[PATCH 7/9] ifcvt: Emit two cmov variants and choose the less expensive one.

[Robin Dapp]

This patch duplicates the previous noce_emit_cmove logic.  First it
passes the canonical comparison emits the sequence and costs it.
Then, a second, separate sequence is created by passing the cc compare
we extracted before.  The costs of both sequences are compared and the
cheaper one is emitted.
---
 gcc/ifcvt.c | 106 +++++++++++++++++++++++++++++++++++++++++++++++-----
 1 file changed, 97 insertions(+), 9 deletions(-)

diff --git a/gcc/ifcvt.c b/gcc/ifcvt.c
index 3db707e1fd1..955f9541f60 100644
--- a/gcc/ifcvt.c
+++ b/gcc/ifcvt.c
@@ -3208,8 +3208,11 @@ noce_convert_multiple_sets (struct noce_if_info *if_info)
 
   hash_map<rtx, bool> need_temps;
 
-  if (!cc_cmp)
-    check_need_temps (then_bb, &need_temps, cond);
+  /* If we newly set a CC before a cmov, we might need a temporary
+     even though the compare will be removed by a later pass.  Costing
+     of a sequence with these will be inaccurate.  */
+
+  check_need_temps (then_bb, &need_temps, cond);
 
   hash_map<rtx, bool> temps_created;
 
@@ -3298,18 +3301,103 @@ noce_convert_multiple_sets (struct noce_if_info *if_info)
 	  old_val = lowpart_subreg (dst_mode, old_val, src_mode);
 	}
 
-      /* Actually emit the conditional move.  */
-      rtx temp_dest = noce_emit_cmove (if_info, temp, cond_code,
-				       x, y, new_val, old_val, cc_cmp);
+      /* Try emitting a conditional move passing the backend the
+	 canonicalized comparison.  This can e.g. be used to recognize
+	 expressions like
+
+	   if (a < b)
+	   {
+	     ...
+	     b = a;
+	   }
+
+	 A backend can recognize this and emit a min/max insn.
+	 We will still emit a superfluous CC comparison before the
+	 min/max, though, which complicates costing.
+      */
+
+      rtx_insn *seq, *seq1 = 0, *seq2 = 0;
+      rtx temp_dest, temp_dest1, temp_dest2;
+      unsigned int cost1 = 0, cost2 = 0;
+      bool speed_p = if_info->speed_p;
+      push_topmost_sequence ();
+
+      {
+	start_sequence ();
+	temp_dest1 = noce_emit_cmove (if_info, temp, cond_code,
+	    x, y, new_val, old_val, NULL_RTX);
+
+	/* If we failed to expand the conditional move, drop out and don't
+	   try to continue.  */
+	if (temp_dest1 == NULL_RTX)
+	  {
+	    seq1 = NULL;
+	    cost1 = COSTS_N_INSNS (100);
+	  }
+	else
+	  {
+	    seq1 = get_insns ();
+	    cost1 = seq_cost (seq1, speed_p);
+	    rtx_insn *ii;
+	    for (ii = seq1; ii; ii = NEXT_INSN (ii))
+	      if (recog_memozied (ii) == -1)
+		cost1 += 10000;
+	  }
+	end_sequence ();
+      }
+
+	/* Now try emitting one passing a non-canonicalized cc comparison.
+	   This allows the backend to emit a cmov directly without additional
+	   compare.  */
+      {
+	start_sequence ();
+	temp_dest2 = noce_emit_cmove (if_info, target, cond_code,
+	    x, y, new_val, old_val, cc_cmp);
+
+	/* If we failed to expand the conditional move, drop out and don't
+	   try to continue.  */
+	if (temp_dest2 == NULL_RTX)
+	  {
+	    seq2 = NULL;
+	    cost2 = COSTS_N_INSNS (100);
+	  }
+	else
+	  {
+	    seq2 = get_insns ();
+	    cost2 = seq_cost (seq2, speed_p);
+	    rtx_insn *ii;
+	    for (ii = seq2; ii; ii = NEXT_INSN (ii))
+	      if (recog_memozied (ii) == -1)
+		cost2 += 10000;
+	  }
+	end_sequence ();
+      }
 
-      /* If we failed to expand the conditional move, drop out and don't
-	 try to continue.  */
-      if (temp_dest == NULL_RTX)
+      /* Check which version is less expensive.  */
+      if (cost1 <= cost2 && seq1 != NULL_RTX)
+	{
+	  temp_dest = temp_dest1;
+	  seq = seq1;
+	}
+      else if (seq2 != NULL_RTX)
+	{
+	  /* We do not require a temp register, so remove it from the list.  */
+	  seq = seq2;
+	  temp_dest = temp_dest2;
+	  temps_created.remove (temp);
+	}
+      else
 	{
+	  /* Nothing worked, bail out.  */
+	  pop_topmost_sequence ();
 	  end_sequence ();
-	  return FALSE;
+	  return false;
 	}
 
+      /* End the sub sequence and emit to the main sequence.  */
+      pop_topmost_sequence ();
+      emit_insn (seq);
+
       /* Bookkeeping.  */
       count++;
       targets.safe_push (target);
-- 
2.17.0