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[C++ PATCH] Implement P0595R1 - so far as __builtin_is_constant_evaluated rather than std::is_constant_evaluated magic builtin


Hi!

As part of the PR86590 discussions that the current libstdc++
__constant_string is extremely costly, because we don't fold the
__builtin_constant_p in the loop early enough and because Richard doesn't
want __builtin_early_constant_p, this patch is an attempt to implement
P0595R1 as a builtin (which will be likely needed anyway, so that libstdc++
will be able to use it even without -std=c++2a).

When evaluating (outermost) constant expressions with !ctx->quiet (i.e.
when we require constant expressions) as well in the special case of
reference initializers, or const non-volatile decl initializers, or
TREE_STATIC decl initializers, the builtin is folded into true (for the
ctx->quiet special cases if the constexpr evaluation doesn't turn a
constant expression, we retry without the special flag), otherwise
folding of it is deferred and it is flagged as non-constant expression,
and folding during gimplification folds it into false.

Not really sure about potential_constant_expression, for if it attempts
to evaluate the condition with ctx->quiet true and the patch doesn't set the
magic flag to fold the builtin to true in that case; it is unclear to me if
in case the condition includes std::is_constant_evaluated () call we need
to fold it also to true or not depending on what will we be evaluating later
on.

Bootstrapped/regtested on x86_64-linux.

2018-07-22  Jakub Jelinek  <jakub@redhat.com>

	P0595R1 - is_constant_evaluated
	* builtins.def (BUILT_IN_IS_CONSTANT_EVALUATED): New C++ builtin.
	* builtins.c (fold_builtin_0): Fold BUILT_IN_IS_CONSTANT_EVALUATED
	to 0.
cp/
	* cp-tree.h (maybe_constant_init): Add const_evaluated argument.
	* typeck2.c (store_init_value): Pass true as new argument to
	maybe_constant_init.
	* constexpr.c (struct constexpr_ctx): Add const_evaluated field.
	(cxx_eval_builtin_function_call): Handle
	BUILT_IN_IS_CONSTANT_EVALUATED.
	(cxx_eval_outermost_constant_expr): Add const_evaluated argument,
	initialize const_evaluated field in ctx.  If the result is
	TREE_CONSTANT and non_constant_p, retry with const_evaluated false
	if it was true.
	(is_sub_constant_expr): Initialize const_evaluated_field in ctx.
	(cxx_constant_value): Pass true as const_evaluated to
	cxx_eval_outermost_constant_expr.
	(maybe_constant_value): Pass false as const_evaluated to
	cxx_eval_outermost_constant_expr.
	(fold_non_dependent_expr): Likewise.
	(maybe_constant_init_1): Add const_evaluated argument, pass it
	down to cxx_eval_outermost_constant_expr.  Pass !allow_non_constant
	instead of false as strict to cxx_eval_outermost_constant_expr.
	(maybe_constant_init): Add const_evaluated argument, pass it down
	to maybe_constant_init_1.
	(cxx_constant_init): Pass true as const_evaluated to
	maybe_constant_init_1.
	* cp-gimplify.c (cp_fold): Don't fold BUILT_IN_IS_CONSTANT_EVALUATED
	calls.
lto/
	* lto-lang.c (c_dialect_cxx): Define.
ada/
	* gcc-interface/utils.c (c_dialect_cxx): Define.
brig/
	* brig-lang.c (c_dialect_cxx): Define.
testsuite/
	* g++.dg/cpp2a/is-constant-evaluated1.C: New test.

--- gcc/builtins.def.jj	2018-06-20 08:15:34.179862153 +0200
+++ gcc/builtins.def	2018-07-20 12:03:10.254453811 +0200
@@ -974,6 +974,11 @@ DEF_EXT_LIB_BUILTIN    (BUILT_IN_PRINTF_
 DEF_EXT_LIB_BUILTIN    (BUILT_IN_VFPRINTF_CHK, "__vfprintf_chk", BT_FN_INT_FILEPTR_INT_CONST_STRING_VALIST_ARG, ATTR_NONNULL_1_FORMAT_PRINTF_3_0)
 DEF_EXT_LIB_BUILTIN    (BUILT_IN_VPRINTF_CHK, "__vprintf_chk", BT_FN_INT_INT_CONST_STRING_VALIST_ARG, ATTR_FORMAT_PRINTF_2_0)
 
