[PATCH 2/2] c++: speculative constexpr and is_constant_evaluated [PR108243]

[Patrick Palka <ppalka@redhat.com>]

This PR illustrates that __builtin_is_constant_evaluated currently acts
as an optimization barrier for our speculative constexpr evaluation,
since we don't want to prematurely fold the builtin to false if the
expression in question would be later manifestly constant evaluated (in
which case it must be folded to true).

This patch fixes this by permitting __builtin_is_constant_evaluated
to get folded as false during cp_fold_function, since at that point
we're sure we're doing manifestly constant evaluation.  To that end
we add a flags parameter to cp_fold that controls what mce_value the
CALL_EXPR case passes to maybe_constant_value.

bootstrapped and rgetsted no x86_64-pc-linux-gnu, does this look OK for
trunk?

	PR c++/108243

gcc/cp/ChangeLog:

	* cp-gimplify.cc (enum fold_flags): Define.
	(cp_fold_data::genericize): Replace this data member with ...
	(cp_fold_data::fold_flags): ... this.
	(cp_fold_r): Adjust cp_fold_data use and cp_fold_calls.
	(cp_fold_function): Likewise.
	(cp_fold_maybe_rvalue): Likewise.
	(cp_fully_fold_init): Likewise.
	(cp_fold): Add fold_flags parameter.  Don't cache if flags
	isn't empty.
	<case CALL_EXPR>: Pass mce_false to maybe_constant_value
	if if ff_genericize is set.

gcc/testsuite/ChangeLog:

	* g++.dg/opt/pr108243.C: New test.
---
 gcc/cp/cp-gimplify.cc               | 76 ++++++++++++++++++-----------
 gcc/testsuite/g++.dg/opt/pr108243.C | 29 +++++++++++
 2 files changed, 76 insertions(+), 29 deletions(-)
 create mode 100644 gcc/testsuite/g++.dg/opt/pr108243.C

diff --git a/gcc/cp/cp-gimplify.cc b/gcc/cp/cp-gimplify.cc
index a35cedd05cc..d023a63768f 100644
--- a/gcc/cp/cp-gimplify.cc
+++ b/gcc/cp/cp-gimplify.cc
@@ -43,12 +43,20 @@ along with GCC; see the file COPYING3.  If not see
 #include "omp-general.h"
 #include "opts.h"
 
+/* Flags for cp_fold and cp_fold_r.  */
+
+enum fold_flags {
+  ff_none = 0,
+  /* Whether we're being called from cp_fold_function.  */
+  ff_genericize = 1 << 0,
+};
+
 /* Forward declarations.  */
 
 static tree cp_genericize_r (tree *, int *, void *);
 static tree cp_fold_r (tree *, int *, void *);
 static void cp_genericize_tree (tree*, bool);
-static tree cp_fold (tree);
+static tree cp_fold (tree, fold_flags);
 
 /* Genericize a TRY_BLOCK.  */
 
@@ -996,9 +1004,8 @@ struct cp_genericize_data
 struct cp_fold_data
 {
   hash_set<tree> pset;
-  bool genericize; // called from cp_fold_function?
-
-  cp_fold_data (bool g): genericize (g) {}
+  fold_flags flags;
+  cp_fold_data (fold_flags flags): flags (flags) {}
 };
 
 static tree
@@ -1039,7 +1046,7 @@ cp_fold_r (tree *stmt_p, int *walk_subtrees, void *data_)
       break;
     }
 
-  *stmt_p = stmt = cp_fold (*stmt_p);
+  *stmt_p = stmt = cp_fold (*stmt_p, data->flags);
 
   if (data->pset.add (stmt))
     {
@@ -1119,12 +1126,12 @@ cp_fold_r (tree *stmt_p, int *walk_subtrees, void *data_)
 	 here rather than in cp_genericize to avoid problems with the invisible
 	 reference transition.  */
     case INIT_EXPR:
-      if (data->genericize)
+      if (data->flags & ff_genericize)
 	cp_genericize_init_expr (stmt_p);
       break;
 
     case TARGET_EXPR:
-      if (data->genericize)
+      if (data->flags & ff_genericize)
 	cp_genericize_target_expr (stmt_p);
 
