[gcc r12-5068] Return NULL for maybe_register_path when unprofitable.

[Aldy Hernandez <aldyh@gcc.gnu.org>]

https://gcc.gnu.org/g:2b59cf475a06f37c2b49631043f2896455923c3c

commit r12-5068-g2b59cf475a06f37c2b49631043f2896455923c3c
Author: Aldy Hernandez <aldyh@redhat.com>
Date:   Tue Nov 9 10:45:26 2021 +0100

    Return NULL for maybe_register_path when unprofitable.
    
    This is a minor cleanup for maybe_register_path to return NULL when
    the path is unprofitable.  It is needed for a follow-up patch to
    generate better dumps from the threader.
    
    There is no change in behavior, since the only call to this function
    bails on !profitable_path_p.
    
    Tested on x86-64 Linux.
    
    gcc/ChangeLog:
    
            * tree-ssa-threadbackward.c (back_threader::maybe_register_path):
            Return NULL when unprofitable.

Diff:
---
 gcc/tree-ssa-threadbackward.c | 38 ++++++++++++++++++++------------------
 1 file changed, 20 insertions(+), 18 deletions(-)

diff --git a/gcc/tree-ssa-threadbackward.c b/gcc/tree-ssa-threadbackward.c
index f9485bf9046..84249544760 100644
--- a/gcc/tree-ssa-threadbackward.c
+++ b/gcc/tree-ssa-threadbackward.c
@@ -186,11 +186,10 @@ back_threader::debug_counter ()
   return true;
 }
 
-// Register the current path for jump threading if it's profitable to
-// do so.
 //
-// Return the known taken edge out of the path, even if the path was
-// not registered, or NULL if the taken edge could not be determined.
+// Return NULL if it is unprofitable to thread this path, or the
+// outgoing edge is unknown.  Return UNREACHABLE_EDGE if the path is
+// unreachable.
 
 edge
 back_threader::maybe_register_path ()
@@ -199,23 +198,26 @@ back_threader::maybe_register_path ()
 
   if (taken_edge && taken_edge != UNREACHABLE_EDGE)
     {
-      // Avoid circular paths.
       if (m_visited_bbs.contains (taken_edge->dest))
-	return UNREACHABLE_EDGE;
-
-      bool irreducible = false;
-      bool profitable
-	= m_profit.profitable_path_p (m_path, m_name, taken_edge, &irreducible);
-
-      if (profitable)
 	{
-	  if (!debug_counter ())
-	    return NULL;
-
-	  m_registry.register_path (m_path, taken_edge);
+	  // Avoid circular paths by indicating there is nothing to
+	  // see in this direction.
+	  taken_edge = UNREACHABLE_EDGE;
+	}
+      else
+	{
+	  bool irreducible = false;
+	  if (m_profit.profitable_path_p (m_path, m_name, taken_edge,
+					  &irreducible)
+	      && debug_counter ())
+	    {
+	      m_registry.register_path (m_path, taken_edge);
 
-	  if (irreducible)
-	    vect_free_loop_info_assumptions (m_path[0]->loop_father);
+	      if (irreducible)
+		vect_free_loop_info_assumptions (m_path[0]->loop_father);
+	    }
+	  else
+	    taken_edge = NULL;
 	}
     }
   return taken_edge;