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[RFC:,2/6] Factor out the comparisons against magic numbers in ifcvt

Message ID 1464886438-17892-3-git-send-email-james.greenhalgh@arm.com
State Superseded
Headers show

Commit Message

James Greenhalgh June 2, 2016, 4:53 p.m. UTC 
Hi,

This patch pulls the comparisons between if_info->branch_cost and a magic
number representing an instruction count to a common function. While I'm
doing it, I've documented the instructions that the magic numbers relate
to, and updated them where they were inconsistent.

If our measure of the cost of a branch is now in rtx costs units, we can
get to an estimate for the cost of an expression from the number of
instructions by multiplying through by COSTS_N_INSNS (1).

This isn't the great revolution in ifcvt costs I hoped for when I sat down
to take on the work. But, it looks like the best I can do short of
constructing ADD and SUB rtx all over the place just to get back a
judgement from the target that they are "cheap". The nicest thing about
doing it this way is that it mostly preserves behaviour, and by factoring
it out we have an upgrade path to a more detailed cost model should we want
it.

OK?

Thanks,
James

---

2016-06-02  James Greenhalgh  <james.greenhalgh@arm.com>

	* ifcvt.c (noce_if_info): New field: rtx_edge_cost.
	(noce_estimate_conversion_profitable_p): New.
	(noce_try_store_flag_constants): Use it.
	(noce_try_addcc): Likewise.
	(noce_try_store_flag_mask): Likewise.
	(noce_try_cmove): Likewise.
	(noce_try_cmove_arith): Likewise.
	(noce_find_if_block): Record targetm.rtx_edge_cost.
diff mbox

Patch

diff --git a/gcc/ifcvt.c b/gcc/ifcvt.c
index 44ae020..22cb5e7 100644
--- a/gcc/ifcvt.c
+++ b/gcc/ifcvt.c
@@ -813,6 +813,7 @@  struct noce_if_info
 
   /* Estimated cost of the particular branch instruction.  */
   unsigned int branch_cost;
+  unsigned int rtx_edge_cost;
 };
 
 static rtx noce_emit_store_flag (struct noce_if_info *, rtx, int, int);
@@ -830,6 +831,17 @@  static int noce_try_minmax (struct noce_if_info *);
 static int noce_try_abs (struct noce_if_info *);
 static int noce_try_sign_mask (struct noce_if_info *);
 
+/* This function is always called when we would expand a number of "cheap"
+   instructions.  Multiply NINSNS by COSTS_N_INSNS (1) to approximate the
+   RTX cost of those cheap instructions.  */
+
+inline static bool
+noce_estimate_conversion_profitable_p (struct noce_if_info *if_info,
+				       unsigned int ninsns)
+{
+  return (if_info->rtx_edge_cost >= ninsns * COSTS_N_INSNS (1));
+}
+
 /* Helper function for noce_try_store_flag*.  */
 
