_alternative. The purpose is to select the Nth type from a
> > list of types, e.g. __builtin_type_pack_element(1, char, int, float) is
> > int.
> > 
> > For a pathological example tuple_element_t<1000, tuple<2000 types...>>
> > the compilation time is reduced by more than 90% and the memory  used by
> > the compiler is reduced by 97%. In realistic examples the gains will be
> > much smaller, but still relevant.
> > 
> > Clang has a similar built-in, __type_pack_element<N, T...>, but that's a
> > "magic template" built-in using <> syntax, which GCC doesn't support. So
> > this provides an equivalent feature, but as a built-in function using
> > parens instead of <>. I don't really like the name "type pack element"
> > (it gives you an element from a pack of types) but the semi-consistency
> > with Clang seems like a reasonable argument in favour of keeping the
> > name. I'd be open to alternative names though, e.g. __builtin_nth_type
> > or __builtin_type_at_index.
> 
> Rather than giving the trait a different name from __type_pack_element,
> I wonder if we could just special case cp_parser_trait to expect <>
> instead of parens for this trait?
> 
> Btw the frontend recently got a generic TRAIT_TYPE tree code, which gets
> rid of much of the boilerplate of adding a new type-yielding built-in
> trait, see e.g. cp-trait.def.

Here's a tested patch based on Jonathan's original patch that implements
the built-in in terms of TRAIT_TYPE, names it __type_pack_element
instead of __builtin_type_pack_element, and treats invocations of it
like a template-id instead of a call (to match Clang).

-- >8 --

Subject: [PATCH] c++: Define built-in for std::tuple_element [PR100157]

This adds a new built-in to replace the recursive class template
instantiations done by traits such as std::tuple_element and
std::variant_alternative.  The purpose is to select the Nth type from a
list of types, e.g. __type_pack_element<1, char, int, float> is int.
We implement it as a special kind of TRAIT_TYPE.

For a pathological example tuple_element_t<1000, tuple<2000 types...>>
the compilation time is reduced by more than 90% and the memory  used by
the compiler is reduced by 97%.  In realistic examples the gains will be
much smaller, but still relevant.

Unlike the other built-in traits, __type_pack_element uses template-id
syntax instead of call syntax and is SFINAE-enabled, matching Clang's
implementation.  And like the other built-in traits, it's not mangleable
so we can't use it directly in function signatures.

Some caveats:

  * Clang's version of the built-in seems to act like a "magic template"
    that can e.g. be used as a template template argument.  For simplicity
    we implement it in a more ad-hoc way.
  * Our parsing of the <>'s in __type_pack_element<...> is currently
    rudimentary and doesn't try to disambiguate a trailing >> vs > >
    as cp_parser_enclosed_template_argument_list does.

Co-authored-by: Jonathan Wakely <jwakely@redhat.com>

	PR c++/100157

gcc/cp/ChangeLog:

	* cp-trait.def (TYPE_PACK_ELEMENT): Define.
	* cp-tree.h (finish_trait_type): Add complain parameter.
	* cxx-pretty-print.cc (pp_cxx_trait): Handle
	CPTK_TYPE_PACK_ELEMENT.
	* parser.cc (cp_parser_constant_expression): Document default
	arguments.
	(cp_parser_trait): Handle CPTK_TYPE_PACK_ELEMENT.  Pass
	tf_warning_or_error to finish_trait_type.
	* pt.cc (tsubst) <case TRAIT_TYPE>: Handle
	CPTK_TYPE_PACK_ELEMENT.
	* semantics.cc (finish_type_pack_element): Define.
	(finish_trait_type): Add complain parameter.  Handle
	CPTK_TYPE_PACK_ELEMENT.
	* tree.cc (strip_typedefs): Pass tf_warning_or_error to
	finish_trait_type.
	* typeck.cc (structural_comptypes) <case TRAIT_TYPE>: Use
	cp_tree_equal instead of same_type_p for TRAIT_TYPE_TYPE.

libstdc++-v3/ChangeLog:

	* include/bits/utility.h (_Nth_type): Conditionally define using
	__type_pack_element.

gcc/testsuite/ChangeLog:

