Avoid threading circular paths.

The backward threader keeps a hash of visited blocks to avoid crossing
the same block twice.  Interestingly, we haven't been checking it for
the final block out of the path.  This may be inherited from the old
code, as it was simple enough that it didn't matter.  With the
upcoming changes enabling the fully resolving threader, it gets
tripped often enough to cause wrong code to be generated.

Tested on x86-64 Linux.

gcc/ChangeLog:

	* tree-ssa-threadbackward.c (back_threader::maybe_register_path):
	Avoid threading circular paths.

diff --git a/gcc/tree-ssa-threadbackward.c b/gcc/tree-ssa-threadbackward.c
index a6b9893abbdf35e8feae1b3eafe38af1de205098..d9ce056b06cec378893c97ef26396ea7dc3674d5 100644 (file)
--- a/gcc/tree-ssa-threadbackward.c
+++ b/gcc/tree-ssa-threadbackward.c
@@ -140,6 +140,10 @@ 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);