[PATCH GCC][4/6]Relax minimal segment length of DR_B for merging alias check

[Bin Cheng]

Hi,
As commented in https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80815#c1,
We can relax minimal segment length of DR_B for merging.  With this change,
the new test can be improved to only one alias check.  Note the
condition is still accurate after this patch, it won't introduce false
alias.
Bootstrap and test on x86_64 and AArch64, is it OK?

2017-05-22  Bin Cheng  <bin.ch...@arm.com>

        * tree-data-ref.c (prune_runtime_alias_test_list): Relax minimal
        segment length for dr_b.

gcc/testsuite/ChangeLog
2017-05-22  Bin Cheng  <bin.ch...@arm.com>

        * gcc.dg/vect/pr80815-3.c: New test.
From 8a570eb93cfaff6fcecdce6b91dd665a81d38e29 Mon Sep 17 00:00:00 2001
From: Bin Cheng <binch...@e108451-lin.cambridge.arm.com>
Date: Mon, 22 May 2017 11:34:18 +0100
Subject: [PATCH 4/6] minimal-seg-length-for-dr_b-20170516.txt

---
 gcc/testsuite/gcc.dg/vect/pr80815-3.c | 45 +++++++++++++++++++++++++++++++++++
 gcc/tree-data-ref.c                   |  5 +++-
 2 files changed, 49 insertions(+), 1 deletion(-)
 create mode 100644 gcc/testsuite/gcc.dg/vect/pr80815-3.c

diff --git a/gcc/testsuite/gcc.dg/vect/pr80815-3.c 
b/gcc/testsuite/gcc.dg/vect/pr80815-3.c
new file mode 100644
index 0000000..dae01fa
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/vect/pr80815-3.c
@@ -0,0 +1,45 @@
+/* { dg-require-effective-target vect_int } */
+
+#include "tree-vect.h"
+int arr[2048];
+int res[100] = { 2148, 2146, 2144, 2142, 2140, 2138, 2136, 2134, 2132, 2130,
+                2128, 2126, 2124, 2122, 2120, 2118, 2116, 2114, 2112, 2110,
+                2108, 2106, 2104, 2102, 2100, 2098, 2096, 2094, 2092, 2090,
+                2088, 2086, 2084, 2082, 2080, 2078, 2076, 2074, 2072, 2070,
+                2068, 2066, 2064, 2062, 2060, 2058, 2056, 2054, 3078, 2050};
+
+__attribute__ ((noinline)) int
+foo (int *a, int *b, int len)
+{
+  int i;
+  int *a1 = a;
+  int *a0 = a1 - 4;
+  for (i = 0; i < len; i++)
+    {
+      *b = *a0 + *a1;
+      b--;
+      a0++;
+      a1++;
+    }
+  return 0;
+}
+
+int main (void)
+{
+  int *a = &arr[1027];
+  int *b = &arr[1024];
+
+  int i;
+  for (i = 0; i < 2048; i++)
+    arr[i] = i;
+
+  foo (a, b, 50);
+
+  for (i = 975; i < 1025; i++)
+    if (arr[i] != res[i - 975])
+      abort ();
+
+  return 0;
+}
+
+/* { dg-final { scan-tree-dump "improved number of alias checks from \[0-9\]* 
to 1" "vect" } } */
diff --git a/gcc/tree-data-ref.c b/gcc/tree-data-ref.c
index f0799d9..5d9054d 100644
--- a/gcc/tree-data-ref.c
+++ b/gcc/tree-data-ref.c
@@ -1286,7 +1286,10 @@ prune_runtime_alias_test_list 
(vec<dr_with_seg_len_pair_t> *alias_pairs,
                min_seg_len_b = 0 - min_seg_len_b;
            }
          else
-           min_seg_len_b = factor;
+           {
+             min_seg_len_b = factor;
+             min_seg_len_b *= absu_hwi (tree_to_shwi (DR_STEP (dr_b1->dr)));
+           }
 
          /* Now we try to merge alias check dr_a1 & dr_b and dr_a2 & dr_b.
 
-- 
1.9.1