[PATCH PR71347][Partial revert r235513]Compute cost for all uses in group

[Bin Cheng <Bin.Cheng@arm.com>]

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Hi,
This patch partially reverts part of r235513 to fix PR71347, the original patch is to improve compilation time for a small amount.  Root cause as analyzed in bugzilla PR is that we can't skip computing cost for sub iv_use if it has different position to the first use in group.  The patch also includes a new test.

Bootstrap and test on x86_64.  Is it OK?

Thanks,
bin

2016-05-31  Bin Cheng  <bin.cheng@arm.com>

	PR tree-optimization/71347
	* tree-ssa-loop-ivopts.c (determine_group_iv_cost_address): Compute
	cost for all uses in group.

gcc/testsuite/ChangeLog
2016-05-31  Bin Cheng  <bin.cheng@arm.com>

	PR tree-optimization/71347
	* gcc.dg/tree-ssa/pr71347.c: New test.

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diff --git a/gcc/tree-ssa-loop-ivopts.c b/gcc/tree-ssa-loop-ivopts.c
index 1e8d637..25b9780 100644
--- a/gcc/tree-ssa-loop-ivopts.c
+++ b/gcc/tree-ssa-loop-ivopts.c
@@ -5115,7 +5115,7 @@ determine_group_iv_cost_address (struct ivopts_data *data,
 {
   unsigned i;
   bitmap depends_on;
-  bool can_autoinc, first = true;
+  bool can_autoinc;
   iv_inv_expr_ent *inv_expr = NULL;
   struct iv_use *use = group->vuses[0];
   comp_cost sum_cost = no_cost, cost;
@@ -5142,30 +5142,11 @@ determine_group_iv_cost_address (struct ivopts_data *data,
     {
       struct iv_use *next = group->vuses[i];
 
-      /* Compute cost for the first use with different offset to the main
-	 use and add it afterwards.  Costs for these uses could be quite
-	 different.  Given below uses in a group:
-	   use 0  : {base + A + offset_0, step}
-	   use 0.1: {base + A + offset_0, step}
-	   use 0.2: {base + A + offset_1, step}
-	   use 0.3: {base + A + offset_2, step}
-	 when we need to compute costs with candidate:
-	   cand 1 : {base + B + offset_0, step}
-
-	 The first use with different offset is use 0.2, its cost is larger
-	 than cost of use 0/0.1 because we need to compute:
-	   A - B + offset_1 - offset_0
-	   rather than:
-	   A - B.  */
-      if (first && next->addr_offset != use->addr_offset)
-	{
-	  first = false;
-	  cost = get_computation_cost (data, next, cand, true,
-				       NULL, &can_autoinc, NULL);
-	  /* Remove setup cost.  */
-	  if (!cost.infinite_cost_p ())
-	    cost -= cost.scratch;
-	}
+      /* TODO: We could skip computing cost for sub iv_use when it has the
+	 same cost as the first iv_use, but the cost really depends on the
+	 offset and where the iv_use is.  */
+	cost = get_computation_cost (data, next, cand, true,
+				     NULL, &can_autoinc, NULL);
       sum_cost += cost;
     }
   set_group_iv_cost (data, group, cand, sum_cost, depends_on,
diff --git a/gcc/testsuite/gcc.dg/tree-ssa/pr71347.c b/gcc/testsuite/gcc.dg/tree-ssa/pr71347.c
new file mode 100644
index 0000000..7e5ad49
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/tree-ssa/pr71347.c
@@ -0,0 +1,17 @@
+/* { dg-do compile } */
+/* { dg-options "-O2 -fdump-tree-optimized" } */
+
+double in;
+extern void Write (double);
+void foo (void)
+{
+  static double X[9];
+  int i;
+        X[1] = in * in; 
+        for (i = 2; i <= 8; i++)
+            X[i] = X[i - 1] * X[1]; 
+        Write (X[5]);
+}
+
+/* Load of X[i - i] can be omitted by reusing X[i] in previous iteration.  */
+/* { dg-final { scan-tree-dump-not ".* = MEM.*;" "optimized"} } */