[PATCH] Remove dead labels to increase superblock scope

[Tom de Vries]

On 11/18/2011 10:29 PM, Eric Botcazou wrote:
>> For the test-case of PR50764, a dead label is introduced by
>> fixup_reorder_chain in cfg_layout_finalize, called from
>> pass_reorder_blocks.
> 
> I presume that there is no reasonable way of preventing fixup_reorder_chain 
> from introducing it or of teaching fixup_reorder_chain to remove it?
> 

This (untested) patch also removes the dead label for the PR, and I think it is
safe.
...
Index: cfglayout.c
===================================================================
--- cfglayout.c (revision 181377)
+++ cfglayout.c (working copy)
@@ -702,6 +702,21 @@ relink_block_chain (bool stay_in_cfglayo
 }
 


+static bool
+forced_label_p (rtx label)
+{
+  rtx insn, forced_label;
+  for (insn = forced_labels; insn; insn = XEXP (insn, 1))
+    {
+      forced_label = XEXP (insn, 0);
+      if (!LABEL_P (forced_label))
+	continue;
+      if (forced_label == label)
+	return true;
+    }
+  return false;
+}
+
 /* Given a reorder chain, rearrange the code to match.  */

 static void
@@ -857,6 +872,12 @@ fixup_reorder_chain (void)
 				       (e_taken->src, e_taken->dest));
 		  e_taken->flags |= EDGE_FALLTHRU;
 		  update_br_prob_note (bb);
+		  if (LABEL_NUSES (ret_label) == 0
+		      && !LABEL_PRESERVE_P (ret_label)
+		      && LABEL_NAME (ret_label) == NULL
+		      && !forced_label_p (ret_label)
+		      && single_pred_p (e_taken->dest))
+		    delete_insn (ret_label);
 		  continue;
 		}
 	    }
...

But I see 2 potential issues:
- it only catches this case in fixup_reorder_chain. I don't know if there are
  more cases, but the earlier catch-all-afterwards patch surely will catch
  those, this patch probably not.
- I'm not sure if the use count will drop always drop to 0 in
  fixup_reorder_chain, that might only happen after rebuild_jump_labels.

Thanks,
- Tom