 static rtx
@@ -1279,7 +1291,8 @@  noce_try_store_flag_constants (struct noce_if_info *if_info)
       && (REG_P (XEXP (a, 0))
 	  || (noce_operand_ok (XEXP (a, 0))
 	      && ! reg_overlap_mentioned_p (if_info->x, XEXP (a, 0))))
-      && if_info->branch_cost >= 2)
+      /* We need one instruction, the ADD of the store flag.  */
+      && noce_estimate_conversion_profitable_p (if_info, 1))
     {
       common = XEXP (a, 0);
       a = XEXP (a, 1);
@@ -1352,22 +1365,32 @@  noce_try_store_flag_constants (struct noce_if_info *if_info)
 	  else
 	    gcc_unreachable ();
 	}
+      /* Is this (cond) ? 2^n : 0?  */
       else if (ifalse == 0 && exact_log2 (itrue) >= 0
 	       && (STORE_FLAG_VALUE == 1
-		   || if_info->branch_cost >= 2))
+		   /* We need ASHIFT, IOR.   */
+		   || noce_estimate_conversion_profitable_p (if_info, 2)))
 	normalize = 1;
+      /* Is this (cond) ? 0 : 2^n?  */
       else if (itrue == 0 && exact_log2 (ifalse) >= 0 && can_reverse
-	       && (STORE_FLAG_VALUE == 1 || if_info->branch_cost >= 2))
+	       && (STORE_FLAG_VALUE == 1
+		   /* We need ASHIFT, IOR.  */
+		   || noce_estimate_conversion_profitable_p (if_info, 2)))
 	{
 	  normalize = 1;
 	  reversep = true;
 	}
+      /* Is this (cond) ? -1 : x?  */
       else if (itrue == -1
 	       && (STORE_FLAG_VALUE == -1
-		   || if_info->branch_cost >= 2))
+		   /* Just an IOR.  */
+		   || noce_estimate_conversion_profitable_p (if_info, 1)))
 	normalize = -1;
+      /* Is this (cond) ? x : -1?  */
       else if (ifalse == -1 && can_reverse
-	       && (STORE_FLAG_VALUE == -1 || if_info->branch_cost >= 2))
+	       && (STORE_FLAG_VALUE == -1
+		   /* Just an IOR.  */
+		   || noce_estimate_conversion_profitable_p (if_info, 1)))
 	{
 	  normalize = -1;
 	  reversep = true;
@@ -1519,8 +1542,8 @@  noce_try_addcc (struct noce_if_info *if_info)
 	}
 
       /* If that fails, construct conditional increment or decrement using
-	 setcc.  */
-      if (if_info->branch_cost >= 2
+	 setcc.  We'd only need an ADD/SUB for this.  */
+      if (noce_estimate_conversion_profitable_p (if_info, 1)
 	  && (XEXP (if_info->a, 1) == const1_rtx
 	      || XEXP (if_info->a, 1) == constm1_rtx))
         {
@@ -1575,7 +1598,9 @@  noce_try_store_flag_mask (struct noce_if_info *if_info)
     return FALSE;
 
   reversep = 0;
-  if ((if_info->branch_cost >= 2
+
+  /* Two insns, AND, NEG.  */
+  if ((noce_estimate_conversion_profitable_p (if_info, 2)
        || STORE_FLAG_VALUE == -1)
       && ((if_info->a == const0_rtx
 	   && rtx_equal_p (if_info->b, if_info->x))
@@ -1778,8 +1803,11 @@  noce_try_cmove (struct noce_if_info *if_info)
 	 approach.  */
       else if (!targetm.have_conditional_execution ()
 		&& CONST_INT_P (if_info->a) && CONST_INT_P (if_info->b)
-		&& ((if_info->branch_cost >= 2 && STORE_FLAG_VALUE == -1)
-		    || if_info->branch_cost >= 3))
+		/* If STORE_FLAG_VALUE is -1, we need SUB, AND, PLUS.  */
+		&& ((noce_estimate_conversion_profitable_p (if_info, 3)
+		     && STORE_FLAG_VALUE == -1)
+		    /* Otherwise, we need NEG, SUB, AND, PLUS.  */
+		    || noce_estimate_conversion_profitable_p (if_info, 4)))
 	{
 	  machine_mode mode = GET_MODE (if_info->x);
 	  HOST_WIDE_INT ifalse = INTVAL (if_info->a);
@@ -2031,7 +2059,7 @@  noce_try_cmove_arith (struct noce_if_info *if_info)
   if (cse_not_expected
       && MEM_P (a) && MEM_P (b)
       && MEM_ADDR_SPACE (a) == MEM_ADDR_SPACE (b)
-      && if_info->branch_cost >= 5)
+      && noce_estimate_conversion_profitable_p (if_info, 5))
     {
       machine_mode address_mode = get_address_mode (a);
 
@@ -3967,6 +3995,9 @@  noce_find_if_block (basic_block test_bb, edge then_edge, edge else_edge,
   if_info.then_else_reversed = then_else_reversed;
   if_info.branch_cost = BRANCH_COST (optimize_bb_for_speed_p (test_bb),
 				     predictable_edge_p (then_edge));
+  if_info.rtx_edge_cost
+    = targetm.rtx_branch_cost (optimize_bb_for_speed_p (test_bb),
+			       predictable_edge_p (then_edge));
 
   /* Do the real work.  */