Re: [PATCH 1/7] ifcvt: Check if cmovs are needed.

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

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Hi Richard,

thanks for going through the patch set.

> A hash_set might be simpler, given that the code only enters insns
> for which the bool is false.  “rtx_insn *” would be better than rtx.

Right, changed that.

> Do you mean the sets might be removed or that the checks might be
> removed?

It's actually the checks that are removed.  I clarified and amended the 
comments.

> The patch is quite hard to review on its own, since nothing actually
> uses this variable.  It's also not obvious how the
> reg_overlap_mentioned_p code works if the old target is referenced
> later.
> 
> Could you refactor the series a bit so that each patch is
> self-contained? It's OK if that means fewer patches.
The attached v2 makes use of need_cmov now, I hope this makes it a bit 
clearer.  Regarding the reg_overlap_mentioned_p: it is used to detect 
the swap idioms as well as when a cmov destination is used in the 
condition as well.  If needed this could be two separate patches (most 
of the second patch would be comments, though).

Regards
  Robin

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From bf7e372d7f48e614f20676c7cf4b2fbde741d4fc Mon Sep 17 00:00:00 2001
From: Robin Dapp <rdapp@linux.ibm.com>
Date: Thu, 24 Jun 2021 16:40:04 +0200
Subject: [PATCH v2 1/7] ifcvt: Check if cmovs are needed.

When if-converting multiple SETs and we encounter a swap-style idiom

  if (a > b)
    {
      tmp = c;   // [1]
      c = d;
      d = tmp;
    }

ifcvt should not generate a conditional move for the instruction at
[1].

In order to achieve that, this patch goes through all relevant SETs
and marks the relevant instructions.  This helps to evaluate costs.

On top, only generate temporaries if the current cmov is going to
overwrite one of the comparands of the initial compare.
---
 gcc/ifcvt.c | 138 ++++++++++++++++++++++++++++++++++++++++++++--------
 1 file changed, 118 insertions(+), 20 deletions(-)

diff --git a/gcc/ifcvt.c b/gcc/ifcvt.c
index 017944f4f79..a5e55456d88 100644
--- a/gcc/ifcvt.c
+++ b/gcc/ifcvt.c
@@ -98,6 +98,7 @@ static int dead_or_predicable (basic_block, basic_block, basic_block,
 			       edge, int);
 static void noce_emit_move_insn (rtx, rtx);
 static rtx_insn *block_has_only_trap (basic_block);
+static void check_need_cmovs (basic_block, hash_set<rtx_insn *> *);
 \f
 /* Count the number of non-jump active insns in BB.  */
 
@@ -3203,6 +3204,10 @@ noce_convert_multiple_sets (struct noce_if_info *if_info)
   auto_vec<rtx_insn *> unmodified_insns;
   int count = 0;
 
+  hash_set<rtx_insn *> need_no_cmov;
+
+  check_need_cmovs (then_bb, &need_no_cmov);
+
   FOR_BB_INSNS (then_bb, insn)
     {
       /* Skip over non-insns.  */
@@ -3213,26 +3218,47 @@ noce_convert_multiple_sets (struct noce_if_info *if_info)
       gcc_checking_assert (set);
 
       rtx target = SET_DEST (set);
-      rtx temp = gen_reg_rtx (GET_MODE (target));
+      rtx temp;
       rtx new_val = SET_SRC (set);
       rtx old_val = target;
 
