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 --git a/gcc/tree-ssa-threadbackward.cc b/gcc/tree-ssa-threadbackward.cc
index 3519aca84cd4ec412048b748f4b8bd33020026ab..90f5331c2657ce0b09a517226cc7b5e8a5bdfc84 100644 (file)
--- 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)
     {