[RFC: Patch 4/6 v2] Modify cost model for noce_cmove_arith

[James Greenhalgh <james.greenhalgh@arm.com>]

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Hi,

This patch clears up the cost model for noce_try_cmove_arith. We lose
the "??? FIXME: Magic number 5" comment, and gain a more realistic cost
model for if-converting memory accesses.

This is the patch that has the chance to cause the largest behavioural
changes for most targets - the current heuristic does not take in to
consideration the cost of a conditional move - once we add that the cost
of the converted sequence often looks higher than we allowed before.

I think that missing the cost of the conditional move from these sequences
is not a good idea, and that the cost model should rely on the target giving
back good information. A target that finds tests failing after this patch
should consider either reducing the cost of a conditional move sequence, or
increasing TARGET_MAX_NOCE_IFCVT_SEQ_COST.

As this ups the cost of if-convert dramatically, I've used the new
parameters to ensure that the tests in the testsuite continue to pass on
all targets.

Bootstrapped in series on aarch64 and x86-64.

OK?

Thanks,
James

---
gcc/

2016-06-21  James Greenhalgh  <james.greenhalgh@arm.com>

	* ifcvt.c (noce_try_cmove_arith): Check costs after constructing
	new sequence.

gcc/testsuite/

2016-06-21  James Greenhalgh  <james.greenhalgh@arm.com>

	* gcc.dg/ifcvt-2.c: Use parameter to guide if-conversion heuristics.
	* gcc.dg/ifcvt-3.c: Use parameter to guide if-conversion heuristics.
	* gcc.dg/pr68435.c: Use parameter to guide if-conversion heuristics.


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diff --git a/gcc/ifcvt.c b/gcc/ifcvt.c
index f4ad037..78906d3 100644
--- a/gcc/ifcvt.c
+++ b/gcc/ifcvt.c
@@ -2092,7 +2092,8 @@ noce_try_cmove_arith (struct noce_if_info *if_info)
   rtx a = if_info->a;
   rtx b = if_info->b;
   rtx x = if_info->x;
-  rtx orig_a, orig_b;
+  rtx orig_a = a;
+  rtx orig_b = b;
   rtx_insn *insn_a, *insn_b;
   bool a_simple = if_info->then_simple;
   bool b_simple = if_info->else_simple;
@@ -2102,16 +2103,15 @@ noce_try_cmove_arith (struct noce_if_info *if_info)
   int is_mem = 0;
   enum rtx_code code;
   rtx_insn *ifcvt_seq;
+  bool speed_p = optimize_bb_for_speed_p (if_info->test_bb);
 
   /* A conditional move from two memory sources is equivalent to a
      conditional on their addresses followed by a load.  Don't do this
      early because it'll screw alias analysis.  Note that we've
      already checked for no side effects.  */
-  /* ??? FIXME: Magic number 5.  */
   if (cse_not_expected
       && MEM_P (a) && MEM_P (b)
-      && MEM_ADDR_SPACE (a) == MEM_ADDR_SPACE (b)
-      && noce_estimate_conversion_profitable_p (if_info, 5))
+      && MEM_ADDR_SPACE (a) == MEM_ADDR_SPACE (b))
     {
       machine_mode address_mode = get_address_mode (a);
 
@@ -2143,25 +2143,6 @@ noce_try_cmove_arith (struct noce_if_info *if_info)
   if (!can_conditionally_move_p (x_mode))
     return FALSE;
 
-  unsigned int then_cost;
-  unsigned int else_cost;
-  if (insn_a)
-    then_cost = if_info->then_cost;
-  else
-    then_cost = 0;
-
-  if (insn_b)
-    else_cost = if_info->else_cost;
-  else
-    else_cost = 0;
-
-  /* We're going to execute one of the basic blocks anyway, so
-     bail out if the most expensive of the two blocks is unacceptable.  */
-
-  /* TODO: Revisit cost model.  */
-  if (MAX (then_cost, else_cost) > if_info->max_seq_cost)
-    return FALSE;
-
   /* Possibly rearrange operands to make things come out more natural.  */
   if (reversed_comparison_code (if_info->cond, if_info->jump) != UNKNOWN)
     {
@@ -2353,6 +2334,12 @@ noce_try_cmove_arith (struct noce_if_info *if_info)
   if (!ifcvt_seq)
     return FALSE;
 
+  /* Check that our cost model will allow the transform.  */
+
+  if (seq_cost (ifcvt_seq, speed_p) > if_info->max_seq_cost)
+    /* Just return false, the sequence has already been finalized.  */
+    return FALSE;
+
   emit_insn_before_setloc (ifcvt_seq, if_info->jump,
 			   INSN_LOCATION (if_info->insn_a));
   if_info->transform_name = "noce_try_cmove_arith";
diff --git a/gcc/testsuite/gcc.dg/ifcvt-2.c b/gcc/testsuite/gcc.dg/ifcvt-2.c
index e0e1728..73e0dcc 100644
--- a/gcc/testsuite/gcc.dg/ifcvt-2.c
+++ b/gcc/testsuite/gcc.dg/ifcvt-2.c
@@ -1,5 +1,5 @@
 /* { dg-do compile { target aarch64*-*-* x86_64-*-* } } */
-/* { dg-options "-fdump-rtl-ce1 -O2" } */
+/* { dg-options "-fdump-rtl-ce1 -O2 --param max-rtl-if-conversion-unpredictable-cost=100" } */
 
 
 typedef unsigned char uint8_t;
diff --git a/gcc/testsuite/gcc.dg/ifcvt-3.c b/gcc/testsuite/gcc.dg/ifcvt-3.c
index 44233d4..b250bc1 100644
--- a/gcc/testsuite/gcc.dg/ifcvt-3.c
+++ b/gcc/testsuite/gcc.dg/ifcvt-3.c
@@ -1,5 +1,5 @@
 /* { dg-do compile { target { { aarch64*-*-* i?86-*-* x86_64-*-* } && lp64 } } } */
-/* { dg-options "-fdump-rtl-ce1 -O2" } */
+/* { dg-options "-fdump-rtl-ce1 -O2 --param max-rtl-if-conversion-unpredictable-cost=100" } */
 
 typedef long long s64;
 
diff --git a/gcc/testsuite/gcc.dg/pr68435.c b/gcc/testsuite/gcc.dg/pr68435.c
index 765699a..f86b7f8 100644
--- a/gcc/testsuite/gcc.dg/pr68435.c
+++ b/gcc/testsuite/gcc.dg/pr68435.c
@@ -1,5 +1,5 @@
 /* { dg-do compile { target aarch64*-*-* x86_64-*-* } } */
-/* { dg-options "-fdump-rtl-ce1 -O2 -w" } */
+/* { dg-options "-fdump-rtl-ce1 -O2 -w --param max-rtl-if-conversion-unpredictable-cost=100" } */
 
 typedef struct cpp_reader cpp_reader;
 enum cpp_ttype