[PATCH PR81620]Don't set has_max_use_after flag for store-store chain

[Bin Cheng]

Hi,
This simple patch fixes the ICE by not setting has_max_use_after flag for
store-store chain because there is no use at all.
Bootstrap and test on x86_64 and AArch64 ongoing.  Is it OK if no failure?

Thanks,
bin
2017-07-31  Bin Cheng  <bin.ch...@arm.com>

        PR tree-optimization/81620
        * tree-predcom.c (add_ref_to_chain): Don't set has_max_use_after
        for store-store chain.

gcc/testsuite/ChangeLog
2017-07-31  Bin Cheng  <bin.ch...@arm.com>

        PR tree-optimization/81620
        * gcc.dg/tree-ssa/pr81620-1.c: New.
        * gcc.dg/tree-ssa/pr81620-2.c: New.
From 4e8f67bb1cc09ef475f9cfbb8e847f9f422c3e44 Mon Sep 17 00:00:00 2001
From: Bin Cheng <binch...@e108451-lin.cambridge.arm.com>
Date: Mon, 31 Jul 2017 10:24:07 +0100
Subject: [PATCH 1/2] pr81620-20170731.txt

---
 gcc/testsuite/gcc.dg/tree-ssa/pr81620-1.c | 20 ++++++++++++++++++++
 gcc/testsuite/gcc.dg/tree-ssa/pr81620-2.c | 25 +++++++++++++++++++++++++
 gcc/tree-predcom.c                        |  4 +++-
 3 files changed, 48 insertions(+), 1 deletion(-)
 create mode 100644 gcc/testsuite/gcc.dg/tree-ssa/pr81620-1.c
 create mode 100644 gcc/testsuite/gcc.dg/tree-ssa/pr81620-2.c

diff --git a/gcc/testsuite/gcc.dg/tree-ssa/pr81620-1.c 
b/gcc/testsuite/gcc.dg/tree-ssa/pr81620-1.c
new file mode 100644
index 0000000..f8f2dd8
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/tree-ssa/pr81620-1.c
@@ -0,0 +1,20 @@
+/* { dg-do run } */
+/* { dg-options "-O3 -fno-tree-loop-vectorize -fdump-tree-pcom-details" } */
+
+int a[7];
+char b;
+void abort (void);
+
+int main() {
+  b = 4;
+  for (; b; b--) {
+    a[b] = b;
+    a[b + 2] = 1;
+  }
+  if (a[0] != 0 || a[1] != 1 || a[2] != 2
+      || a[3] != 1 || a[4] != 1 || a[5] != 1 || a[6] != 1)
+    abort ();
+  return 0;
+}
+
+/* { dg-final { scan-tree-dump-times "Store-stores chain" 1 "pcom" } } */
diff --git a/gcc/testsuite/gcc.dg/tree-ssa/pr81620-2.c 
b/gcc/testsuite/gcc.dg/tree-ssa/pr81620-2.c
new file mode 100644
index 0000000..85a8e35
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/tree-ssa/pr81620-2.c
@@ -0,0 +1,25 @@
+/* { dg-do run } */
+/* { dg-options "-O3 -fno-tree-loop-vectorize -fdump-tree-pcom-details" } */
+
+int a[200];
+char b;
+void abort (void);
+
+int main() {
+  int i;
+  b = 100;
+  for (; b; b--) {
+    a[b] = 2;
+    a[b + 2] = 1;
+  }
+
+  if (a[0] != 0 || a[1] != 2 || a[2] != 2)
+    abort ();
+  for (i = 3; i < 103; i++)
+    if (a[i] != 1)
+    abort ();
+
+  return 0;
+}
+
+/* { dg-final { scan-tree-dump-times "Store-stores chain" 1 "pcom" } } */
diff --git a/gcc/tree-predcom.c b/gcc/tree-predcom.c
index a4011bf..f7a57a4 100644
--- a/gcc/tree-predcom.c
+++ b/gcc/tree-predcom.c
@@ -1069,7 +1069,9 @@ add_ref_to_chain (chain_p chain, dref ref)
       chain->has_max_use_after = false;
     }
 
-  if (ref->distance == chain->length
+  /* Don't set the flag for store-store chain since there is no use.  */
+  if (chain->type != CT_STORE_STORE
+      && ref->distance == chain->length
       && ref->pos > root->pos)
     chain->has_max_use_after = true;
 
-- 
1.9.1