diff --git a/gcc/ChangeLog b/gcc/ChangeLog
index a7f3afdba76..ac121a83660 100644
--- a/gcc/ChangeLog
+++ b/gcc/ChangeLog
@@ -1,3 +1,8 @@
+2018-11-04  Bernd Edlinger  <bernd.edlinger@hotmail.de>
+
+	PR tree-optimization/86572
+	* builtins.c (c_strlen): Handle negative offsets in a safe way.
+
 2018-11-04  Bernd Edlinger  <bernd.edlinger@hotmail.de>
 
 	PR tree-optimization/87672
diff --git a/gcc/builtins.c b/gcc/builtins.c
index f64b3d4065b..9f2a90a4edf 100644
--- a/gcc/builtins.c
+++ b/gcc/builtins.c
@@ -734,11 +734,14 @@ c_strlen (tree src, int only_value, c_strlen_data *data, unsigned eltsize)
 	 of the string subtract the offset from the length of the string,
 	 and return that.  Otherwise the length is zero.  Take care to
 	 use SAVE_EXPR in case the OFFSET has side-effects.  */
-      tree offsave = TREE_SIDE_EFFECTS (byteoff) ? save_expr (byteoff) : byteoff;
-      offsave = fold_convert (ssizetype, offsave);
+      tree offsave = TREE_SIDE_EFFECTS (byteoff) ? save_expr (byteoff)
+						 : byteoff;
+      offsave = fold_convert_loc (loc, sizetype, offsave);
       tree condexp = fold_build2_loc (loc, LE_EXPR, boolean_type_node, offsave,
-				      build_int_cst (ssizetype, len));
-      tree lenexp = size_diffop_loc (loc, ssize_int (len), offsave);
+				      size_int (len));
+      tree lenexp = fold_build2_loc (loc, MINUS_EXPR, sizetype, size_int (len),
+				     offsave);
+      lenexp = fold_convert_loc (loc, ssizetype, lenexp);
       return fold_build3_loc (loc, COND_EXPR, ssizetype, condexp, lenexp,
 			      build_zero_cst (ssizetype));
     }
diff --git a/gcc/testsuite/ChangeLog b/gcc/testsuite/ChangeLog
index 79a7859e537..a28d9a043eb 100644
--- a/gcc/testsuite/ChangeLog
+++ b/gcc/testsuite/ChangeLog
@@ -1,3 +1,8 @@
+2018-11-04  Bernd Edlinger  <bernd.edlinger@hotmail.de>
+
+	PR tree-optimization/86572
+	* gcc.dg/pr86572.c: New test.
+
 2018-11-04  Bernd Edlinger  <bernd.edlinger@hotmail.de>
 
 	PR tree-optimization/87672
diff --git a/gcc/testsuite/gcc.dg/pr86572.c b/gcc/testsuite/gcc.dg/pr86572.c
new file mode 100644
index 00000000000..6cda55bae22
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/pr86572.c
@@ -0,0 +1,11 @@
+/* { dg-do compile } */
+/* { dg-options "-O2 -fdump-tree-optimized" } */
+
+const char buf[40] = "test";
+void test (int x)
+{
+  if (__builtin_strlen (buf + x) > 4)
+    __builtin_abort ();
+}
+
+/* { dg-final { scan-tree-dump-not "abort" "optimized" } } */