Loosen loop crossing restriction in threader.

Crossing loops is generally discouraged from the threader, but we can
make an exception when we don't cross the latch or enter another loop,
since this is just an early exit out of the loop.

In fact, the whole threaded path is logically outside the loop.  This
has nice secondary effects.  For example, objects on the threaded path
will no longer necessarily be live throughout the loop, so we can get
register allocation improvements.  The threaded path can physically
move outside the loop resulting in better icache efficiency, etc.

Tested on x86-64 Linux, and on a visium-elf cross making sure that the
following tests do not have an abort in the final assembly:

gcc.c-torture/execute/960218-1.c
gcc.c-torture/execute/visium-pending-4.c
gcc.c-torture/execute/pr58209.c

gcc/ChangeLog:

	* tree-ssa-threadupdate.c (jt_path_registry::cancel_invalid_paths):
	Loosen restrictions

gcc/testsuite/ChangeLog:

	* gcc.dg/tree-ssa/ssa-thread-valid.c: New test.

1 files changed, 29 insertions, 11 deletions

diff --git a/gcc/tree-ssa-threadupdate.c b/gcc/tree-ssa-threadupdate.c
index dcabfdb..32ce1e3 100644
--- a/gcc/tree-ssa-threadupdate.c
+++ b/gcc/tree-ssa-threadupdate.c
@@ -2766,10 +2766,17 @@ bool
 jt_path_registry::cancel_invalid_paths (vec<jump_thread_edge *> &path)
 {
   gcc_checking_assert (!path.is_empty ());
-  edge taken_edge = path[path.length () - 1]->e;
-  loop_p loop = taken_edge->src->loop_father;
+  edge entry = path[0]->e;
+  edge exit = path[path.length () - 1]->e;
   bool seen_latch = false;
-  bool path_crosses_loops = false;
+  int loops_crossed = 0;
+  bool crossed_latch = false;
+  // Use ->dest here instead of ->src to ignore the first block.  The
+  // first block is allowed to be in a different loop, since it'll be
+  // redirected.  See similar comment in profitable_path_p: "we don't
+  // care about that block...".
+  loop_p loop = entry->dest->loop_father;
+  loop_p curr_loop = loop;
 
   for (unsigned int i = 0; i < path.length (); i++)
     {
@@ -2784,19 +2791,30 @@ jt_path_registry::cancel_invalid_paths (vec<jump_thread_edge *> &path)
 	}
 
       if (loop->latch == e->src || loop->latch == e->dest)
-	seen_latch = true;
+	{
+	  seen_latch = true;
+	  // Like seen_latch, but excludes the first block.
+	  if (e->src != entry->src)
+	    crossed_latch = true;
+	}
 
-      // The first entry represents the block with an outgoing edge
-      // that we will redirect to the jump threading path.  Thus we
-      // don't care about that block's loop father.
-      if ((i > 0 && e->src->loop_father != loop)
-	  || e->dest->loop_father != loop)
-	path_crosses_loops = true;
+      if (e->dest->loop_father != curr_loop)
+	{
+	  curr_loop = e->dest->loop_father;
+	  ++loops_crossed;
+	}
 
       if (flag_checking && !m_backedge_threads)
 	gcc_assert ((path[i]->e->flags & EDGE_DFS_BACK) == 0);
     }
 
+  // If we crossed a loop into an outer loop without crossing the
+  // latch, this is just an early exit from the loop.
+  if (loops_crossed == 1
+      && !crossed_latch
+      && flow_loop_nested_p (exit->dest->loop_father, exit->src->loop_father))
+    return false;
+
   if (cfun->curr_properties & PROP_loop_opts_done)
     return false;
 
@@ -2806,7 +2824,7 @@ jt_path_registry::cancel_invalid_paths (vec<jump_thread_edge *> &path)
 		     "would create non-empty latch");
       return true;
     }
-  if (path_crosses_loops)
+  if (loops_crossed)
     {
       cancel_thread (&path, "Path crosses loops");
       return true;