+/* C++ __builtin_is_constant_evaluated.  */
+DEF_BUILTIN (BUILT_IN_IS_CONSTANT_EVALUATED, "__builtin_is_constant_evaluated",
+	     BUILT_IN_NORMAL, BT_FN_BOOL, BT_LAST, false, false, false,
+	     ATTR_CONST_NOTHROW_LEAF_LIST, true, c_dialect_cxx ())
+
 /* Profiling hooks.  */
 DEF_BUILTIN (BUILT_IN_PROFILE_FUNC_ENTER, "__cyg_profile_func_enter", BUILT_IN_NORMAL, BT_FN_VOID_PTR_PTR, BT_LAST,
 	     false, false, false, ATTR_NULL, true, true)
--- gcc/builtins.c.jj	2018-07-16 23:24:51.306429546 +0200
+++ gcc/builtins.c	2018-07-20 12:09:13.278818768 +0200
@@ -9104,6 +9104,10 @@ fold_builtin_0 (location_t loc, tree fnd
     case BUILT_IN_CLASSIFY_TYPE:
       return fold_builtin_classify_type (NULL_TREE);
 
+    case BUILT_IN_IS_CONSTANT_EVALUATED:
+      /* The C++ FE can evaluate this to something other than false.  */
+      return boolean_false_node;     
+
     default:
       break;
     }
--- gcc/cp/cp-tree.h.jj	2018-07-18 22:57:10.657780293 +0200
+++ gcc/cp/cp-tree.h	2018-07-20 18:10:33.416409798 +0200
@@ -7536,7 +7536,7 @@ extern bool require_potential_rvalue_con
 extern tree cxx_constant_value			(tree, tree = NULL_TREE);
 extern tree cxx_constant_init			(tree, tree = NULL_TREE);
 extern tree maybe_constant_value		(tree, tree = NULL_TREE);
-extern tree maybe_constant_init			(tree, tree = NULL_TREE);
+extern tree maybe_constant_init			(tree, tree = NULL_TREE, bool = false);
 extern tree fold_non_dependent_expr		(tree, tsubst_flags_t = tf_warning_or_error);
 extern tree fold_simple				(tree);
 extern bool is_sub_constant_expr                (tree);
--- gcc/cp/typeck2.c.jj	2018-06-29 09:38:17.786306395 +0200
+++ gcc/cp/typeck2.c	2018-07-20 18:11:36.649492893 +0200
@@ -837,7 +837,7 @@ store_init_value (tree decl, tree init,
 	    value = cxx_constant_init (value, decl);
 	}
       else
-	value = maybe_constant_init (value, decl);
+	value = maybe_constant_init (value, decl, true);
       if (TREE_CODE (value) == CONSTRUCTOR && cp_has_mutable_p (type))
 	/* Poison this CONSTRUCTOR so it can't be copied to another
 	   constexpr variable.  */
--- gcc/cp/constexpr.c.jj	2018-06-25 14:51:23.094989194 +0200
+++ gcc/cp/constexpr.c	2018-07-20 19:16:40.504036595 +0200
@@ -1007,6 +1007,8 @@ struct constexpr_ctx {
   /* Whether we are strictly conforming to constant expression rules or
      trying harder to get a constant value.  */
   bool strict;
+  /* Whether __builtin_is_constant_evaluated () should be true.  */
+  bool const_evaluated;
 };
 
 /* A table of all constexpr calls that have been evaluated by the
@@ -1184,6 +1186,18 @@ cxx_eval_builtin_function_call (const co
       return t;
     }
 
+  /* For __builtin_is_constant_evaluated, defer it if not ctx->const_evaluated,
+     otherwise fold it to true.  */
+  if (DECL_FUNCTION_CODE (fun) == BUILT_IN_IS_CONSTANT_EVALUATED)
+    {
+      if (!ctx->const_evaluated)
+	{
+	  *non_constant_p = true;
+	  return t;
+	}
+      return boolean_true_node;
+    }
+
   /* Be permissive for arguments to built-ins; __builtin_constant_p should
      return constant false for a non-constant argument.  */
   constexpr_ctx new_ctx = *ctx;
@@ -4884,7 +4898,9 @@ instantiate_constexpr_fns (tree t)
 
 static tree
 cxx_eval_outermost_constant_expr (tree t, bool allow_non_constant,
-				  bool strict = true, tree object = NULL_TREE)
+				  bool strict = true,
+				  bool const_evaluated = false,
+				  tree object = NULL_TREE)
 {
   auto_timevar time (TV_CONSTEXPR);
 
