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


On Wed, Aug 01, 2018 at 12:35:09AM +1000, Jason Merrill wrote:
> On Mon, Jul 23, 2018 at 8:50 PM, Richard Biener
> <richard.guenther@gmail.com> wrote:
> > On Mon, Jul 23, 2018 at 12:28 PM Jakub Jelinek <jakub@redhat.com> wrote:
> >>
> >> On Mon, Jul 23, 2018 at 12:17:42PM +0200, Richard Biener wrote:
> >> > > Bootstrapped/regtested on x86_64-linux.
> >> >
> >> > Thanks for working on this.  I wonder if we can completely hide this
> >> > from the middle-end, without requiring defining of c_dialect_cxx.
> >> > There is the BUILT_IN_FRONTEND class so you could somewhere
> >> > manually inject a decl in that class for C++?
> >>
> >> But then I couldn't handle folding of the builtin in the middle-end to false,
> >> which is what I need (because in the FE it needs to be either folded to
> >> true, or its folding deferred until later).
> >> Or maybe in the C++ gimplification langhook?
> >
> > Yes, I was thinking the C++ langhook or its fully_fold routine.
> 
> fully_fold is too soon (until constexpr evaluation uses the
> pre-genericize form), but the gimplification hook should work.
> 
> >> Seems we have a single BUILT_IN_FRONTEND builtin in the whole compiler,
> >> __integer_pack, but it doesn't act as a normal builtin, given it is a
> >> templatish magic.
> >
> > Yeah, I think at some point we considered removing BUILT_IN_FRONTEND ...
> >
> > Nowadays internal-use builtins can easily be internal-functions but of couse
> > this one will eventually be used from libstdc++.
> 
> Immediately, I'd think.

Here is an updated patch that uses BUILT_IN_FRONTEND for this rather than
BUILT_IN_NORMAL builtin.

Bootstrapped/regtested on x86_64-linux and i686-linux.

2018-08-01  Jakub Jelinek  <jakub@redhat.com>

	P0595R1 - is_constant_evaluated
cp/
	* cp-tree.h (enum cp_built_in_function): New.
	(maybe_constant_init): Add const_evaluated argument.
	* typeck2.c (store_init_value): Pass true as new argument to
	maybe_constant_init.
	* constexpr.c (constexpr_fn_retval): Check also DECL_BUILT_IN_CLASS
	for BUILT_IN_UNREACHABLE.
	(struct constexpr_ctx): Add const_evaluated field.
	(cxx_eval_builtin_function_call): Use DECL_IS_BUILTIN_CONSTANT_P
	macro.  Handle CP_BUILT_IN_IS_CONSTANT_EVALUATED.  Check also
	DECL_BUILT_IN_CLASS for BUILT_IN_UNREACHABLE.
	(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_gimplify_expr): Handle CALL_EXPRs to
	CP_BUILT_IN_IS_CONSTANT_EVALUATED.
	(cp_fold): Don't fold CP_BUILT_IN_IS_CONSTANT_EVALUATED calls.
	* decl.c: Include langhooks.h.
	(cxx_init_decl_processing): Register __builtin_is_constant_evaluated
	built-in.
	* tree.c (builtin_valid_in_constant_expr_p): Return true for
	CP_BUILT_IN_IS_CONSTANT_EVALUATED.
	* pt.c (declare_integer_pack): Initialize DECL_FUNCTION_CODE.
testsuite/
	* g++.dg/cpp2a/is-constant-evaluated1.C: New test.

