Re: [PATCH 3/7] ifcvt: Improve costs handling for noce_convert_multiple.

[Robin Dapp <rdapp@linux.ibm.com>]

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> It looks like this is an existing (potential) problem,
> but default_noce_conversion_profitable_p uses seq_cost, which in turn
> uses insn_cost.  And insn_cost has an optional target hook behind it,
> which allows for costing based on insn attributes etc.  For a true
> apples-with-apples comparison we should use insn_cost here too.

Good point, fixed that.

> I think the detail that COSTS_N_INSNS (2) is the default is useful here.
> (In other words, I'd forgotten by the time I'd poked around other bits of
> ifcvt and was about to ask why we didn't cost the condition “properly”.)
> So how about something like:
> 
>     The original costs already include a base cost of COSTS_N_INSNS (2):
>     one instruction for the compare (which we will be needing either way)
>     and one instruction for the branch.

Yes, this is much clearer. I went with that wording in the attached v2.

Regards
  Robin

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From 54508fa00fbee082fa62f4e1a8167f477938e781 Mon Sep 17 00:00:00 2001
From: Robin Dapp <rdapp@linux.ibm.com>
Date: Wed, 27 Nov 2019 13:46:17 +0100
Subject: [PATCH v2 3/7] ifcvt: Improve costs handling for
 noce_convert_multiple.

When noce_convert_multiple is called the original costs are not yet
initialized.  Therefore, up to now, costs were only ever unfairly
compared against COSTS_N_INSNS (2).  This would lead to
default_noce_conversion_profitable_p () rejecting all but the most
contrived of sequences.

This patch temporarily initializes the original costs by counting
adding costs for all sets inside the then_bb.
---
 gcc/ifcvt.c | 31 +++++++++++++++++++++++++++----
 1 file changed, 27 insertions(+), 4 deletions(-)

diff --git a/gcc/ifcvt.c b/gcc/ifcvt.c
index f9d4478ec18..91b54dbea9c 100644
--- a/gcc/ifcvt.c
+++ b/gcc/ifcvt.c
@@ -3404,14 +3404,17 @@ noce_convert_multiple_sets (struct noce_if_info *if_info)
    (SET (REG) (REG)) insns suitable for conversion to a series
    of conditional moves.  Also check that we have more than one set
    (other routines can handle a single set better than we would), and
-   fewer than PARAM_MAX_RTL_IF_CONVERSION_INSNS sets.  */
+   fewer than PARAM_MAX_RTL_IF_CONVERSION_INSNS sets.  While going
+   through the insns store the sum of their potential costs in COST.  */
 
 static bool
-bb_ok_for_noce_convert_multiple_sets (basic_block test_bb)
+bb_ok_for_noce_convert_multiple_sets (basic_block test_bb, unsigned *cost)
 {
   rtx_insn *insn;
   unsigned count = 0;
   unsigned param = param_max_rtl_if_conversion_insns;
+  bool speed_p = optimize_bb_for_speed_p (test_bb);
+  unsigned potential_cost = 0;
 
   FOR_BB_INSNS (test_bb, insn)
     {
@@ -3447,9 +3450,13 @@ bb_ok_for_noce_convert_multiple_sets (basic_block test_bb)
       if (!can_conditionally_move_p (GET_MODE (dest)))
 	return false;
 
+      potential_cost += insn_cost (insn, speed_p);
+
       count++;
     }
 
+  *cost += potential_cost;
+
   /* If we would only put out one conditional move, the other strategies
      this pass tries are better optimized and will be more appropriate.
      Some targets want to strictly limit the number of conditional moves
@@ -3497,11 +3504,24 @@ noce_process_if_block (struct noce_if_info *if_info)
      to calculate a value for x.
      ??? For future expansion, further expand the "multiple X" rules.  */
 
-  /* First look for multiple SETS.  */
+  /* First look for multiple SETS.  The original costs already include
+     a base cost of COSTS_N_INSNS (2): one instruction for the compare
+     (which we will be needing either way) and one instruction for the
+     branch.  When comparing costs we want to use the branch instruction
+     cost and the sets vs. the cmovs generated here.  Therefore subtract
+     the costs of the compare before checking.
+     ??? Actually, instead of the branch instruction costs we might want
+     to use COSTS_N_INSNS (BRANCH_COST ()) as in other places.  */
+
+  unsigned potential_cost = if_info->original_cost - COSTS_N_INSNS (1);
+  unsigned old_cost = if_info->original_cost;
   if (!else_bb
       && HAVE_conditional_move
-      && bb_ok_for_noce_convert_multiple_sets (then_bb))
+      && bb_ok_for_noce_convert_multiple_sets (then_bb, &potential_cost))
     {
+      /* Temporarily set the original costs to what we estimated so
+	 we can determine if the transformation is worth it.  */
+      if_info->original_cost = potential_cost;
       if (noce_convert_multiple_sets (if_info))
 	{
 	  if (dump_file && if_info->transform_name)
@@ -3509,6 +3529,9 @@ noce_process_if_block (struct noce_if_info *if_info)
 		     if_info->transform_name);
 	  return TRUE;
 	}
+
+      /* Restore the original costs.  */
+      if_info->original_cost = old_cost;
     }
 
   bool speed_p = optimize_bb_for_speed_p (test_bb);
-- 
2.31.1