cse.c: preferable()

[Dimitrios Apostolou <jimis@gmx.net>]

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Attached patch is also posted at bug #19832 and I think resolves it, as 
well as /maybe/ offers a negligible speedup of 3-4 M instr or a couple 
milliseconds. I also post it here for comments.


2011-08-13  Dimitrios Apostolou  <jimis@gmx.net>

     * cse.c (preferable): Make it more readable and slightly faster,
     without affecting its logic.


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=== modified file 'gcc/cse.c'
--- gcc/cse.c	2011-06-02 21:52:46 +0000
+++ gcc/cse.c	2011-08-13 00:54:06 +0000
@@ -720,32 +720,25 @@ approx_reg_cost (rtx x)
 static int
 preferable (int cost_a, int regcost_a, int cost_b, int regcost_b)
 {
-  /* First, get rid of cases involving expressions that are entirely
-     unwanted.  */
-  if (cost_a != cost_b)
-    {
-      if (cost_a == MAX_COST)
-	return 1;
-      if (cost_b == MAX_COST)
-	return -1;
-    }
+  int cost_diff = cost_a - cost_b;
+  int regcost_diff = regcost_a - regcost_b;
 
-  /* Avoid extending lifetimes of hardregs.  */
-  if (regcost_a != regcost_b)
+  if (cost_diff != 0)
     {
-      if (regcost_a == MAX_COST)
-	return 1;
-      if (regcost_b == MAX_COST)
-	return -1;
+      /* If none of the expressions are entirely unwanted */
+      if ((cost_a != MAX_COST) && (cost_b != MAX_COST)
+	  /* AND only one of the regs is HARD_REG */
+	  && (regcost_diff != 0)
+	  && ((regcost_a == MAX_COST) || (regcost_b == MAX_COST))
+	  )
+	/* Then avoid extending lifetime of HARD_REG */
+	return regcost_diff;
+
+      return cost_diff;
     }
 
-  /* Normal operation costs take precedence.  */
-  if (cost_a != cost_b)
-    return cost_a - cost_b;
-  /* Only if these are identical consider effects on register pressure.  */
-  if (regcost_a != regcost_b)
-    return regcost_a - regcost_b;
-  return 0;
+  /* cost_a == costb, consider effects on register pressure */
+  return regcost_diff;
 }
 
 /* Internal function, to compute cost when X is not a register; called