--- gcc/cp/decl.c.jj	2018-07-25 18:46:56.159149394 +0200
+++ gcc/cp/decl.c	2018-07-31 19:54:56.637180751 +0200
@@ -52,6 +52,7 @@ along with GCC; see the file COPYING3.
 #include "gimplify.h"
 #include "asan.h"
 #include "gcc-rich-location.h"
+#include "langhooks.h"
 
 /* Possible cases of bad specifiers type used by bad_specifiers. */
 enum bad_spec_place {
@@ -4125,6 +4126,13 @@ cxx_init_decl_processing (void)
 
   c_common_nodes_and_builtins ();
 
+  tree bool_ftype = build_function_type_list (boolean_type_node, NULL_TREE);
+  tree decl
+    = add_builtin_function ("__builtin_is_constant_evaluated",
+			    bool_ftype, CP_BUILT_IN_IS_CONSTANT_EVALUATED,
+			    BUILT_IN_FRONTEND, NULL, NULL_TREE);
+  set_call_expr_flags (decl, ECF_CONST | ECF_NOTHROW | ECF_LEAF);
+
   integer_two_node = build_int_cst (NULL_TREE, 2);
 
   /* Guess at the initial static decls size.  */
--- gcc/cp/typeck2.c.jj	2018-07-23 09:46:51.145991660 +0200
+++ gcc/cp/typeck2.c	2018-07-31 18:32:23.526305749 +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	2018-07-23 09:46:51.193991708 +0200
+++ gcc/cp/constexpr.c	2018-07-31 20:06:04.176221114 +0200
@@ -713,6 +713,7 @@ constexpr_fn_retval (tree body)
 	{
 	  tree fun = get_function_named_in_call (body);
 	  if (fun != NULL_TREE
+	      && DECL_BUILT_IN_CLASS (fun) == BUILT_IN_NORMAL
 	      && DECL_FUNCTION_CODE (fun) == BUILT_IN_UNREACHABLE)
 	    return NULL_TREE;
 	}
@@ -1007,6 +1008,8 @@ 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
@@ -1171,7 +1174,7 @@ cxx_eval_builtin_function_call (const co
   int i;
 
   /* Don't fold __builtin_constant_p within a constexpr function.  */
-  bool bi_const_p = (DECL_FUNCTION_CODE (fun) == BUILT_IN_CONSTANT_P);
+  bool bi_const_p = DECL_IS_BUILTIN_CONSTANT_P (fun);
 
   /* If we aren't requiring a constant expression, defer __builtin_constant_p
      in a constexpr function until we have values for the parameters.  */
@@ -1184,6 +1187,19 @@ 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_BUILT_IN_CLASS (fun) == BUILT_IN_FRONTEND
+      && (int) DECL_FUNCTION_CODE (fun) == CP_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;
@@ -1217,7 +1233,8 @@ cxx_eval_builtin_function_call (const co
 	  /* Do not allow__builtin_unreachable in constexpr function.
 	     The __builtin_unreachable call with BUILTINS_LOCATION
 	     comes from cp_maybe_instrument_return.  */
-	  if (DECL_FUNCTION_CODE (fun) == BUILT_IN_UNREACHABLE
+	  if (DECL_BUILT_IN_CLASS (fun) == BUILT_IN_NORMAL
+	      && DECL_FUNCTION_CODE (fun) == BUILT_IN_UNREACHABLE
 	      && EXPR_LOCATION (t) == BUILTINS_LOCATION)
 	    error ("%<constexpr%> call flows off the end of the function");
 	  else
@@ -4884,7 +4901,9 @@ 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 +4912,8 @@ 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 +5002,12 @@ 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 +5053,8 @@ 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 +5069,7 @@ 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 +5175,7 @@ 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 +5249,8 @@ 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 +5274,8 @@ 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 +5293,9 @@ 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 +5308,9 @@ 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 +5318,7 @@ 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-tree.h.jj	2018-07-25 18:46:56.156149388 +0200
+++ gcc/cp/cp-tree.h	2018-07-31 18:44:25.078455768 +0200
@@ -5966,6 +5966,13 @@ struct GTY((chain_next ("%h.next"))) tin
   static const unsigned short refcount_infinity = (unsigned short) ~0;
 };
 
+/* BUILT_IN_FRONTEND function codes.  */
+enum cp_built_in_function {
+  CP_BUILT_IN_IS_CONSTANT_EVALUATED,
+  CP_BUILT_IN_INTEGER_PACK,
+  CP_BUILT_IN_LAST
+};
+
 bool decl_spec_seq_has_spec_p (const cp_decl_specifier_seq *, cp_decl_spec);
 
