C++ PATCH for c++/65970 (constexpr infinite loop)

diff --git a/gcc/c-family/c.opt b/gcc/c-family/c.opt
index 8c70152..a5358ed 100644
--- a/gcc/c-family/c.opt
+++ b/gcc/c-family/c.opt
@@ -1174,6 +1174,10 @@  fconstexpr-depth=
 C++ ObjC++ Joined RejectNegative UInteger Var(max_constexpr_depth) Init(512)
 -fconstexpr-depth=<number>	Specify maximum constexpr recursion depth.
 
+fconstexpr-loop-limit=
+C++ ObjC++ Joined RejectNegative UInteger Var(constexpr_loop_limit) Init(262144)
+-fconstexpr-loop-limit=<number>	Specify maximum constexpr loop iteration count.
+
 fdebug-cpp
 C ObjC C++ ObjC++
 Emit debug annotations during preprocessing.
diff --git a/gcc/cp/constexpr.c b/gcc/cp/constexpr.c
index 47fb39b..6bcb41a 100644
--- a/gcc/cp/constexpr.c
+++ b/gcc/cp/constexpr.c
@@ -3468,6 +3468,7 @@  cxx_eval_loop_expr (const constexpr_ctx *ctx, tree t,
   constexpr_ctx new_ctx = *ctx;
 
   tree body = TREE_OPERAND (t, 0);
+  int count = 0;
   do
     {
       hash_set<tree> save_exprs;
@@ -3480,6 +3481,16 @@  cxx_eval_loop_expr (const constexpr_ctx *ctx, tree t,
       for (hash_set<tree>::iterator iter = save_exprs.begin();
 	   iter != save_exprs.end(); ++iter)
 	new_ctx.values->remove (*iter);
+      if (++count >= constexpr_loop_limit)
+	{
+	  if (!ctx->quiet)
+	    error_at (EXPR_LOC_OR_LOC (t, input_location),
+		      "constexpr loop iteration count exceeds limit of %d "
+		      "(use -fconstexpr-loop-limit= to increase the limit)",
+		      constexpr_loop_limit);
+	  *non_constant_p = true;
+	  break;
+	}
     }
   while (!returns (jump_target) && !breaks (jump_target) && !*non_constant_p);
 
diff --git a/gcc/cp/cp-gimplify.c b/gcc/cp/cp-gimplify.c
index 59953a6..d9f7cea 100644
--- a/gcc/cp/cp-gimplify.c
+++ b/gcc/cp/cp-gimplify.c
@@ -264,14 +264,7 @@  genericize_cp_loop (tree *stmt_p, location_t start_locus, tree cond, tree body,
 	loop = stmt_list;
     }
   else
-    {
-      location_t loc = start_locus;
-      if (!cond || integer_nonzerop (cond))
-	loc = EXPR_LOCATION (expr_first (body));
-      if (loc == UNKNOWN_LOCATION)
-	loc = start_locus;
-      loop = build1_loc (loc, LOOP_EXPR, void_type_node, stmt_list);
-    }
+    loop = build1_loc (start_locus, LOOP_EXPR, void_type_node, stmt_list);
 
   stmt_list = NULL;
   append_to_statement_list (loop, &stmt_list);
diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
index 9e0f07e..79c842d 100644
--- a/gcc/doc/invoke.texi
+++ b/gcc/doc/invoke.texi
@@ -191,7 +191,8 @@  in the following sections.
 @xref{C++ Dialect Options,,Options Controlling C++ Dialect}.
 @gccoptlist{-fabi-version=@var{n}  -fno-access-control @gol
 -fargs-in-order=@var{n} -fcheck-new @gol
--fconstexpr-depth=@var{n}  -ffriend-injection @gol
+-fconstexpr-depth=@var{n} -fconstexpr-loop-limit=@var{n} @gol
+-ffriend-injection @gol
 -fno-elide-constructors @gol
 -fno-enforce-eh-specs @gol
 -ffor-scope  -fno-for-scope  -fno-gnu-keywords @gol
@@ -2265,6 +2266,12 @@  to @var{n}.  A limit is needed to detect endless recursion during
 constant expression evaluation.  The minimum specified by the standard
 is 512.
 
+@item -fconstexpr-loop-limit=@var{n}
+@opindex fconstexpr-loop-limit
+Set the maximum number of iterations for a loop in C++14 constexpr functions
+to @var{n}.  A limit is needed to detect infinite loops during
+constant expression evaluation.  The default is 262144 (1<<18).
+
 @item -fdeduce-init-list
 @opindex fdeduce-init-list
 Enable deduction of a template type parameter as
diff --git a/gcc/testsuite/g++.dg/cpp1y/constexpr-loop6.C b/gcc/testsuite/g++.dg/cpp1y/constexpr-loop6.C
new file mode 100644
index 0000000..e49e531
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1y/constexpr-loop6.C
@@ -0,0 +1,11 @@ 
+// PR c++/65970
+// { dg-do compile { target c++14 } }
+// { dg-options -fconstexpr-loop-limit=5 }
+
+constexpr int foo() {
+  while (true)			// { dg-error "-fconstexpr-loop-limit" }
+    ;
+  return 0;
+}
+
+constexpr int i = foo();	// { dg-message "" }