[PATCH] Fix bogus warning with -Wimplicit-fallthrough (PR c/79152)

[Marek Polacek]

We already have code to handle

  case 0:
    foo ();
  label:
    bar ();

where we don't want to warn, but the code didn't handle consecutive non-case
labels.  Thus fixed.

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

2017-01-20  Marek Polacek  <pola...@redhat.com>

        PR c/79152
        * gimplify.c (should_warn_for_implicit_fallthrough): Handle consecutive
        non-case labels.

        * c-c++-common/Wimplicit-fallthrough-35.c: New test.

diff --git gcc/gimplify.c gcc/gimplify.c
index 2777a23..d382eea 100644
--- gcc/gimplify.c
+++ gcc/gimplify.c
@@ -1985,7 +1985,7 @@ should_warn_for_implicit_fallthrough 
(gimple_stmt_iterator *gsi_p, tree label)
   if (FALLTHROUGH_LABEL_P (label))
     return false;
 
-  /* Don't warn for a non-case label followed by a statement:
+  /* Don't warn for non-case labels followed by a statement:
        case 0:
         foo ();
        label:
@@ -1993,7 +1993,12 @@ should_warn_for_implicit_fallthrough 
(gimple_stmt_iterator *gsi_p, tree label)
      as these are likely intentional.  */
   if (!case_label_p (&gimplify_ctxp->case_labels, label))
     {
-      gsi_next (&gsi);
+      tree l;
+      while (!gsi_end_p (gsi)
+            && gimple_code (gsi_stmt (gsi)) == GIMPLE_LABEL
+            && (l = gimple_label_label (as_a <glabel *> (gsi_stmt (gsi))))
+            && !case_label_p (&gimplify_ctxp->case_labels, l))
+       gsi_next (&gsi);
       if (gsi_end_p (gsi) || gimple_code (gsi_stmt (gsi)) != GIMPLE_LABEL)
        return false;
     }
diff --git gcc/testsuite/c-c++-common/Wimplicit-fallthrough-35.c 
gcc/testsuite/c-c++-common/Wimplicit-fallthrough-35.c
index e69de29..9a0aba6 100644
--- gcc/testsuite/c-c++-common/Wimplicit-fallthrough-35.c
+++ gcc/testsuite/c-c++-common/Wimplicit-fallthrough-35.c
@@ -0,0 +1,61 @@
+/* PR c/79152 */
+/* { dg-do compile } */
+/* { dg-options "-Wimplicit-fallthrough" } */
+
+extern void foo (int);
+
+void
+f (int i)
+{
+  switch (i)
+    {
+    case 0:
+      foo (0);
+l1:
+      foo (1);
+    }
+
+  switch (i)
+    {
+    case 0:
+      foo (0);
+l2:;
+    }
+
+  switch (i)
+    {
+    case 0:
+      foo (0);
+l3:
+l4:
+      foo (1);
+    }
+
+  switch (i)
+    {
+    case 0:
+      foo (0);
+l5:
+l6:;
+    }
+
+  switch (i)
+    {
+    case 0:
+      foo (0); /* { dg-warning "statement may fall through" } */
+l7:
+l8:
+    case 1:
+      foo (1);
+    }
+
+  switch (i)
+    {
+    case 0:
+      foo (0); /* { dg-warning "statement may fall through" } */
+l9:
+    case 1:
+l10:
+      foo (1);
+    }
+}

        Marek