PATCH to fix bogus warning with -Wstringop-truncation -g (PR tree-optimization/84228)

[Marek Polacek <polacek at redhat dot com>]

When -Wstringop-truncation sees a strncpy call where the specified bound
is equal to the size of the destination, it looks at the next statement
to see if it's dst[i] = '\0';, and if it is, it doesn't warn.  But it
needs to look at the next nondebug statement, otherwise we can get a
false positive with -g, as this testcase shows.

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

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

	PR tree-optimization/84228
	* tree-ssa-strlen.c (maybe_diag_stxncpy_trunc): Skip debug statements.

	* c-c++-common/Wstringop-truncation-3.c: New test.

diff --git gcc/testsuite/c-c++-common/Wstringop-truncation-3.c gcc/testsuite/c-c++-common/Wstringop-truncation-3.c
index e69de29bb2d..e15e6a42b07 100644
--- gcc/testsuite/c-c++-common/Wstringop-truncation-3.c
+++ gcc/testsuite/c-c++-common/Wstringop-truncation-3.c
@@ -0,0 +1,20 @@
+/* PR tree-optimization/84228 */
+/* { dg-do compile } */
+/* { dg-options "-Wstringop-truncation -O2 -g" } */
+
+char *strncpy (char *, const char *, __SIZE_TYPE__);
+struct S
+{
+  char arr[64];
+};
+
+int
+foo (struct S *p1, const char *a)
+{
+  if (a)
+    goto err;
+  strncpy (p1->arr, a, sizeof p1->arr); /* { dg-bogus "specified bound" } */
+  p1->arr[3] = '\0';
+err:
+  return 0;
+}
diff --git gcc/tree-ssa-strlen.c gcc/tree-ssa-strlen.c
index c3cf432a921..f0f6535017b 100644
--- gcc/tree-ssa-strlen.c
+++ gcc/tree-ssa-strlen.c
@@ -1849,7 +1849,7 @@ maybe_diag_stxncpy_trunc (gimple_stmt_iterator gsi, tree src, tree cnt)
 
   /* Look for dst[i] = '\0'; after the stxncpy() call and if found
      avoid the truncation warning.  */
-  gsi_next (&gsi);
+  gsi_next_nondebug (&gsi);
   gimple *next_stmt = gsi_stmt (gsi);
 
   if (!gsi_end_p (gsi) && is_gimple_assign (next_stmt))

	Marek