rs6000: Improve comparison rtx_cost (PR81288)

The current rs6000 rtx_cost for comparisons against 0 is very high if
TARGET_ISEL && !TARGET_MFCRF, much higher than for reg-reg comparisons,
much higher than a load of 0 and such a reg-reg-comparison.  This leads
to infinite recursion in CSE (see PR81288).

This patch removes the too-high cost, also simplifying this code.


	PR 81288/target
	* config/rs6000/rs6000.c (rs6000_rtx_costs): Do not handle
	TARGET_ISEL && !TARGET_MFCRF differently.  Simplify code.


git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@255188 138bc75d-0d04-0410-961f-82ee72b054a4

Author: segher

gcc/ChangeLog

@@ -1,3 +1,9 @@
+2017-11-28  Segher Boessenkool  <segher@kernel.crashing.org>
+
+	PR 81288/target
+	* config/rs6000/rs6000.c (rs6000_rtx_costs): Do not handle
+	TARGET_ISEL && !TARGET_MFCRF differently.  Simplify code.
+
 2017-11-27  Segher Boessenkool  <segher@kernel.crashing.org>
 
 	* config/rs6000/rs6000.md (<code><GPR:mode><GPR2:mode>2_isel): Change

gcc/config/rs6000/rs6000.c

@@ -34889,14 +34889,16 @@ rs6000_rtx_costs (rtx x, machine_mode mode, int outer_code,
 	  *total = COSTS_N_INSNS (1);
 	  return true;
 	}
+      /* FALLTHRU */
+
+    case GT:
+    case LT:
+    case UNORDERED:
       if (outer_code == SET)
 	{
 	  if (XEXP (x, 1) == const0_rtx)
 	    {
-	      if (TARGET_ISEL && !TARGET_MFCRF)
-		*total = COSTS_N_INSNS (8);
-	      else
-		*total = COSTS_N_INSNS (2);
+	      *total = COSTS_N_INSNS (2);
 	      return true;
 	    }
 	  else
@@ -34905,19 +34907,6 @@ rs6000_rtx_costs (rtx x, machine_mode mode, int outer_code,
 	      return false;
 	    }
 	}
-      /* FALLTHRU */
-
-    case GT:
-    case LT:
-    case UNORDERED:
-      if (outer_code == SET && (XEXP (x, 1) == const0_rtx))
-	{
-	  if (TARGET_ISEL && !TARGET_MFCRF)
-	    *total = COSTS_N_INSNS (8);
-	  else
-	    *total = COSTS_N_INSNS (2);
-	  return true;
-	}
       /* CC COMPARE.  */
       if (outer_code == COMPARE)
 	{