debug/105158 - improve debug stmt retaining for forwarder removal

Currently when we cannot move debug stmt from a forwarder to the
destination block we drop/reset them.  But in some cases as for
the testcase we can move them to the predecessor when that has
a single successor and we can insert after the last stmt of the
block.  That allows us to preserve debug info here.

2022-04-05  Richard Biener  <rguenther@suse.de>

	PR debug/105158
	* tree-cfgcleanup.cc (move_debug_stmts_from_forwarder):
	Move debug stmts to the predecessor if moving to the
	destination is not possible.
	(remove_forwarder_block): Adjust.
	(remove_forwarder_block_with_phi): Likewise.

diff --git a/gcc/tree-cfgcleanup.cc b/gcc/tree-cfgcleanup.cc
index 6c05ef2464e621884576f746dc9c3a2f1d85aaf5..893619eaaa43b3c055f79721ee0c65250a3854c1 100644 (file)
--- a/gcc/tree-cfgcleanup.cc
+++ b/gcc/tree-cfgcleanup.cc
@@ -468,15 +468,34 @@ phi_alternatives_equal (basic_block dest, edge e1, edge e2)
   return true;
 }
 
-/* Move debug stmts from the forwarder block SRC to DEST.  */
+/* Move debug stmts from the forwarder block SRC to DEST or PRED.  */
 
 static void
-move_debug_stmts_from_forwarder (basic_block src, basic_block dest,
-                                bool dest_single_pred_p)
+move_debug_stmts_from_forwarder (basic_block src,
+                                basic_block dest, bool dest_single_pred_p,
+                                basic_block pred, bool pred_single_succ_p)
 {
   if (!MAY_HAVE_DEBUG_STMTS)
     return;
 
+  /* If we cannot move to the destination but to the predecessor do that.  */
+  if (!dest_single_pred_p && pred_single_succ_p)
+    {
+      gimple_stmt_iterator gsi_to = gsi_last_bb (pred);
+      if (gsi_end_p (gsi_to) || !stmt_ends_bb_p (gsi_stmt (gsi_to)))
+       {
+         for (gimple_stmt_iterator gsi = gsi_after_labels (src);
+              !gsi_end_p (gsi);)
+           {
+             gimple *debug = gsi_stmt (gsi);
+             gcc_assert (is_gimple_debug (debug));
+             gsi_move_after (&gsi, &gsi_to);
+           }
+         return;
+       }
+    }
+
+  /* Else move to DEST or drop/reset them.  */
   gimple_stmt_iterator gsi_to = gsi_after_labels (dest);
   for (gimple_stmt_iterator gsi = gsi_after_labels (src); !gsi_end_p (gsi);)
     {
@@ -627,7 +646,8 @@ remove_forwarder_block (basic_block bb)
 
   /* Move debug statements.  Reset them if the destination does not
      have a single predecessor.  */
-  move_debug_stmts_from_forwarder (bb, dest, dest_single_pred_p);
+  move_debug_stmts_from_forwarder (bb, dest, dest_single_pred_p,
+                                  pred, pred && single_succ_p (pred));
 
   bitmap_set_bit (cfgcleanup_altered_bbs, dest->index);
 
@@ -1309,7 +1329,8 @@ remove_forwarder_block_with_phi (basic_block bb)
 
   /* Move debug statements.  Reset them if the destination does not
      have a single predecessor.  */
-  move_debug_stmts_from_forwarder (bb, dest, dest_single_pred_p);
+  move_debug_stmts_from_forwarder (bb, dest, dest_single_pred_p,
+                                  pred, pred && single_succ_p (pred));
 
   /* Update the dominators.  */
   dombb = get_immediate_dominator (CDI_DOMINATORS, bb);