	* g++.dg/ext/type_pack_element1.C: New test.
	* g++.dg/ext/type_pack_element2.C: New test.
	* g++.dg/ext/type_pack_element3.C: New test.
---
 gcc/cp/cp-trait.def                           |  1 +
 gcc/cp/cp-tree.h                              |  2 +-
 gcc/cp/cxx-pretty-print.cc                    | 17 ++++++--
 gcc/cp/parser.cc                              | 36 ++++++++++++-----
 gcc/cp/pt.cc                                  |  8 +++-
 gcc/cp/semantics.cc                           | 39 ++++++++++++++++++-
 gcc/cp/tree.cc                                |  3 +-
 gcc/cp/typeck.cc                              |  2 +-
 gcc/testsuite/g++.dg/ext/type_pack_element1.C | 19 +++++++++
 gcc/testsuite/g++.dg/ext/type_pack_element2.C | 14 +++++++
 gcc/testsuite/g++.dg/ext/type_pack_element3.C | 14 +++++++
 libstdc++-v3/include/bits/utility.h           |  6 +++
 12 files changed, 142 insertions(+), 19 deletions(-)
 create mode 100644 gcc/testsuite/g++.dg/ext/type_pack_element1.C
 create mode 100644 gcc/testsuite/g++.dg/ext/type_pack_element2.C
 create mode 100644 gcc/testsuite/g++.dg/ext/type_pack_element3.C

diff --git a/gcc/cp/cp-trait.def b/gcc/cp/cp-trait.def
index 823899a26c5..63f6b101eeb 100644
--- a/gcc/cp/cp-trait.def
+++ b/gcc/cp/cp-trait.def
@@ -89,6 +89,7 @@ DEFTRAIT_TYPE (REMOVE_CV, "__remove_cv", 1)
 DEFTRAIT_TYPE (REMOVE_REFERENCE, "__remove_reference", 1)
 DEFTRAIT_TYPE (REMOVE_CVREF, "__remove_cvref", 1)
 DEFTRAIT_TYPE (UNDERLYING_TYPE,  "__underlying_type", 1)
+DEFTRAIT_TYPE (TYPE_PACK_ELEMENT, "__type_pack_element", -1)
 
 /* These traits yield a type pack, not a type, and are represented by
    cp_parser_trait as a special BASES tree instead of a TRAIT_TYPE tree.  */
diff --git a/gcc/cp/cp-tree.h b/gcc/cp/cp-tree.h
index 1f4967c2ba0..f3cc2c5874c 100644
--- a/gcc/cp/cp-tree.h
+++ b/gcc/cp/cp-tree.h
@@ -7764,7 +7764,7 @@ extern tree finish_decltype_type                (tree, bool, tsubst_flags_t);
 extern tree fold_builtin_is_corresponding_member (location_t, int, tree *);
 extern tree fold_builtin_is_pointer_inverconvertible_with_class (location_t, int, tree *);
 extern tree finish_trait_expr			(location_t, enum cp_trait_kind, tree, tree);
-extern tree finish_trait_type			(enum cp_trait_kind, tree, tree);
+extern tree finish_trait_type			(enum cp_trait_kind, tree, tree, tsubst_flags_t);
 extern tree build_lambda_expr                   (void);
 extern tree build_lambda_object			(tree);
 extern tree begin_lambda_type                   (tree);
diff --git a/gcc/cp/cxx-pretty-print.cc b/gcc/cp/cxx-pretty-print.cc
index 8ca1b8f234a..07a0406e491 100644
--- a/gcc/cp/cxx-pretty-print.cc
+++ b/gcc/cp/cxx-pretty-print.cc
@@ -2625,8 +2625,16 @@ pp_cxx_trait (cxx_pretty_printer *pp, tree t)
 #undef DEFTRAIT
     }
 
-  pp_cxx_left_paren (pp);
-  pp->type_id (type1);
+  if (kind == CPTK_TYPE_PACK_ELEMENT)
+    {
+      pp_cxx_begin_template_argument_list (pp);
+      pp->expression (type1);
+    }
+  else
+    {
+      pp_cxx_left_paren (pp);
+      pp->type_id (type1);
+    }
   if (type2)
     {
       if (TREE_CODE (type2) != TREE_LIST)
@@ -2641,7 +2649,10 @@ pp_cxx_trait (cxx_pretty_printer *pp, tree t)
 	    pp->type_id (TREE_VALUE (arg));
 	  }
     }
-  pp_cxx_right_paren (pp);
+  if (kind == CPTK_TYPE_PACK_ELEMENT)
+    pp_cxx_end_template_argument_list (pp);
+  else
+    pp_cxx_right_paren (pp);
 }
 
