[PATCH] Allow loop crossing paths in back threader copier.

[Aldy Hernandez via Gcc-patches]

We are currently restricting loop crossing paths in the generic copier
used by the back threader, but we should be able to handle them after
loop_done has completed.

This fixes the PR at -O2, though the problem remains at -O1 because we
have no threaders smart enough to elide the undefined read.  DOM3 could
be a candidate when it is converted to either a hybrid threader or
replaced with the backward threader (when ranger can handle floats).

Tested on x86-64 Linux.

OK for trunk?

        PR tree-optimization/80548

gcc/ChangeLog:

        * attribs.c (sorted_attr_string): Add assert for -Wstringop-overread.
        * tree-ssa-threadupdate.c
        (back_jt_path_registry::duplicate_thread_path): Allow paths that
        cross loops after loop_done.
        (back_jt_path_registry::update_cfg): Diagnose dropped threads
        after duplicate_thread_path.

gcc/testsuite/ChangeLog:

        * gcc.dg/pr80548.c: New test.
---
 gcc/attribs.c                  |  1 +
 gcc/testsuite/gcc.dg/pr80548.c | 23 +++++++++++++++++++++++
 gcc/tree-ssa-threadupdate.c    | 19 +++++++++++--------
 3 files changed, 35 insertions(+), 8 deletions(-)
 create mode 100644 gcc/testsuite/gcc.dg/pr80548.c

diff --git a/gcc/attribs.c b/gcc/attribs.c
index c252f5af07b..9a079b8405a 100644
--- a/gcc/attribs.c
+++ b/gcc/attribs.c
@@ -1035,6 +1035,7 @@ sorted_attr_string (tree arglist)
       attr_str[str_len_sum + len] = TREE_CHAIN (arg) ? ',' : '\0';
       str_len_sum += len + 1;
     }
+  gcc_assert (arglist);
 
   /* Replace "=,-" with "_".  */
   for (i = 0; i < strlen (attr_str); i++)
diff --git a/gcc/testsuite/gcc.dg/pr80548.c b/gcc/testsuite/gcc.dg/pr80548.c
new file mode 100644
index 00000000000..2327111143e
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/pr80548.c
@@ -0,0 +1,23 @@
+// { dg-do compile }
+// { dg-options "-O2 -Wuninitialized" }
+
+int g (void);
+void h (int, int);
+
+void f (int b)
+{
+  int x, y;
+
+  if (b)
+    {
+      x = g ();
+      y = g ();
+    }
+
+  while (g ())
+    if (b)
+      {
+        h (x, y); // { dg-bogus "uninit" }
+        y = g ();
+      }
+}
diff --git a/gcc/tree-ssa-threadupdate.c b/gcc/tree-ssa-threadupdate.c
index 8aac733ac25..b194c11e23d 100644
--- a/gcc/tree-ssa-threadupdate.c
+++ b/gcc/tree-ssa-threadupdate.c
@@ -2410,13 +2410,14 @@ back_jt_path_registry::duplicate_thread_path (edge 
entry,
      missuses of the functions.  I.e. if you ask to copy something weird,
      it will work, but the state of structures probably will not be
      correct.  */
-  for (i = 0; i < n_region; i++)
-    {
-      /* We do not handle subloops, i.e. all the blocks must belong to the
-        same loop.  */
-      if (region[i]->loop_father != loop)
-       return false;
-    }
+  if (!(cfun->curr_properties & PROP_loop_opts_done))
+    for (i = 0; i < n_region; i++)
+      {
+       /* We do not handle subloops, i.e. all the blocks must belong to the
+          same loop.  */
+       if (region[i]->loop_father != loop)
+         return false;
+      }
 
   initialize_original_copy_tables ();
 
@@ -2651,9 +2652,11 @@ back_jt_path_registry::update_cfg (bool 
/*peel_loop_headers*/)
          visited_starting_edges.add (entry);
          retval = true;
          m_num_threaded_edges++;
+         path->release ();
        }
+      else
+       cancel_thread (path, "Failure in duplicate_thread_path");
 
-      path->release ();
       m_paths.unordered_remove (0);
       free (region);
     }
-- 
2.31.1