[gcc r17-3348] Report failed block copies in the backward threader [PR126906]

https://gcc.gnu.org/g:454e3ba345c2263563562b489bdb81fcc8e71105

commit r17-3348-g454e3ba345c2263563562b489bdb81fcc8e71105
Author: Aldy Hernandez <[email protected]>
Date:   Sun Aug 16 18:50:54 2026 +0000

    Report failed block copies in the backward threader [PR126906]
    
    When back_jt_path_registry::duplicate_thread_path fails, the
    registered path is dropped silently: the dump shows "Registering jump
    thread" with no matching outcome.  Every other way a thread dies is
    reported, the profitability FAILs, and the registry's other drops via
    cancel_thread.  Report why the copy failed through cancel_thread like
    everywhere else.
    
    gcc/ChangeLog:
            PR tree-optimization/126906
    
            * tree-ssa-threadupdate.h
            (back_jt_path_registry::duplicate_thread_path): Add failure_reason
            argument.
            * tree-ssa-threadupdate.cc
            (back_jt_path_registry::duplicate_thread_path): Set
            FAILURE_REASON on the two failing exits.
            (back_jt_path_registry::update_cfg): Cancel the path with the
            reason when duplicate_thread_path fails.

Diff:
---
 gcc/tree-ssa-threadupdate.cc | 24 ++++++++++++++++++------
 gcc/tree-ssa-threadupdate.h  |  3 ++-
 2 files changed, 20 insertions(+), 7 deletions(-)

diff --git a/gcc/tree-ssa-threadupdate.cc b/gcc/tree-ssa-threadupdate.cc
index 91c2c1788efb..54d2c4194237 100644
--- a/gcc/tree-ssa-threadupdate.cc
+++ b/gcc/tree-ssa-threadupdate.cc
@@ -2382,14 +2382,16 @@ back_jt_path_registry::adjust_paths_after_duplication (unsigned curr_path_num)
    CURRENT_PATH_NO is an index into the global paths[] table
    specifying the jump-thread path.
 
-   Returns false if it is unable to copy the region, true otherwise.  */
+   Returns false if it is unable to copy the region, true otherwise.
+   On failure *FAILURE_REASON says why.  */
 
 bool
 back_jt_path_registry::duplicate_thread_path (edge entry,
 					      edge exit,
 					      basic_block *region,
 					      unsigned n_region,
-					      unsigned current_path_no)
+					      unsigned current_path_no,
+					      const char **failure_reason)
 {
   unsigned i;
   class loop *loop = entry->dest->loop_father;
@@ -2398,7 +2400,10 @@ back_jt_path_registry::duplicate_thread_path (edge entry,
   profile_count curr_count;
 
   if (!can_copy_bbs_p (region, n_region))
-    return false;
+    {
+      *failure_reason = "Cannot copy the blocks in the path";
+      return false;
+    }
 
   /* Some sanity checking.  Note that we do not check for all possible
      missuses of the functions.  I.e. if you ask to copy something weird,
@@ -2416,7 +2421,10 @@ back_jt_path_registry::duplicate_thread_path (edge entry,
 	  || (loop->header == entry->dest
 	      && region[i] != exit->src
 	      && EDGE_COUNT (region[i]->succs) > 1))
-	return false;
+	{
+	  *failure_reason = "Path crosses loops";
+	  return false;
+	}
     }
 
   initialize_original_copy_tables ();
@@ -2645,16 +2653,20 @@ back_jt_path_registry::update_cfg (bool /*peel_loop_headers*/)
       for (unsigned int j = 0; j < len - 1; j++)
 	region[j] = (*path)[j]->e->dest;
 
-      if (duplicate_thread_path (entry, exit, region, len - 1, 0))
+      const char *failure_reason = NULL;
+      if (duplicate_thread_path (entry, exit, region, len - 1, 0,
+				 &failure_reason))
 	{
 	  /* We do not update dominance info.  */
 	  free_dominance_info (CDI_DOMINATORS);
 	  visited_starting_edges.add (entry);
 	  retval = true;
 	  m_num_threaded_edges++;
+	  path->release ();
 	}
+      else
+	cancel_thread (path, failure_reason);
 
-      path->release ();
       m_paths.unordered_remove (0);
       free (region);
     }
diff --git a/gcc/tree-ssa-threadupdate.h b/gcc/tree-ssa-threadupdate.h
index 7faac34fb8fc..624bc10a64d4 100644
--- a/gcc/tree-ssa-threadupdate.h
+++ b/gcc/tree-ssa-threadupdate.h
@@ -116,7 +116,8 @@ private:
   bool update_cfg (bool peel_loop_headers) override;
   void adjust_paths_after_duplication (unsigned curr_path_num);
   bool duplicate_thread_path (edge entry, edge exit, basic_block *region,
-			      unsigned n_region, unsigned current_path_no);
+			      unsigned n_region, unsigned current_path_no,
+			      const char **failure_reason);
   bool rewire_first_differing_edge (unsigned path_num, unsigned edge_num);
 };