PATCH to fix ICE with -Wstringop-overflow and VLA (PR tree-optimization/84238)

[Marek Polacek]

Here we ICE because get_range_strlen's result might not be an array of
two integer constants -- for a VLA the array might contain a non-constant.
So beef up the check before converting to wide_int.

Bootstrapped/regtested on x86_64-linux, ok for trunk?

2018-02-06  Marek Polacek  <polacek@redhat.com>

	PR tree-optimization/84238
	* tree-ssa-strlen.c (maybe_diag_stxncpy_trunc): Verify the result of
	get_range_strlen.

	* gcc.dg/Wstringop-overflow-3.c: New test.

diff --git gcc/testsuite/gcc.dg/Wstringop-overflow-3.c gcc/testsuite/gcc.dg/Wstringop-overflow-3.c
index e69de29bb2d..8e2531f6b4e 100644
--- gcc/testsuite/gcc.dg/Wstringop-overflow-3.c
+++ gcc/testsuite/gcc.dg/Wstringop-overflow-3.c
@@ -0,0 +1,13 @@
+/* PR tree-optimization/84238 */
+/* { dg-do compile } */
+/* { dg-options "-O2" } */
+
+char a[1];
+int b;
+char *strncpy (char *, char *, __SIZE_TYPE__);
+void
+c ()
+{
+  char d[b];
+  strncpy (a, &d[3], 3); /* { dg-warning "writing 3 bytes into a region of size 1" } */
+}
diff --git gcc/tree-ssa-strlen.c gcc/tree-ssa-strlen.c
index f0f6535017b..94ed2bedc03 100644
--- gcc/tree-ssa-strlen.c
+++ gcc/tree-ssa-strlen.c
@@ -1899,7 +1899,10 @@ maybe_diag_stxncpy_trunc (gimple_stmt_iterator gsi, tree src, tree cnt)
     {
       tree range[2];
       get_range_strlen (src, range);
-      if (range[0])
+      if (range[0] != NULL_TREE
+	  && TREE_CODE (range[0]) == INTEGER_CST
+	  && range[1] != NULL_TREE
+	  && TREE_CODE (range[1]) == INTEGER_CST)
 	{
 	  lenrange[0] = wi::to_wide (range[0], prec);
 	  lenrange[1] = wi::to_wide (range[1], prec);

	Marek