commit dd5a0f8d7d39447025d36ed5305709d38fe3f16b
Author: Robin Dapp <rdapp@linux.ibm.com>
Date:   Fri Sep 17 20:22:10 2021 +0200

    ifcvt: Run second pass if it is possible to omit a temporary.
    
    If one of the to-be-converted SETs requires the original comparison
    (i.e. in order to generate a min/max insn) but no other insn after it
    does, we can omit creating temporaries, thus facilitating costing.

diff --git a/gcc/ifcvt.c b/gcc/ifcvt.c
index 4f3af5e1b00..2243157e32c 100644
--- a/gcc/ifcvt.c
+++ b/gcc/ifcvt.c
@@ -3261,6 +3261,11 @@ noce_convert_multiple_sets (struct noce_if_info *if_info)
 
   need_cmov_or_rewire (then_bb, &need_no_cmov, &rewired_src);
 
+  int last_needs_comparison = -1;
+  bool second_try = false;
+
+restart:
+
   FOR_BB_INSNS (then_bb, insn)
     {
       /* Skip over non-insns.  */
@@ -3302,8 +3307,12 @@ noce_convert_multiple_sets (struct noce_if_info *if_info)
 	 Therefore we introduce a temporary every time we are about to
 	 overwrite a variable used in the check.  Costing of a sequence with
 	 these is going to be inaccurate so only use temporaries when
-	 needed.  */
-      if (reg_overlap_mentioned_p (target, cond))
+	 needed.
+
+	 If performing a second try, we know how many insns require a
+	 temporary.  For the last of these, we can omit creating one.  */
+      if (reg_overlap_mentioned_p (target, cond)
+	  && (!second_try || count < last_needs_comparison))
 	temp = gen_reg_rtx (GET_MODE (target));
       else
 	temp = target;
@@ -3386,6 +3395,8 @@ noce_convert_multiple_sets (struct noce_if_info *if_info)
 	{
 	  seq = seq1;
 	  temp_dest = temp_dest1;
+	  if (!second_try)
+	    last_needs_comparison = count;
 	}
       else if (seq2 != NULL_RTX)
 	{
@@ -3409,6 +3420,24 @@ noce_convert_multiple_sets (struct noce_if_info *if_info)
       unmodified_insns.safe_push (insn);
     }
 
+    /* If there are insns that overwrite part of the initial
+       comparison, we can still omit creating temporaries for
+       the last of them.
+       As the second try will always create a less expensive,
+       valid sequence, we do not need to compare and can discard
+       the first one.  */
+    if (!second_try && last_needs_comparison >= 0)
+      {
+	end_sequence ();
+	start_sequence ();
+	count = 0;
+	targets.truncate (0);
+	temporaries.truncate (0);
+	unmodified_insns.truncate (0);
+	second_try = true;
+	goto restart;
+      }
+
   /* We must have seen some sort of insn to insert, otherwise we were
      given an empty BB to convert, and we can't handle that.  */
   gcc_assert (!unmodified_insns.is_empty ());