       /* Folding might replace e.g. a COND_EXPR with a TARGET_EXPR; in
@@ -1157,7 +1164,7 @@ cp_fold_r (tree *stmt_p, int *walk_subtrees, void *data_)
 void
 cp_fold_function (tree fndecl)
 {
-  cp_fold_data data (/*genericize*/true);
+  cp_fold_data data (ff_genericize);
   cp_walk_tree (&DECL_SAVED_TREE (fndecl), cp_fold_r, &data, NULL);
 }
 
@@ -2375,7 +2382,7 @@ cp_fold_maybe_rvalue (tree x, bool rval)
 {
   while (true)
     {
-      x = cp_fold (x);
+      x = cp_fold (x, ff_none);
       if (rval)
 	x = mark_rvalue_use (x);
       if (rval && DECL_P (x)
@@ -2434,7 +2441,7 @@ cp_fully_fold_init (tree x)
   if (processing_template_decl)
     return x;
   x = cp_fully_fold (x);
-  cp_fold_data data (/*genericize*/false);
+  cp_fold_data data (ff_none);
   cp_walk_tree (&x, cp_fold_r, &data, NULL);
   return x;
 }
@@ -2469,7 +2476,7 @@ clear_fold_cache (void)
     Function returns X or its folded variant.  */
 
 static tree
-cp_fold (tree x)
+cp_fold (tree x, fold_flags flags)
 {
   tree op0, op1, op2, op3;
   tree org_x = x, r = NULL_TREE;
@@ -2490,8 +2497,11 @@ cp_fold (tree x)
   if (fold_cache == NULL)
     fold_cache = hash_map<tree, tree>::create_ggc (101);
 
-  if (tree *cached = fold_cache->get (x))
-    return *cached;
+  bool cache_p = (flags == ff_none);
+
+  if (cache_p)
+    if (tree *cached = fold_cache->get (x))
+      return *cached;
 
   uid_sensitive_constexpr_evaluation_checker c;
 
@@ -2526,7 +2536,7 @@ cp_fold (tree x)
 	     Don't create a new tree if op0 != TREE_OPERAND (x, 0), the
 	     folding of the operand should be in the caches and if in cp_fold_r
 	     it will modify it in place.  */
-	  op0 = cp_fold (TREE_OPERAND (x, 0));
+	  op0 = cp_fold (TREE_OPERAND (x, 0), flags);
 	  if (op0 == error_mark_node)
 	    x = error_mark_node;
 	  break;
@@ -2571,7 +2581,7 @@ cp_fold (tree x)
 	{
 	  tree p = maybe_undo_parenthesized_ref (x);
 	  if (p != x)
-	    return cp_fold (p);
+	    return cp_fold (p, flags);
 	}
       goto unary;
 
@@ -2763,8 +2773,8 @@ cp_fold (tree x)
     case COND_EXPR:
       loc = EXPR_LOCATION (x);
       op0 = cp_fold_rvalue (TREE_OPERAND (x, 0));
-      op1 = cp_fold (TREE_OPERAND (x, 1));
-      op2 = cp_fold (TREE_OPERAND (x, 2));
+      op1 = cp_fold (TREE_OPERAND (x, 1), flags);
+      op2 = cp_fold (TREE_OPERAND (x, 2), flags);
 
       if (TREE_CODE (TREE_TYPE (x)) == BOOLEAN_TYPE)
 	{
@@ -2854,7 +2864,7 @@ cp_fold (tree x)
 	      {
 		if (!same_type_p (TREE_TYPE (x), TREE_TYPE (r)))
 		  r = build_nop (TREE_TYPE (x), r);
-		x = cp_fold (r);
+		x = cp_fold (r, flags);
 		break;
 	      }
 	  }
@@ -2908,7 +2918,7 @@ cp_fold (tree x)
 	int m = call_expr_nargs (x);
 	for (int i = 0; i < m; i++)
 	  {
-	    r = cp_fold (CALL_EXPR_ARG (x, i));
+	    r = cp_fold (CALL_EXPR_ARG (x, i), flags);
 	    if (r != CALL_EXPR_ARG (x, i))
 	      {
 		if (r == error_mark_node)
@@ -2931,7 +2941,7 @@ cp_fold (tree x)
 
 	if (TREE_CODE (r) != CALL_EXPR)
 	  {
-	    x = cp_fold (r);
+	    x = cp_fold (r, flags);
 	    break;
 	  }
 
