tree-cfg: check placement of returns_twice calls

When a returns_twice call has an associated abnormal edge, the edge
corresponds to the "second return" from the call. It wouldn't make sense
if any executable statements appeared between the call and the
destination of the edge (they wouldn't be re-executed upon the "second
return"), so verify that.

gcc/ChangeLog:

	* tree-cfg.cc (gimple_verify_flow_info): Check placement of
	returns_twice calls.

1 files changed, 33 insertions, 0 deletions

diff --git a/gcc/tree-cfg.cc b/gcc/tree-cfg.cc
index a365aad..5bcf781 100644
--- a/gcc/tree-cfg.cc
+++ b/gcc/tree-cfg.cc
@@ -5653,6 +5653,7 @@ gimple_verify_flow_info (void)
 	}
 
       /* Verify that body of basic block BB is free of control flow.  */
+      bool seen_nondebug_stmt = false;
       for (; !gsi_end_p (gsi); gsi_next (&gsi))
 	{
 	  gimple *stmt = gsi_stmt (gsi);
@@ -5673,6 +5674,38 @@ gimple_verify_flow_info (void)
 		     gimple_label_label (label_stmt), bb->index);
 	      err = 1;
 	    }
+
+	  /* Check that no statements appear between a returns_twice call
+	     and its associated abnormal edge.  */
+	  if (gimple_code (stmt) == GIMPLE_CALL
+	      && gimple_call_flags (stmt) & ECF_RETURNS_TWICE)
+	    {
+	      const char *misplaced = NULL;
+	      /* TM is an exception: it points abnormal edges just after the
+		 call that starts a transaction, i.e. it must end the BB.  */
+	      if (gimple_call_builtin_p (stmt, BUILT_IN_TM_START))
+		{
+		  if (single_succ_p (bb)
+		      && bb_has_abnormal_pred (single_succ (bb))
+		      && !gsi_one_nondebug_before_end_p (gsi))
+		    misplaced = "not last";
+		}
+	      else
+		{
+		  if (seen_nondebug_stmt
+		      && bb_has_abnormal_pred (bb))
+		    misplaced = "not first";
+		}
+	      if (misplaced)
+		{
+		  error ("returns_twice call is %s in basic block %d",
+			 misplaced, bb->index);
+		  print_gimple_stmt (stderr, stmt, 0, TDF_SLIM);
+		  err = 1;
+		}
+	    }
+	  if (!is_gimple_debug (stmt))
+	    seen_nondebug_stmt = true;
 	}
 
       gsi = gsi_last_nondebug_bb (bb);