 // requires-clause:
diff --git a/gcc/cp/parser.cc b/gcc/cp/parser.cc
index 8b1658decba..c6e31aa6f3f 100644
--- a/gcc/cp/parser.cc
+++ b/gcc/cp/parser.cc
@@ -10683,9 +10683,9 @@ cp_parser_expression (cp_parser* parser, cp_id_kind * pidk,
 
 static cp_expr
 cp_parser_constant_expression (cp_parser* parser,
-			       int allow_non_constant_p,
-			       bool *non_constant_p,
-			       bool strict_p)
+			       int allow_non_constant_p /* = 0 */,
+			       bool *non_constant_p /* = NULL */,
+			       bool strict_p /* = false */)
 {
   bool saved_integral_constant_expression_p;
   bool saved_allow_non_integral_constant_expression_p;
@@ -10912,12 +10912,25 @@ cp_parser_trait (cp_parser* parser, enum rid keyword)
   cp_lexer_consume_token (parser->lexer);
 
   matching_parens parens;
-  parens.require_open (parser);
 
-  {
-    type_id_in_expr_sentinel s (parser);
-    type1 = cp_parser_type_id (parser);
-  }
+  /* Unlike the other built-in traits, __type_pack_element is unique in that
+     it takes an expression as its first argument and it uses template-id
+     syntax instead of function call syntax (for compatibility with Clang).
+     We special case these properties of __type_pack_element here and elsewhere.
+
+     TODO: Generalize the trait infrastructure so that __type_pack_element
+     is no longer a special case.  */
+  if (kind == CPTK_TYPE_PACK_ELEMENT)
+    {
+      cp_parser_require (parser, CPP_LESS, RT_LESS);
+      type1 = cp_parser_constant_expression (parser);
+    }
+  else
+    {
+      parens.require_open (parser);
+      type_id_in_expr_sentinel s (parser);
+      type1 = cp_parser_type_id (parser);
+    }
 
   if (type1 == error_mark_node)
     return error_mark_node;
@@ -10953,7 +10966,10 @@ cp_parser_trait (cp_parser* parser, enum rid keyword)
     }
 
   location_t finish_loc = cp_lexer_peek_token (parser->lexer)->location;
-  parens.require_close (parser);
+  if (kind == CPTK_TYPE_PACK_ELEMENT)
+    cp_parser_require (parser, CPP_GREATER, RT_GREATER);
+  else
+    parens.require_close (parser);
 
   /* Construct a location of the form:
        __is_trivially_copyable(_Tp)
@@ -10971,7 +10987,7 @@ cp_parser_trait (cp_parser* parser, enum rid keyword)
       return cp_expr (finish_bases (type1, true), trait_loc);
     default:
       if (type)
-	return finish_trait_type (kind, type1, type2);
+	return finish_trait_type (kind, type1, type2, tf_warning_or_error);
       else
 	return finish_trait_expr (trait_loc, kind, type1, type2);
     }
diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
index cbe5898b553..2e83970a252 100644
--- a/gcc/cp/pt.cc
+++ b/gcc/cp/pt.cc
@@ -16576,9 +16576,13 @@ tsubst (tree t, tree args, tsubst_flags_t complain, tree in_decl)
 
     case TRAIT_TYPE:
       {
-	tree type1 = tsubst (TRAIT_TYPE_TYPE1 (t), args, complain, in_decl);
+	tree type1 = TRAIT_TYPE_TYPE1 (t);
+	if (TRAIT_TYPE_KIND (t) != CPTK_TYPE_PACK_ELEMENT)
+	  type1 = tsubst (type1, args, complain, in_decl);
+	else
+	  type1 = tsubst_copy_and_build (type1, args, complain, in_decl);
 	tree type2 = tsubst (TRAIT_TYPE_TYPE2 (t), args, complain, in_decl);
-	type = finish_trait_type (TRAIT_TYPE_KIND (t), type1, type2);
+	type = finish_trait_type (TRAIT_TYPE_KIND (t), type1, type2, complain);
 	return cp_build_qualified_type (type,
 					cp_type_quals (t) | cp_type_quals (type),
 					complain | tf_ignore_bad_quals);
diff --git a/gcc/cp/semantics.cc b/gcc/cp/semantics.cc
index ef5bf2430b1..af9edf59b3b 100644
--- a/gcc/cp/semantics.cc
+++ b/gcc/cp/semantics.cc
@@ -4457,6 +4457,36 @@ finish_underlying_type (tree type)
   return underlying_type;
 }
 
