[rtlanal] Do a better job of costing parallel sets containing flag-setting operations.

[Richard Earnshaw (lists)]

Many parallel set insns are of the form of a single set that also sets
the condition code flags.  In this case the cost of such an insn is
normally the cost of the part that doesn't set the flags, since updating
the condition flags is simply a side effect.

At present all such insns are treated as having unknown cost (ie 0) and
combine assumes that such insns are infinitely more expensive than any
other insn sequence with a non-zero cost.

This patch addresses this problem by allowing insn_rtx_cost to ignore
the condition setting part of a PARALLEL iff there is exactly one
comparison set and one non-comparison set.  If the only set operation is
a comparison we still use that as the basis of the insn cost.

        * rtlanal.c (insn_rtx_cost): If a parallel contains exactly one
        comparison set and one other set, use the cost of the
        non-comparison set.

Bootstrapped on aarch64-none-linuxgnu

OK?

R.

diff --git a/gcc/rtlanal.c b/gcc/rtlanal.c
index d9f57c3..5cae793 100644
--- a/gcc/rtlanal.c
+++ b/gcc/rtlanal.c
@@ -5260,23 +5260,41 @@ insn_rtx_cost (rtx pat, bool speed)
   int i, cost;
   rtx set;
 
-  /* Extract the single set rtx from the instruction pattern.
-     We can't use single_set since we only have the pattern.  */
+  /* Extract the single set rtx from the instruction pattern.  We
+     can't use single_set since we only have the pattern.  We also
+     consider PARALLELs of a normal set and and a single comparison.
+     In that case we use the cost of the non-comparison SET operation,
+     which is most-likely to be the real cost of this operation.  */
   if (GET_CODE (pat) == SET)
     set = pat;
   else if (GET_CODE (pat) == PARALLEL)
     {
       set = NULL_RTX;
+      rtx comparison = NULL_RTX;
+
       for (i = 0; i < XVECLEN (pat, 0); i++)
 	{
 	  rtx x = XVECEXP (pat, 0, i);
 	  if (GET_CODE (x) == SET)
 	    {
-	      if (set)
-		return 0;
-	      set = x;
+	      if (GET_CODE (SET_SRC (x)) == COMPARE)
+		{
+		  if (comparison)
+		    return 0;
+		  comparison = x;
+		}
+	      else
+		{
+		  if (set)
+		    return 0;
+		  set = x;
+		}
 	    }
 	}
+
+      if (!set && comparison)
+	set = comparison;
+
       if (!set)
 	return 0;
     }