-      /* If we were supposed to read from an earlier write in this block,
-	 we've changed the register allocation.  Rewire the read.  While
-	 we are looking, also try to catch a swap idiom.  */
-      for (int i = count - 1; i >= 0; --i)
-	if (reg_overlap_mentioned_p (new_val, targets[i]))
-	  {
-	    /* Catch a "swap" style idiom.  */
-	    if (find_reg_note (insn, REG_DEAD, new_val) != NULL_RTX)
-	      /* The write to targets[i] is only live until the read
-		 here.  As the condition codes match, we can propagate
-		 the set to here.  */
-	      new_val = SET_SRC (single_set (unmodified_insns[i]));
-	    else
-	      new_val = temporaries[i];
-	    break;
-	  }
+      /* As we are transforming
+	 if (x > y)
+	   {
+	     a = b;
+	     c = d;
+	   }
+	 into
+	   a = (x > y) ...
+	   c = (x > y) ...
+
+	 we potentially check x > y before every set.
+	 Even though the check might be removed by subsequent passes, this means
+	 that we cannot transform
+	   if (x > y)
+	     {
+	       x = y;
+	       ...
+	     }
+	 into
+	   x = (x > y) ...
+	   ...
+	 since this would invalidate x and the following to-be-removed checks.
+	 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))
+	temp = gen_reg_rtx (GET_MODE (target));
+      else
+	temp = target;
+
+      /* We have identified swap-style idioms in check_need_cmovs.  A normal
+	 set will need to be a cmov while the first instruction of a swap-style
+	 idiom can be a regular move.  This helps with costing.  */
+      bool need_cmov = true;
+      if (need_no_cmov.contains (insn))
+	need_cmov = false;
 
       /* If we had a non-canonical conditional jump (i.e. one where
 	 the fallthrough is to the "else" case) we need to reverse
@@ -3275,9 +3301,22 @@ 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);
+      rtx temp_dest = NULL_RTX;
+
+      if (need_cmov)
+	{
+	  /* Actually emit the conditional move.  */
+	  temp_dest = noce_emit_cmove (if_info, temp, cond_code,
+					   x, y, new_val, old_val);
+	}
+      else
+	{
+	  if (if_info->then_else_reverse)
+	    noce_emit_move_insn (temp, old_val);
+	  else
+	    noce_emit_move_insn (temp, new_val);
+	  temp_dest = temp;
+	}
 
       /* If we failed to expand the conditional move, drop out and don't
 	 try to continue.  */
@@ -3808,6 +3847,65 @@ check_cond_move_block (basic_block bb,
   return TRUE;
 }
 
+/* Find local swap-style idioms in BB and mark the first insn (1)
+   that is only a temporary as not needing a conditional move as
+   it is going to be dead afterwards anyway.
+
+     (1) int tmp = a;
+	 a = b;
+	 b = tmp;
+
+	 ifcvt
+	 -->
+
+	 load tmp,a
+	 cmov a,b
+	 cmov b,tmp   */
+
+static void
+check_need_cmovs (basic_block bb, hash_set<rtx_insn *> *need_no_cmov)
+{
+  rtx_insn *insn;
+  int count = 0;
+  auto_vec<rtx_insn *> insns;
+  auto_vec<rtx> dests;
+
+  /* Iterate over all SETs, storing the destinations
+     in DEST.  If we hit a SET that reads from a destination
+     that we have seen before and the corresponding register
+     is dead afterwards, it must be a swap.  */
+  FOR_BB_INSNS (bb, insn)
+    {
+      rtx set, src, dest;
+
+      if (!active_insn_p (insn))
+	continue;
+
+      set = single_set (insn);
+      if (set == NULL_RTX)
+	continue;
+
+      src = SET_SRC (set);
+      dest = SET_DEST (set);
+
+      /* Check if the current SET's source is the same
+	 as any previously seen destination.
+	 This is quadratic but the number of insns in BB
+	 is bounded by PARAM_MAX_RTL_IF_CONVERSION_INSNS.  */
+      for (int i = count - 1; i >= 0; --i)
+	{
+	  if (reg_overlap_mentioned_p (src, dests[i])
+	      && find_reg_note (insn, REG_DEAD, src) != NULL_RTX)
+	    need_no_cmov->add (insns[i]);
+	}
+
+      insns.safe_push (insn);
+      dests.safe_push (dest);
+
+      count++;
+    }
+}
+
 /* Given a basic block BB suitable for conditional move conversion,
    a condition COND, and pointer maps THEN_VALS and ELSE_VALS containing
    the register values depending on COND, emit the insns in the block as
-- 
2.31.1