 /* Return the type of the `this' parameter of FNTYPE.  */
@@ -7572,7 +7579,7 @@ 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/cp-gimplify.c.jj	2018-07-23 09:48:01.638068411 +0200
+++ gcc/cp/cp-gimplify.c	2018-07-31 20:05:48.925174439 +0200
@@ -793,6 +793,15 @@ cp_gimplify_expr (tree *expr_p, gimple_s
 		ret = GS_ERROR;
 	    }
 	}
+      if (ret != GS_ERROR)
+	{
+	  tree decl = cp_get_callee_fndecl_nofold (*expr_p);
+	  if (decl
+	      && DECL_BUILT_IN_CLASS (decl) == BUILT_IN_FRONTEND
+	      && ((int) DECL_FUNCTION_CODE (decl)
+		  == CP_BUILT_IN_IS_CONSTANT_EVALUATED))
+	    *expr_p = boolean_false_node;
+	}
       break;
 
     case RETURN_EXPR:
@@ -2483,6 +2492,13 @@ 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_FRONTEND
+	    && ((int) DECL_FUNCTION_CODE (callee)
+		== CP_BUILT_IN_IS_CONSTANT_EVALUATED))
+	  break;
+
 	x = copy_node (x);
 
 	m = call_expr_nargs (x);
--- gcc/cp/tree.c.jj	2018-07-17 20:08:09.482226661 +0200
+++ gcc/cp/tree.c	2018-07-31 19:14:26.520425530 +0200
@@ -415,10 +415,18 @@ cp_stabilize_reference (tree ref)
 bool
 builtin_valid_in_constant_expr_p (const_tree decl)
 {
-  if (!(TREE_CODE (decl) == FUNCTION_DECL
-	&& DECL_BUILT_IN_CLASS (decl) == BUILT_IN_NORMAL))
-    /* Not a built-in.  */
+  if (TREE_CODE (decl) != FUNCTION_DECL)
+    /* Not a function.  */
     return false;
+  if (DECL_BUILT_IN_CLASS (decl) != BUILT_IN_NORMAL)
+    {
+      if (DECL_BUILT_IN_CLASS (decl) == BUILT_IN_FRONTEND
+	  && ((int) DECL_FUNCTION_CODE (decl)
+	      == CP_BUILT_IN_IS_CONSTANT_EVALUATED))
+	return true;
+      /* Not a built-in.  */
+      return false;
+    }
   switch (DECL_FUNCTION_CODE (decl))
     {
       /* These always have constant results like the corresponding
--- gcc/cp/pt.c.jj	2018-07-18 22:57:10.806780476 +0200
+++ gcc/cp/pt.c	2018-07-31 19:55:48.090337614 +0200
@@ -27537,6 +27537,8 @@ declare_integer_pack (void)
 			       NULL_TREE, ECF_CONST);
   DECL_DECLARED_CONSTEXPR_P (ipfn) = true;
   DECL_BUILT_IN_CLASS (ipfn) = BUILT_IN_FRONTEND;
+  DECL_FUNCTION_CODE (ipfn)
+    = (enum built_in_function) (int) CP_BUILT_IN_INTEGER_PACK;
 }
 
 /* Set up the hash tables for template instantiations.  */
--- gcc/testsuite/g++.dg/cpp2a/is-constant-evaluated1.C.jj	2018-07-31 18:32:23.529305758 +0200
+++ gcc/testsuite/g++.dg/cpp2a/is-constant-evaluated1.C	2018-07-31 20:13:03.311489944 +0200
@@ -0,0 +1,66 @@
+// 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
+int d = __builtin_is_constant_evaluated (); // initializes d to 1
+int e = d + __builtin_is_constant_evaluated (); // initializes e to 0
+
+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 || d != 1 || e != 0 || 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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