[PATCH] Stop backwards thread discovery when leaving a loop

[Richard Biener <rguenther@suse.de>]

The backward threader copier cannot deal with the situation of
copying blocks belonging to different loops and will reject those
paths late.  The following uses this to prune path discovery,
saving on compile-time.  Note the off-loop block is still considered
as entry edge origin.

Bootstrapped and tested on x86_64-unknown-linux-gnu.

I've split this out from the 'Support threading of just the exit edge'
patch under discussion - it's really an independent change.

Pushed.

	* tree-ssa-threadbackward.cc (back_threader::find_paths_to_names):
	Do not walk further if we are leaving the current loop.
---
 gcc/tree-ssa-threadbackward.cc | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/gcc/tree-ssa-threadbackward.cc b/gcc/tree-ssa-threadbackward.cc
index b886027fccf..1c362839fab 100644
--- a/gcc/tree-ssa-threadbackward.cc
+++ b/gcc/tree-ssa-threadbackward.cc
@@ -355,6 +355,12 @@ back_threader::find_paths_to_names (basic_block bb, bitmap interesting,
 	  || maybe_register_path ()))
     ;
 
+  // The backwards thread copier cannot copy blocks that do not belong
+  // to the same loop, so when the new source of the path entry no
+  // longer belongs to it we don't need to search further.
+  else if (m_path[0]->loop_father != bb->loop_father)
+    ;
+
   // Continue looking for ways to extend the path but limit the
   // search space along a branch
   else if ((overall_paths = overall_paths * EDGE_COUNT (bb->preds))
-- 
2.35.3