@@ -2944,7 +2954,15 @@ cp_fold (tree x)
 	   constant, but the call followed by an INDIRECT_REF is.  */
 	if (callee && DECL_DECLARED_CONSTEXPR_P (callee)
 	    && !flag_no_inline)
-	  r = maybe_constant_value (x);
+	  {
+	    mce_value manifestly_const_eval = mce_unknown;
+	    if (flags & ff_genericize)
+	      /* At genericization time it's safe to fold
+		 __builtin_is_constant_evaluated to false.  */
+	      manifestly_const_eval = mce_false;
+	    r = maybe_constant_value (x, /*decl=*/NULL_TREE,
+				      manifestly_const_eval);
+	  }
 	optimize = sv;
 
         if (TREE_CODE (r) != CALL_EXPR)
@@ -2971,7 +2989,7 @@ cp_fold (tree x)
 	vec<constructor_elt, va_gc> *nelts = NULL;
 	FOR_EACH_VEC_SAFE_ELT (elts, i, p)
 	  {
-	    tree op = cp_fold (p->value);
+	    tree op = cp_fold (p->value, flags);
 	    if (op != p->value)
 	      {
 		if (op == error_mark_node)
@@ -3002,7 +3020,7 @@ cp_fold (tree x)
 
 	for (int i = 0; i < n; i++)
 	  {
-	    tree op = cp_fold (TREE_VEC_ELT (x, i));
+	    tree op = cp_fold (TREE_VEC_ELT (x, i), flags);
 	    if (op != TREE_VEC_ELT (x, i))
 	      {
 		if (!changed)
@@ -3019,10 +3037,10 @@ cp_fold (tree x)
     case ARRAY_RANGE_REF:
 
       loc = EXPR_LOCATION (x);
-      op0 = cp_fold (TREE_OPERAND (x, 0));
-      op1 = cp_fold (TREE_OPERAND (x, 1));
-      op2 = cp_fold (TREE_OPERAND (x, 2));
-      op3 = cp_fold (TREE_OPERAND (x, 3));
+      op0 = cp_fold (TREE_OPERAND (x, 0), flags);
+      op1 = cp_fold (TREE_OPERAND (x, 1), flags);
+      op2 = cp_fold (TREE_OPERAND (x, 2), flags);
+      op3 = cp_fold (TREE_OPERAND (x, 3), flags);
 
       if (op0 != TREE_OPERAND (x, 0)
 	  || op1 != TREE_OPERAND (x, 1)
@@ -3050,7 +3068,7 @@ cp_fold (tree x)
       /* A SAVE_EXPR might contain e.g. (0 * i) + (0 * j), which, after
 	 folding, evaluates to an invariant.  In that case no need to wrap
 	 this folded tree with a SAVE_EXPR.  */
-      r = cp_fold (TREE_OPERAND (x, 0));
+      r = cp_fold (TREE_OPERAND (x, 0), flags);
       if (tree_invariant_p (r))
 	x = r;
       break;
@@ -3069,7 +3087,7 @@ cp_fold (tree x)
       copy_warning (x, org_x);
     }
 
-  if (!c.evaluation_restricted_p ())
+  if (cache_p && !c.evaluation_restricted_p ())
     {
       fold_cache->put (org_x, x);
       /* Prevent that we try to fold an already folded result again.  */
diff --git a/gcc/testsuite/g++.dg/opt/pr108243.C b/gcc/testsuite/g++.dg/opt/pr108243.C
new file mode 100644
index 00000000000..4c45dbba13c
--- /dev/null
+++ b/gcc/testsuite/g++.dg/opt/pr108243.C
@@ -0,0 +1,29 @@
+// PR c++/108243
+// { dg-do compile { target c++11 } }
+// { dg-additional-options "-O -fdump-tree-original" }
+
+constexpr int foo() {
+  return __builtin_is_constant_evaluated() + 1;
+}
+
+#if __cpp_if_consteval
+constexpr int bar() {
+  if consteval {
+    return 5;
+  } else {
+    return 4;
+  }
+}
+#endif
+
+int p, q;
+
+int main() {
+  p = foo();
+#if __cpp_if_consteval
+  q = bar();
+#endif
+}
+
+// { dg-final { scan-tree-dump-not "= foo" "original" } }
+// { dg-final { scan-tree-dump-not "= bar" "original" } }
-- 
2.39.1.348.g5dec958dcf