@@ -4893,7 +4909,8 @@ cxx_eval_outermost_constant_expr (tree t
   hash_map<tree,tree> map;
 
   constexpr_ctx ctx = { NULL, &map, NULL, NULL, NULL, NULL,
-			allow_non_constant, strict };
+			allow_non_constant, strict,
+			const_evaluated || !allow_non_constant };
 
   tree type = initialized_type (t);
   tree r = t;
@@ -4982,6 +4999,12 @@ cxx_eval_outermost_constant_expr (tree t
     return error_mark_node;
   else if (non_constant_p && TREE_CONSTANT (r))
     {
+      /* If __builtin_is_constant_evaluated () was evaluated to true
+	 and the result is not a valid constant expression, we need to
+	 punt.  */
+      if (const_evaluated)
+	return cxx_eval_outermost_constant_expr (t, true, strict,
+						 false, object);
       /* This isn't actually constant, so unset TREE_CONSTANT.
 	 Don't clear TREE_CONSTANT on ADDR_EXPR, as the middle-end requires
 	 it to be set if it is invariant address, even when it is not
@@ -5027,7 +5049,8 @@ is_sub_constant_expr (tree t)
   bool overflow_p = false;
   hash_map <tree, tree> map;
 
-  constexpr_ctx ctx = { NULL, &map, NULL, NULL, NULL, NULL, true, true };
+  constexpr_ctx ctx
+    = { NULL, &map, NULL, NULL, NULL, NULL, true, true, false };
 
   instantiate_constexpr_fns (t);
   cxx_eval_constant_expression (&ctx, t, false, &non_constant_p,
@@ -5042,7 +5065,7 @@ is_sub_constant_expr (tree t)
 tree
 cxx_constant_value (tree t, tree decl)
 {
-  return cxx_eval_outermost_constant_expr (t, false, true, decl);
+  return cxx_eval_outermost_constant_expr (t, false, true, true, decl);
 }
 
 /* Helper routine for fold_simple function.  Either return simplified
@@ -5148,7 +5171,7 @@ maybe_constant_value (tree t, tree decl)
   if (tree *cached = cv_cache->get (t))
     return *cached;
 
-  r = cxx_eval_outermost_constant_expr (t, true, true, decl);
+  r = cxx_eval_outermost_constant_expr (t, true, true, false, decl);
   gcc_checking_assert (r == t
 		       || CONVERT_EXPR_P (t)
 		       || TREE_CODE (t) == VIEW_CONVERT_EXPR
@@ -5222,7 +5245,8 @@ fold_non_dependent_expr (tree t,
 	      return t;
 	    }
 
-	  tree r = cxx_eval_outermost_constant_expr (t, true, true, NULL_TREE);
+	  tree r = cxx_eval_outermost_constant_expr (t, true, true, false,
+						     NULL_TREE);
 	  /* cp_tree_equal looks through NOPs, so allow them.  */
 	  gcc_checking_assert (r == t
 			       || CONVERT_EXPR_P (t)
@@ -5246,7 +5270,8 @@ fold_non_dependent_expr (tree t,
    than wrapped in a TARGET_EXPR.  */
 
 static tree
-maybe_constant_init_1 (tree t, tree decl, bool allow_non_constant)
+maybe_constant_init_1 (tree t, tree decl, bool allow_non_constant,
+		       bool const_evaluated)
 {
   if (!t)
     return t;
@@ -5264,7 +5289,9 @@ maybe_constant_init_1 (tree t, tree decl
   else if (CONSTANT_CLASS_P (t) && allow_non_constant)
     /* No evaluation needed.  */;
   else
-    t = cxx_eval_outermost_constant_expr (t, allow_non_constant, false, decl);
+    t = cxx_eval_outermost_constant_expr (t, allow_non_constant,
+					  !allow_non_constant,
+					  const_evaluated, decl);
   if (TREE_CODE (t) == TARGET_EXPR)
     {
       tree init = TARGET_EXPR_INITIAL (t);
@@ -5277,9 +5304,9 @@ maybe_constant_init_1 (tree t, tree decl
 /* Wrapper for maybe_constant_init_1 which permits non constants.  */
 
 tree
-maybe_constant_init (tree t, tree decl)
+maybe_constant_init (tree t, tree decl, bool const_evaluated)
 {
-  return maybe_constant_init_1 (t, decl, true);
+  return maybe_constant_init_1 (t, decl, true, const_evaluated);
 }
 
 /* Wrapper for maybe_constant_init_1 which does not permit non constants.  */
@@ -5287,7 +5314,7 @@ maybe_constant_init (tree t, tree decl)
 tree
 cxx_constant_init (tree t, tree decl)
 {
-  return maybe_constant_init_1 (t, decl, false);
+  return maybe_constant_init_1 (t, decl, false, true);
 }
 
 #if 0
--- gcc/cp/cp-gimplify.c.jj	2018-07-20 11:39:15.543037497 +0200
+++ gcc/cp/cp-gimplify.c	2018-07-20 12:21:29.869568404 +0200
@@ -2478,6 +2478,12 @@ cp_fold (tree x)
 	    && DECL_DECLARED_CONSTEXPR_P (current_function_decl))
 	  nw = 1;
 
