[gcc r13-1891] tree-optimization/105679 - disable backward threading of unlikely entry

[Richard Biener]

https://gcc.gnu.org/g:49ba4fdeb648c149fa7d964ba812084262c3d06f

commit r13-1891-g49ba4fdeb648c149fa7d964ba812084262c3d06f
Author: Richard Biener <rguenther@suse.de>
Date:   Fri Jul 29 10:40:34 2022 +0200

    tree-optimization/105679 - disable backward threading of unlikely entry
    
    The following makes the backward threader reject threads whose entry
    edge is probably never executed according to the profile.  That in
    particular, for the testcase, avoids threading the irq == 1 check
    on the path where irq > 31, thereby avoiding spurious -Warray-bounds
    diagnostics
    
      if (irq_1(D) > 31)
        goto <bb 3>; [0.00%]
      else
        goto <bb 4>; [100.00%]
    
    ;;   basic block 3, loop depth 0, count 0 (precise), probably never executed
      _2 = (unsigned long) irq_1(D);
      __builtin___ubsan_handle_shift_out_of_bounds (&*.Lubsan_data0, 1, _2);
    
      _3 = 1 << irq_1(D);
      mask_4 = (u32) _3;
      entry = instance_5(D)->array[irq_1(D)];
      capture (mask_4);
      if (level_6(D) != 0)
        goto <bb 7>; [34.00%]
      else
        goto <bb 5>; [66.00%]
    
    ;;   basic block 5, loop depth 0, count 708669600 (estimated locally), maybe hot  if (irq_1(D) == 1)
        goto <bb 7>; [20.97%]
      else
        goto <bb 6>; [79.03%]
    
            PR tree-optimization/105679
            * tree-ssa-threadbackward.cc
            (back_threader_profitability::profitable_path_p): Avoid threading
            when the entry edge is probably never executed.

Diff:
---
 gcc/tree-ssa-threadbackward.cc | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/gcc/tree-ssa-threadbackward.cc b/gcc/tree-ssa-threadbackward.cc
index 3519aca84cd..90f5331c265 100644
--- a/gcc/tree-ssa-threadbackward.cc
+++ b/gcc/tree-ssa-threadbackward.cc
@@ -777,6 +777,15 @@ back_threader_profitability::profitable_path_p (const vec<basic_block> &m_path,
 		     "exceeds PARAM_MAX_FSM_THREAD_PATH_INSNS.\n");
 	  return false;
 	}
+      edge entry = find_edge (m_path[m_path.length () - 1],
+			      m_path[m_path.length () - 2]);
+      if (probably_never_executed_edge_p (cfun, entry))
+	{
+	  if (dump_file && (dump_flags & TDF_DETAILS))
+	    fprintf (dump_file, "  FAIL: Jump-thread path not considered: "
+		     "path entry is probably never executed.\n");
+	  return false;
+	}
     }
   else if (!m_speed_p && n_insns > 1)
     {