+/* Implement the __type_pack_element keyword: Return the type
+   at index IDX within TYPES.  */
+
+static tree
+finish_type_pack_element (tree idx, tree types, tsubst_flags_t complain)
+{
+  idx = maybe_constant_value (idx);
+  if (TREE_CODE (idx) != INTEGER_CST || !INTEGRAL_TYPE_P (TREE_TYPE (idx)))
+    {
+      if (complain & tf_error)
+	error ("%<__type_pack_element%> index is not an integral constant");
+      return error_mark_node;
+    }
+  HOST_WIDE_INT val = tree_to_shwi (idx);
+  if (val < 0)
+    {
+      if (complain & tf_error)
+	error ("%<__type_pack_element%> index is negative");
+      return error_mark_node;
+    }
+  tree result = chain_index (val, types);
+  if (!result)
+    {
+      if (complain & tf_error)
+	error ("%<__type_pack_element%> index is out of range");
+      return error_mark_node;
+    }
+  return TREE_VALUE (result);
+}
+
 /* Implement the __direct_bases keyword: Return the direct base classes
    of type.  */
 
@@ -12213,7 +12243,8 @@ finish_trait_expr (location_t loc, cp_trait_kind kind, tree type1, tree type2)
 /* Process a trait type.  */
 
 tree
-finish_trait_type (cp_trait_kind kind, tree type1, tree type2)
+finish_trait_type (cp_trait_kind kind, tree type1, tree type2,
+		   tsubst_flags_t complain)
 {
   if (type1 == error_mark_node
       || type2 == error_mark_node)
@@ -12237,17 +12268,23 @@ finish_trait_type (cp_trait_kind kind, tree type1, tree type2)
     {
     case CPTK_UNDERLYING_TYPE:
       return finish_underlying_type (type1);
+
     case CPTK_REMOVE_CV:
       return cv_unqualified (type1);
+
     case CPTK_REMOVE_REFERENCE:
       if (TYPE_REF_P (type1))
 	type1 = TREE_TYPE (type1);
       return type1;
+
     case CPTK_REMOVE_CVREF:
       if (TYPE_REF_P (type1))
 	type1 = TREE_TYPE (type1);
       return cv_unqualified (type1);
 
