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[RFC:,5/6] Improve the cost model for multiple-sets

Message ID 1464886438-17892-6-git-send-email-james.greenhalgh@arm.com
State New
Headers show

Commit Message

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

This patch is a small rewrite to the cost model for
bb_ok_for_noce_multiple_sets to use the new noce_cmove_estimate_cost
function added in the previous patches.

Thanks,
James

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

	* ifcvt.c (bb_of_for_noce_convert_multiple_sets): Change cost model.
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Patch

diff --git a/gcc/ifcvt.c b/gcc/ifcvt.c
index bd3f55d..f71889e 100644
--- a/gcc/ifcvt.c
+++ b/gcc/ifcvt.c
@@ -3373,12 +3373,7 @@  noce_convert_multiple_sets (struct noce_if_info *if_info)
 /* Return true iff basic block TEST_BB is comprised of only
    (SET (REG) (REG)) insns suitable for conversion to a series
    of conditional moves.  FORNOW: Use II to find the expected cost of
-   the branch into/over TEST_BB.
-
-   TODO: This creates an implicit "magic number" for if conversion.
-   II->rtx_edge_cost now guides the maximum number of set instructions in
-   a basic block which is considered profitable to completely
-   if-convert.  */
+   the branch into/over TEST_BB.  */
 
 static bool
 bb_ok_for_noce_convert_multiple_sets (basic_block test_bb,
@@ -3387,8 +3382,11 @@  bb_ok_for_noce_convert_multiple_sets (basic_block test_bb,
   rtx_insn *insn;
   unsigned count = 0;
   unsigned param = PARAM_VALUE (PARAM_MAX_RTL_IF_CONVERSION_INSNS);
-  /* TODO:  Revisit this cost model.  */
-  unsigned limit = MIN (ii->rtx_edge_cost / COSTS_N_INSNS (1), param);
+  unsigned cost_limit = ii->rtx_edge_cost;
+  unsigned cost = 0;
+  bool speed_p = optimize_bb_for_speed_p (ii->test_bb);
+  rtx_code code = GET_CODE (ii->cond);
+  machine_mode cmode = GET_MODE (XEXP (ii->cond, 0));
 
   FOR_BB_INSNS (test_bb, insn)
     {
@@ -3404,6 +3402,9 @@  bb_ok_for_noce_convert_multiple_sets (basic_block test_bb,
       rtx dest = SET_DEST (set);
       rtx src = SET_SRC (set);
 
+      cost += noce_cmove_estimate_cost (cmode, GET_MODE (dest),
+					code, speed_p);
+
       /* We can possibly relax this, but for now only handle REG to REG
 	 moves.  This avoids any issues that might come from introducing
 	 loads/stores that might violate data-race-freedom guarantees.  */
@@ -3418,14 +3419,14 @@  bb_ok_for_noce_convert_multiple_sets (basic_block test_bb,
       if (!can_conditionally_move_p (GET_MODE (dest)))
 	return false;
 
-      /* FORNOW: Our cost model is a count of the number of instructions we
-	 would if-convert.  This is suboptimal, and should be improved as part
-	 of a wider rework of branch_cost.  */
-      if (++count > limit)
-	return false;
+      count++;
     }
 
-  return count > 1;
+  /* If we would only put out one conditional move, the other strategies
+     this pass tries are better optimized and will be more appropriate.
+     If the cost in instructions is higher than the limit we've imposed,
+     also give up.  */
+  return (count > 1 && cost <= cost_limit && count <= param);
 }
 
 /* Given a simple IF-THEN-JOIN or IF-THEN-ELSE-JOIN block, attempt to convert