[gcc r12-4700] Avoid threading circular paths.

[gcc r12-4700] Avoid threading circular paths.

[Aldy Hernandez]

https://gcc.gnu.org/g:e53fbb17839723ea1697fcbaf76b1c092675eaaf

commit r12-4700-ge53fbb17839723ea1697fcbaf76b1c092675eaaf
Author: Aldy Hernandez <aldyh@redhat.com>
Date:   Wed Oct 20 18:52:45 2021 +0200

    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:
---
 gcc/tree-ssa-threadbackward.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/gcc/tree-ssa-threadbackward.c b/gcc/tree-ssa-threadbackward.c
index a6b9893abbd..d9ce056b06c 100644
--- 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);