+    case CPTK_TYPE_PACK_ELEMENT:
+      return finish_type_pack_element (type1, type2, complain);
+
 #define DEFTRAIT_EXPR(CODE, NAME, ARITY) \
     case CPTK_##CODE:
 #include "cp-trait.def"
diff --git a/gcc/cp/tree.cc b/gcc/cp/tree.cc
index faf01616f87..dd5b0662235 100644
--- a/gcc/cp/tree.cc
+++ b/gcc/cp/tree.cc
@@ -1799,7 +1799,8 @@ strip_typedefs (tree t, bool *remove_attributes /* = NULL */,
 	if (type1 == TRAIT_TYPE_TYPE1 (t) && type2 == TRAIT_TYPE_TYPE2 (t))
 	  result = NULL_TREE;
 	else
-	  result = finish_trait_type (TRAIT_TYPE_KIND (t), type1, type2);
+	  result = finish_trait_type (TRAIT_TYPE_KIND (t), type1, type2,
+				      tf_warning_or_error);
       }
       break;
     case TYPE_PACK_EXPANSION:
diff --git a/gcc/cp/typeck.cc b/gcc/cp/typeck.cc
index 69b1268cfec..f5548737e4c 100644
--- a/gcc/cp/typeck.cc
+++ b/gcc/cp/typeck.cc
@@ -1632,7 +1632,7 @@ structural_comptypes (tree t1, tree t2, int strict)
     case TRAIT_TYPE:
       if (TRAIT_TYPE_KIND (t1) != TRAIT_TYPE_KIND (t2))
 	return false;
-      if (!same_type_p (TRAIT_TYPE_TYPE1 (t1), TRAIT_TYPE_TYPE1 (t2))
+      if (!cp_tree_equal (TRAIT_TYPE_TYPE1 (t1), TRAIT_TYPE_TYPE1 (t2))
 	  || !cp_tree_equal (TRAIT_TYPE_TYPE2 (t1), TRAIT_TYPE_TYPE2 (t2)))
 	return false;
       break;
diff --git a/gcc/testsuite/g++.dg/ext/type_pack_element1.C b/gcc/testsuite/g++.dg/ext/type_pack_element1.C
new file mode 100644
index 00000000000..46858555502
--- /dev/null
+++ b/gcc/testsuite/g++.dg/ext/type_pack_element1.C
@@ -0,0 +1,19 @@
+// { dg-do compile { target c++11 } }
+
+using ty0 = __type_pack_element<0, int>;
+using ty0 = __type_pack_element<0, int, char>;
+using ty0 = int;
+
+using ty1 = __type_pack_element<1, int, char>;
+using ty1 = __type_pack_element<(6 - 5) * 1, int, char>;
+using ty1 = char;
+
+template<int N, class... Ts>
+using __const_type_pack_element_t = const __type_pack_element<N, Ts...>;
+
+using ty2 = __const_type_pack_element_t<2, int, char, long>;
+using ty2 = const long;
+
+template<class T> struct A { };
+using ty3 = __type_pack_element<3, int, int, int, A<int>>;
+using ty3 = A<int>;
diff --git a/gcc/testsuite/g++.dg/ext/type_pack_element2.C b/gcc/testsuite/g++.dg/ext/type_pack_element2.C
new file mode 100644
index 00000000000..363cfa92514
--- /dev/null
+++ b/gcc/testsuite/g++.dg/ext/type_pack_element2.C
@@ -0,0 +1,14 @@
+// { dg-do compile { target c++11 } }
+
+int p;
+
+using type = __type_pack_element<&p, int>; // { dg-error "not an integral constant" }
+using type = __type_pack_element<1, int>; // { dg-error "out of range" }
+using type = __type_pack_element<-1, int>; // { dg-error "negative" }
+using type = __type_pack_element<2, int, char>; // { dg-error "out of range" }
+
+template<int N, class... Ts>
+using __type_pack_element_t = __type_pack_element<N, Ts...>;
+// { dg-error "out of range" "" { target *-*-* } .-1 }
+
+using type = __type_pack_element_t<3, int, char, long>; // { dg-message "here" }
diff --git a/gcc/testsuite/g++.dg/ext/type_pack_element3.C b/gcc/testsuite/g++.dg/ext/type_pack_element3.C
new file mode 100644
index 00000000000..5cba03d4b81
--- /dev/null
+++ b/gcc/testsuite/g++.dg/ext/type_pack_element3.C
@@ -0,0 +1,14 @@
+// { dg-do compile { target c++11 } }
+
+template<class T, T N, class... Ts, class = __type_pack_element<N, Ts...> >
+constexpr int f(int) { return 1; }
+
+template<class T, T N, class... Ts>
+constexpr int f(...) { return 2; };
+
+int p;
+
+static_assert(f<int, 0, void, char>(0) == 1, "");
+static_assert(f<int, 1, void, char>(0) == 1, "");
+static_assert(f<int, 2, void, char>(0) == 2, "");
+static_assert(f<int*, &p, void, char>(0) == 2, "");
diff --git a/libstdc++-v3/include/bits/utility.h b/libstdc++-v3/include/bits/utility.h
index 6a192e27836..5d524d33b4c 100644
--- a/libstdc++-v3/include/bits/utility.h
+++ b/libstdc++-v3/include/bits/utility.h
@@ -224,6 +224,11 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 #endif // C++17
 #endif // C++14
 
+#if __has_builtin(__type_pack_element)
+  template<size_t _Np, typename... _Types>
+    struct _Nth_type
+    { using type = __type_pack_element<_Np, _Types...>; };
+#else
   template<size_t _Np, typename... _Types>
     struct _Nth_type
     { };
@@ -262,6 +267,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     struct _Nth_type<1, _Tp0, _Tp1, _Tp2, _Rest...>
     { using type = _Tp1; };
 #endif
+#endif
 
 _GLIBCXX_END_NAMESPACE_VERSION
 } // namespace
-- 
2.39.0.198.ga38d39a4c5