[gcc(refs/users/aoliva/heads/testme)] detect infinite loops earlier in returning_call_p

[Alexandre Oliva]

https://gcc.gnu.org/g:6269d7a415f9fad7a49f03c8b4a2387d21a9ccc9

commit 6269d7a415f9fad7a49f03c8b4a2387d21a9ccc9
Author: Alexandre Oliva <oliva@adacore.com>
Date:   Thu Jun 8 05:55:04 2023 -0300

    detect infinite loops earlier in returning_call_p
    
    An infinite loop could create a path as long as the block count in
    returning_call_p, and then fail the backwards check if a call is found
    before emptying the path.
    
    Return as soon as the path exceeds the block count, and search for
    duplicate blocks before allocating more memory for the path, so as to
    cut the looping short.
    
    
    for  gcc/ChangeLog
    
            * gimple-harden-control-flow.cc (returning_call_p): Detect
            infinite loops sooner.

Diff:
---
 gcc/gimple-harden-control-flow.cc | 16 +++++++++++-----
 1 file changed, 11 insertions(+), 5 deletions(-)

diff --git a/gcc/gimple-harden-control-flow.cc b/gcc/gimple-harden-control-flow.cc
index 3998fd0d293..044671cab79 100644
--- a/gcc/gimple-harden-control-flow.cc
+++ b/gcc/gimple-harden-control-flow.cc
@@ -19,6 +19,7 @@ along with GCC; see the file COPYING3.  If not see
 <http://www.gnu.org/licenses/>.  */
 
 #include "config.h"
+#define INCLUDE_ALGORITHM /* find */
 #include "system.h"
 #include "coretypes.h"
 #include "backend.h"
@@ -211,14 +212,19 @@ returning_call_p (gcall *call)
     return false;
 
   /* Quickly check that there's a path to exit compatible with a
-     returning call.  Detect infinite loops through the counter.  */
-  basic_block bb = gimple_bb (call);
+     returning call.  Detect infinite loops by limiting the path
+     length to the basic block count, and by looking for duplicate
+     blocks before allocating more memory for the path, for amortized
+     O(n).  */
   auto_vec<basic_block, 10> path;
-  for (int i = n_basic_blocks_for_fn (cfun);
-       bb != EXIT_BLOCK_PTR_FOR_FN (cfun) && i--;
+  for (basic_block bb = gimple_bb (call);
+       bb != EXIT_BLOCK_PTR_FOR_FN (cfun);
        bb = single_succ (bb))
     if (!single_succ_p (bb)
-	|| (single_succ_edge (bb)->flags & EDGE_EH) != 0)
+	|| (single_succ_edge (bb)->flags & EDGE_EH) != 0
+	|| n_basic_blocks_for_fn (cfun) - path.length () <= NUM_FIXED_BLOCKS
+	|| (path.length () == path.allocated ()
+	    && std::find (path.begin (), path.end (), bb) != path.end ()))
       return false;
     else
       path.safe_push (bb);