+	/* Defer folding __builtin_is_constant_evaluated.  */
+	if (callee
+	    && DECL_BUILT_IN_CLASS (callee) == BUILT_IN_NORMAL
+	    && DECL_FUNCTION_CODE (callee) == BUILT_IN_IS_CONSTANT_EVALUATED)
+	  break;
+
 	x = copy_node (x);
 
 	m = call_expr_nargs (x);
--- gcc/lto/lto-lang.c.jj	2018-06-13 10:05:49.991124932 +0200
+++ gcc/lto/lto-lang.c	2018-07-20 12:29:28.087207392 +0200
@@ -246,6 +246,7 @@ static GTY(()) tree signed_size_type_nod
 int flag_isoc94;
 int flag_isoc99;
 int flag_isoc11;
+#define c_dialect_cxx() 0
 
 /* Attribute handlers.  */
 
--- gcc/ada/gcc-interface/utils.c.jj	2018-07-17 12:48:21.096585583 +0200
+++ gcc/ada/gcc-interface/utils.c	2018-07-21 09:26:01.365363959 +0200
@@ -6458,6 +6458,7 @@ def_builtin_1 (enum built_in_function fn
 static int flag_isoc94 = 0;
 static int flag_isoc99 = 0;
 static int flag_isoc11 = 0;
+#define c_dialect_cxx() 0
 
 /* Install what the common builtins.def offers.  */
 
--- gcc/brig/brig-lang.c.jj	2018-05-06 23:12:54.400623542 +0200
+++ gcc/brig/brig-lang.c	2018-07-21 09:25:37.121341436 +0200
@@ -587,6 +587,7 @@ static GTY(()) tree signed_size_type_nod
 int flag_isoc94;
 int flag_isoc99;
 int flag_isoc11;
+#define c_dialect_cxx() 0
 
 static void
 def_fn_type (builtin_type def, builtin_type ret, bool var, int n, ...)
--- gcc/testsuite/g++.dg/cpp2a/is-constant-evaluated1.C.jj	2018-07-20 18:43:58.240798640 +0200
+++ gcc/testsuite/g++.dg/cpp2a/is-constant-evaluated1.C	2018-07-20 19:09:44.712391964 +0200
@@ -0,0 +1,64 @@
+// P0595R1
+// { dg-do compile { target c++14 } }
+
+template<int N> struct X { int v = N; };
+X<__builtin_is_constant_evaluated ()> x; // type X<true>
+int y = 4;
+int a = __builtin_is_constant_evaluated () ? y : 1; // initializes a to 1
+int b = __builtin_is_constant_evaluated () ? 2 : y; // initializes b to 2
+int c = y + (__builtin_is_constant_evaluated () ? 2 : y); // initializes c to 2*y
+  
+struct false_type { static constexpr bool value = false; };
+struct true_type { static constexpr bool value = true; };
+template<class T, class U>
+struct is_same : false_type {};
+template<class T>
+struct is_same<T, T> : true_type {};
+
+constexpr int
+foo (int x)
+{
+  const int n = __builtin_is_constant_evaluated () ? 13 : 17; // n == 13
+  int m = __builtin_is_constant_evaluated () ? 13 : 17; // m might be 13 or 17 (see below)
+  char arr[n] = {}; // char[13]
+  return m + sizeof (arr) + x;
+}
+
+constexpr int
+bar ()
+{
+  const int n = __builtin_is_constant_evaluated() ? 13 : 17;
+  X<n> x1;
+  X<__builtin_is_constant_evaluated() ? 13 : 17> x2;
+  static_assert (is_same<decltype (x1), decltype (x2)>::value, "x1/x2's type");
+  return x1.v + x2.v;
+}
+
+int p = foo (0); // m == 13; initialized to 26
+int q = p + foo (0); // m == 17 for this call; initialized to 56
+static_assert (bar () == 26, "bar");
+
+struct S { int a, b; };
+
+S s = { __builtin_is_constant_evaluated () ? 2 : 3, y };
+S t = { __builtin_is_constant_evaluated () ? 2 : 3, 4 };
+
+static_assert (is_same<decltype (x), X<true> >::value, "x's type");
+
+int
+main ()
+{
+  if (a != 1 || b != 2 || c != 8 || p != 26 || q != 56)
+    __builtin_abort ();
+  if (s.a != 3 || s.b != 4 || t.a != 2 || t.b != 4)
+    __builtin_abort ();
+  if (foo (y) != 34)
+    __builtin_abort ();
+#if __cplusplus >= 201703L
+  if constexpr (foo (0) != 26)
+    __builtin_abort ();
+#endif
+  constexpr int w = foo (0);
+  if (w != 26)
+    __builtin_abort ();
+}

	Jakub


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