The following allows the entry and exit block of a jump thread path
to be equal, which can easily happen when there isn't a forwarder
on the interesting edge for an FSM thread conditional.  We just
don't want to enlarge the path from such a block.

Bootstrapped and tested on x86_64-unknown-linux-gnu, tested the
gcc.dg/tree-ssa/ssa-dom-thread-7.c adjustment on aarch64-linux with
a cross, pushed to trunk.

Richard.

        PR tree-optimization/120003
        * tree-ssa-threadbackward.cc (back_threader::find_paths_to_names):
        Allow block re-use but do not enlarge the path beyond such a
        re-use.

        * gcc.dg/tree-ssa/ssa-thread-23.c: New testcase.
        * gcc.dg/tree-ssa/ssa-dom-thread-7.c: Adjust.
---
 .../gcc.dg/tree-ssa/ssa-dom-thread-7.c        |  4 ++--
 gcc/testsuite/gcc.dg/tree-ssa/ssa-thread-23.c | 19 +++++++++++++++++++
 gcc/tree-ssa-threadbackward.cc                |  8 +++-----
 3 files changed, 24 insertions(+), 7 deletions(-)
 create mode 100644 gcc/testsuite/gcc.dg/tree-ssa/ssa-thread-23.c

diff --git a/gcc/testsuite/gcc.dg/tree-ssa/ssa-dom-thread-7.c 
b/gcc/testsuite/gcc.dg/tree-ssa/ssa-dom-thread-7.c
index d84aceebc5d..8be9878e0cf 100644
--- a/gcc/testsuite/gcc.dg/tree-ssa/ssa-dom-thread-7.c
+++ b/gcc/testsuite/gcc.dg/tree-ssa/ssa-dom-thread-7.c
@@ -11,8 +11,8 @@
    to change decisions in switch expansion which in turn can expose new
    jump threading opportunities.  Skip the later tests on aarch64.  */
 /* { dg-final { scan-tree-dump-not "Jumps threaded"  "dom3" { target { ! 
aarch64*-*-* } } } } */
-/* { dg-final { scan-tree-dump "Jumps threaded: 9"  "thread2" { target { ! 
aarch64*-*-* } } } } */
-/* { dg-final { scan-tree-dump "Jumps threaded: 17"  "thread2" { target { 
aarch64*-*-* } } } } */
+/* { dg-final { scan-tree-dump "Jumps threaded: 10"  "thread2" { target { ! 
aarch64*-*-* } } } } */
+/* { dg-final { scan-tree-dump "Jumps threaded: 14"  "thread2" { target { 
aarch64*-*-* } } } } */
 
 enum STATE {
   S0=0,
diff --git a/gcc/testsuite/gcc.dg/tree-ssa/ssa-thread-23.c 
b/gcc/testsuite/gcc.dg/tree-ssa/ssa-thread-23.c
new file mode 100644
index 00000000000..930360a33b5
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/tree-ssa/ssa-thread-23.c
@@ -0,0 +1,19 @@
+/* PR120003 */
+/* { dg-do compile } */
+/* { dg-options "-O2 -fdump-tree-cddce3-details" } */
+
+extern _Bool g(int);
+
+_Bool f()
+{
+  _Bool retval = 0;
+  for(int i=0; i<1000000; ++i)
+    retval = retval || g(i);
+  return retval;
+}
+
+/* Jump threading after loop optimization should get the counting loop
+   separated from the loop until retval is true and CD-DCE elide it.
+   It's difficult to check for the fact that a true retval terminates
+   the loop so check CD-DCE eliminates one loop instead.  */
+/* { dg-final { scan-tree-dump "fix_loop_structure: removing loop" "cddce3" } 
} */
diff --git a/gcc/tree-ssa-threadbackward.cc b/gcc/tree-ssa-threadbackward.cc
index 23bfc14c8f0..ce765cb5ded 100644
--- a/gcc/tree-ssa-threadbackward.cc
+++ b/gcc/tree-ssa-threadbackward.cc
@@ -349,9 +349,6 @@ back_threader::find_paths_to_names (basic_block bb, bitmap 
interesting,
                                    unsigned overall_paths,
                                    back_threader_profitability &profit)
 {
-  if (m_visited_bbs.add (bb))
-    return;
-
   m_path.safe_push (bb);
 
   // Try to resolve the path without looking back.  Avoid resolving paths
@@ -377,7 +374,8 @@ back_threader::find_paths_to_names (basic_block bb, bitmap 
interesting,
   // 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))
-          <= (unsigned)param_max_jump_thread_paths)
+          <= (unsigned)param_max_jump_thread_paths
+          && !m_visited_bbs.add (bb))
     {
       // For further greedy searching we want to remove interesting
       // names defined in BB but add ones on the PHI edges for the
@@ -489,6 +487,7 @@ back_threader::find_paths_to_names (basic_block bb, bitmap 
interesting,
         backtracking we have to restore it.  */
       for (int j : new_imports)
        bitmap_clear_bit (m_imports, j);
+      m_visited_bbs.remove (bb);
     }
   else if (dump_file && (dump_flags & TDF_DETAILS))
     fprintf (dump_file, "  FAIL: Search space limit %d reached.\n",
@@ -496,7 +495,6 @@ back_threader::find_paths_to_names (basic_block bb, bitmap 
interesting,
 
   // Reset things to their original state.
   m_path.pop ();
-  m_visited_bbs.remove (bb);
 }
 
 // Search backwards from BB looking for paths where the final
-- 
2.43.0

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