[SFN] sync up debug-only stmt list's side effects with empty stmts too

for  gcc/c-family/ChangeLog

	PR debug/83527
	PR debug/83419
	* c-semantics.c (only_debug_stmts_after_p): New.
	(pop_stmt_list): Clear side effects in debug-only stmt list.
	Check for single nondebug stmt followed by debug stmts only.

for  gcc/testsuite/ChangeLog

	PR debug/83527
	PR debug/83419
	* gcc.dg/pr83527.c: New.

From-SVN: r255966

gcc/c-family/ChangeLog

+2017-12-22  Alexandre Oliva <aoliva@redhat.com>
+
+	PR debug/83527
+	PR debug/83419
+	* c-semantics.c (only_debug_stmts_after_p): New.
+	(pop_stmt_list): Clear side effects in debug-only stmt list.
+	Check for single nondebug stmt followed by debug stmts only.
+
 2017-12-21  Alexandre Oliva  <aoliva@redhat.com>
 
 	PR debug/83419

gcc/c-family/c-semantics.c

@@ -35,6 +35,17 @@ push_stmt_list (void)
   return t;
 }
 
+/* Return TRUE if, after I, there are any nondebug stmts.  */
+
+static inline bool
+only_debug_stmts_after_p (tree_stmt_iterator i)
+{
+  for (tsi_next (&i); !tsi_end_p (i); tsi_next (&i))
+    if (TREE_CODE (tsi_stmt (i)) != DEBUG_BEGIN_STMT)
+      return false;
+  return true;
+}
+
 /* Finish the statement tree rooted at T.  */
 
 tree
@@ -99,11 +110,15 @@ pop_stmt_list (tree t)
 	  while (!tsi_end_p (i)
 		 && TREE_CODE (tsi_stmt (i)) == DEBUG_BEGIN_STMT)
 	    tsi_next (&i);
-	  /* If there's only one nondebug stmt in the list, we'd have
-	     extracted the stmt and dropped the list, and we'd take
-	     TREE_SIDE_EFFECTS from that statement, so keep the list's
+	  /* If there are only debug stmts in the list, without them
+	     we'd have an empty stmt without side effects.  If there's
+	     only one nondebug stmt, we'd have extracted the stmt and
+	     dropped the list, and we'd take TREE_SIDE_EFFECTS from
+	     that statement.  In either case, keep the list's
 	     TREE_SIDE_EFFECTS in sync.  */
-	  if (tsi_one_before_end_p (i))
+	  if (tsi_end_p (i))
+	    TREE_SIDE_EFFECTS (t) = 0;
+	  else if (only_debug_stmts_after_p (i))
 	    TREE_SIDE_EFFECTS (t) = TREE_SIDE_EFFECTS (tsi_stmt (i));
 	}
     }

gcc/testsuite/ChangeLog

+2017-12-22  Alexandre Oliva <aoliva@redhat.com>
+
+	PR debug/83527
+	PR debug/83419
+	* gcc.dg/pr83527.c: New.
+
 2017-12-21  Martin Sebor  <msebor@redhat.com>
 
 	PR testsuite/83462

gcc/testsuite/gcc.dg/pr83527.c

+/* PR debug/83527 */
+/* { dg-do compile } */
+/* { dg-options "-O2 -fcompare-debug" } */
+
+extern void fn2(void);
+extern void fn3(void);
+int a, b;
+void fn1() {
+  int c;
+  short d;
+  switch (a) {
+  case 32800:
+    fn2();
+  case 32769:
+    b = 0;
+  case 32771:
+  case 32772:
+  case 32782:
+    fn3();
+  }
+  if (d || c) {
+    do
+      ;
+    while (0);
+  }
+}