Make more bad uses of fallthrough attribute into pedwarns.

Various bad uses of the [[fallthrough]] attribute are constraint
violations in C2x, so need pedwarns rather than warnings.

This patch duly turns the relevant warnings into pedwarns.  The
relevant code is not specific to C, and does not know which form the
attribute was given in ([[fallthrough]] or [[gnu::fallthrough]] or
__attribute__((fallthrough))), but as I understand it these usages are
also erroneous for C++ and it seems reasonable to give a pedwarn here
even when a form other than [[fallthrough]] is being used.

The precise meaning of the standard wording about "The next statement
that would be executed" seems a but unclear in some corner cases; the
tests added keep to cases where it is clear whether or not the next
statement executed is of the required form.

Bootstrapped with no regressions for x86_64-pc-linux-gnu.

gcc:
	* gimplify.c (expand_FALLTHROUGH_r, expand_FALLTHROUGH): Use
	pedwarn instead of warning_at for fallthrough not preceding a case
	or default label.

gcc/c-family:
	* c-attribs.c (handle_fallthrough_attribute): Use pedwarn instead
	of warning.

gcc/testsuite:
	* gcc.dg/c2x-attr-fallthrough-6.c: New test.  Split out from
	c2x-attr-fallthrough-3.c.
	* gcc.dg/c2x-attr-fallthrough-1.c: Add more tests.
	* gcc.dg/c2x-attr-fallthrough-2.c: Update expected diagnostics.
	* gcc.dg/c2x-attr-fallthrough-3.c: Split inside-switch part of
	test out to c2x-attr-fallthrough-6.c.

From-SVN: r278599

gcc/ChangeLog

+2019-11-21  Joseph Myers  <joseph@codesourcery.com>
+
+	* gimplify.c (expand_FALLTHROUGH_r, expand_FALLTHROUGH): Use
+	pedwarn instead of warning_at for fallthrough not preceding a case
+	or default label.
+
 2019-11-22  Prathamesh Kulkarni  <prathamesh.kulkarni@linaro.org>
 
 	PR tree-optimization/92608

gcc/c-family/ChangeLog

+2019-11-21  Joseph Myers  <joseph@codesourcery.com>
+
+	* c-attribs.c (handle_fallthrough_attribute): Use pedwarn instead
+	of warning.
+
 2019-11-19  Joseph Myers  <joseph@codesourcery.com>
 
 	* c-common.c (attribute_fallthrough_p): In C, use pedwarn not

gcc/c-family/c-attribs.c

@@ -4117,7 +4117,7 @@ tree
 handle_fallthrough_attribute (tree *, tree name, tree, int,
 			      bool *no_add_attrs)
 {
-  warning (OPT_Wattributes, "%qE attribute ignored", name);
+  pedwarn (input_location, OPT_Wattributes, "%qE attribute ignored", name);
   *no_add_attrs = true;
   return NULL_TREE;
 }

gcc/gimplify.c

@@ -2405,8 +2405,8 @@ expand_FALLTHROUGH_r (gimple_stmt_iterator *gsi_p, bool *handled_ops_p,
 	      gsi_next (&gsi2);
 	    }
 	  if (!found)
-	    warning_at (loc, 0, "attribute %<fallthrough%> not preceding "
-			"a case label or default label");
+	    pedwarn (loc, 0, "attribute %<fallthrough%> not preceding "
+		     "a case label or default label");
 	}
       break;
     default:
@@ -2428,8 +2428,8 @@ expand_FALLTHROUGH (gimple_seq *seq_p)
   if (wi.callback_result == integer_zero_node)
     /* We've found [[fallthrough]]; at the end of a switch, which the C++
        standard says is ill-formed; see [dcl.attr.fallthrough].  */
-    warning_at (loc, 0, "attribute %<fallthrough%> not preceding "
-		"a case label or default label");
+    pedwarn (loc, 0, "attribute %<fallthrough%> not preceding "
+	     "a case label or default label");
 }
 
 

gcc/testsuite/ChangeLog

+2019-11-21  Joseph Myers  <joseph@codesourcery.com>
+
+	* gcc.dg/c2x-attr-fallthrough-6.c: New test.  Split out from
+	c2x-attr-fallthrough-3.c.
+	* gcc.dg/c2x-attr-fallthrough-1.c: Add more tests.
+	* gcc.dg/c2x-attr-fallthrough-2.c: Update expected diagnostics.
+	* gcc.dg/c2x-attr-fallthrough-3.c: Split inside-switch part of
+	test out to c2x-attr-fallthrough-6.c.
+
 2019-11-22  Prathamesh Kulkarni  <prathamesh.kulkarni@linaro.org>
 
 	PR tree-optimization/92608

gcc/testsuite/gcc.dg/c2x-attr-fallthrough-1.c

@@ -3,7 +3,7 @@
 /* { dg-options "-std=c2x -pedantic-errors -Wextra" } */
 
 int
-f (int a)
+f (int a, int c)
 {
   int b = 2;
   switch (a)
@@ -22,6 +22,21 @@ f (int a)
     case 5:
       b += 1;
       break;
+    case 6:
+      if (c == 2)
+	{
+	  [[fallthrough]];
+	}
+      else
+	{
+	  [[fallthrough]];
+	}
+    case 7:
+      b += 3;
+      [[fallthrough]];
+    default:
+      b += 8;
+      break;
     }
   return b;
 }

gcc/testsuite/gcc.dg/c2x-attr-fallthrough-2.c

@@ -15,7 +15,8 @@ int z = sizeof (int [[fallthrough]]); /* { dg-error "ignored" } */
 int
 f (int a)
 {
-  [[fallthrough]] int b = 2; /* { dg-warning "not followed by|ignored" } */
+  [[fallthrough]] int b = 2; /* { dg-warning "not followed by" } */
+  /* { dg-error "ignored" "ignored" { target *-*-* } .-1 } */
   switch (a)
     {
     case 1:

gcc/testsuite/gcc.dg/c2x-attr-fallthrough-3.c

 /* Test C2x attribute syntax.  Invalid use of fallthrough attribute
-   outside switch or in bad context inside switch.  */
+   outside switch.  */
 /* { dg-do compile } */
 /* { dg-options "-std=c2x -pedantic-errors -Wextra" } */
 
@@ -7,12 +7,5 @@ int
 f (int a)
 {
   [[fallthrough]]; /* { dg-error "invalid use of attribute 'fallthrough'" } */
-  switch (a)
-    {
-    case 1:
-      a++;
-      [[fallthrough]]; /* { dg-warning "attribute 'fallthrough' not preceding a case label or default label" } */
-      a++;
-    }
   return a;
 }

gcc/testsuite/gcc.dg/c2x-attr-fallthrough-6.c

+/* Test C2x attribute syntax.  Invalid use of fallthrough attribute in
+   bad context inside switch.  */
+/* { dg-do compile } */
+/* { dg-options "-std=c2x -pedantic-errors -Wextra" } */
+
+int
+f (int a)
+{
+  switch (a)
+    {
+    case 1:
+      a++;
+      [[fallthrough]]; /* { dg-error "attribute 'fallthrough' not preceding a case label or default label" } */
+      a++;
+      [[fallthrough]]; /* { dg-error "attribute 'fallthrough' not preceding a case label or default label" } */
+    }
+  return a;
+}