[PATCH] tree-optimization/105679 - disable backward threading of unlikely entry

[Richard Biener <rguenther@suse.de>]

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%]

Bootstrap and regtest running on x86_64-unknown-linux-gnu.

The testcase in the PR requries both ubsan and sancov so I'm not sure
where to put it but IIRC there were quite some duplicate PRs wrt
threading unlikely paths exposing diagnostics, eventually some
testcase will come out of those (when we identify them).  Note
the patch is quite conservative in only disabling likely never
executed paths rather than requiring maybe_hot_edge_p (OTOH those
are somewhat similar in the end).

I'm going to push it when testing finishes but maybe there are some
testcases to adjust.

	PR tree-optimization/105679
	* tree-ssa-threadbackwards.cc
	(back_threader_profitability::profitable_path_p): Avoid threading
	when the entry edge is probably never executed.
---
 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)
     {
-- 
2.35.3