From 00a03e7acf5509396c8a1ac90d3d661250750d9c Mon Sep 17 00:00:00 2001
Date: Fri, 22 Jan 2021 16:29:29 +0800
Subject: 8243670:Unexpected test result caused by C2
 MergeMemNode::Ideal

Summary: <c2>: <Unexpected test result caused by C2 MergeMemNode>
LLT: hotspot/test/compiler/c2/TestReplaceEquivPhis.java
Bug url: https://bugs.openjdk.java.net/browse/JDK-8243670
---
 hotspot/src/share/vm/opto/cfgnode.cpp         | 24 ++++++
 hotspot/src/share/vm/opto/memnode.cpp         | 20 +----
 hotspot/src/share/vm/opto/type.hpp            |  7 ++
 .../compiler/c2/TestReplaceEquivPhis.java     | 77 +++++++++++++++++++
 4 files changed, 109 insertions(+), 19 deletions(-)
 create mode 100644 hotspot/test/compiler/c2/TestReplaceEquivPhis.java

diff --git a/hotspot/src/share/vm/opto/cfgnode.cpp b/hotspot/src/share/vm/opto/cfgnode.cpp
index dd5a5ec41..f3d223fbd 100644
--- a/hotspot/src/share/vm/opto/cfgnode.cpp
+++ b/hotspot/src/share/vm/opto/cfgnode.cpp
@@ -1156,6 +1156,30 @@ Node *PhiNode::Identity( PhaseTransform *phase ) {
     if (id != NULL)  return id;
   }
 
+  // Looking for phis with identical inputs.  If we find one that has
+  // type TypePtr::BOTTOM, replace the current phi with the bottom phi.
+  if (phase->is_IterGVN() && type() == Type::MEMORY && adr_type() !=
+      TypePtr::BOTTOM && !adr_type()->is_known_instance()) {
+    uint phi_len = req();
+    Node* phi_reg = region();
+    for (DUIterator_Fast imax, i = phi_reg->fast_outs(imax); i < imax; i++) {
+      Node* u = phi_reg->fast_out(i);
+      if (u->is_Phi() && u->as_Phi()->type() == Type::MEMORY &&
+          u->adr_type() == TypePtr::BOTTOM && u->in(0) == phi_reg &&
+          u->req() == phi_len) {
+        for (uint j = 1; j < phi_len; j++) {
+          if (in(j) != u->in(j)) {
+            u = NULL;
+            break;
+          }
+        }
+        if (u != NULL) {
+          return u;
+        }
+      }
+    }
+  }
+
   return this;                     // No identity
 }
 
diff --git a/hotspot/src/share/vm/opto/memnode.cpp b/hotspot/src/share/vm/opto/memnode.cpp
index 68a13dbd8..f2a2ee6a2 100644
--- a/hotspot/src/share/vm/opto/memnode.cpp
+++ b/hotspot/src/share/vm/opto/memnode.cpp
@@ -4418,25 +4418,7 @@ Node *MergeMemNode::Ideal(PhaseGVN *phase, bool can_reshape) {
       new_mem = old_mmem->memory_at(i);
     }
     // else preceding memory was not a MergeMem
-
-    // replace equivalent phis (unfortunately, they do not GVN together)
-    if (new_mem != NULL && new_mem != new_base &&
-        new_mem->req() == phi_len && new_mem->in(0) == phi_reg) {
-      if (new_mem->is_Phi()) {
-        PhiNode* phi_mem = new_mem->as_Phi();
-        for (uint i = 1; i < phi_len; i++) {
-          if (phi_base->in(i) != phi_mem->in(i)) {
-            phi_mem = NULL;
-            break;
-          }
-        }
-        if (phi_mem != NULL) {
-          // equivalent phi nodes; revert to the def
-          new_mem = new_base;
-        }
-      }
-    }
-
+    
     // maybe store down a new value
     Node* new_in = new_mem;
     if (new_in == new_base)  new_in = empty_mem;
diff --git a/hotspot/src/share/vm/opto/type.hpp b/hotspot/src/share/vm/opto/type.hpp
index ec06f8d69..8da4a766d 100644
--- a/hotspot/src/share/vm/opto/type.hpp
+++ b/hotspot/src/share/vm/opto/type.hpp
@@ -432,6 +432,8 @@ public:
     return exact_kls != NULL;
   }
 
+  virtual bool is_known_instance() const { return false; }
+
 private:
   // support arrays
   static const Type*        _zero_type[T_CONFLICT+1];
@@ -1333,6 +1335,11 @@ public:
     return _ptrtype;
   }
 
+  bool is_known_instance() const {
+    return _ptrtype->is_known_instance();
+  }
+
+
 #ifndef PRODUCT
   virtual void dump2( Dict &d, uint depth, outputStream *st ) const;
 #endif
diff --git a/hotspot/test/compiler/c2/TestReplaceEquivPhis.java b/hotspot/test/compiler/c2/TestReplaceEquivPhis.java
new file mode 100644
index 000000000..d4c93b390
--- /dev/null
+++ b/hotspot/test/compiler/c2/TestReplaceEquivPhis.java
@@ -0,0 +1,77 @@
+/*
+ * Copyright (c) 2020, Huawei Technologies Co. Ltd. All rights reserved.
+ * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
+ *
+ * This code is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License version 2 only, as
+ * published by the Free Software Foundation.
+ *
+ * This code is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+ * version 2 for more details (a copy is included in the LICENSE file that
+ * accompanied this code).
+ *
+ * You should have received a copy of the GNU General Public License version
+ * 2 along with this work; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
+ *
+ * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
+ * or visit www.oracle.com if you need additional information or have any
+ * questions.
+ */
+
+/**
+ * @test
+ * @bug 8243670
+ * @summary Unexpected test result caused by C2 MergeMemNode::Ideal
+ *
+ * @run main/othervm -Xcomp -XX:-SplitIfBlocks
+ *      -XX:CompileOnly=compiler.c2.TestReplaceEquivPhis::test
+ *      -XX:-BackgroundCompilation compiler.c2.TestReplaceEquivPhis
+ */
+
+package compiler.c2;
+
+public class TestReplaceEquivPhis {
+
+    public static final int N = 400;
+    public static volatile int instanceCount = 0;
+    public int iFld = 0;
+    public static int iArrFld[] = new int[N];
+
+    public int test() {
+        int v = 0;
+        boolean bArr[] = new boolean[N];
+
+        for (int i = 1; i < 344; i++) {
+            iFld = i;
+            for (int j = 2; j <177 ; j++) {
+                v = iFld;
+                iFld = TestReplaceEquivPhis.instanceCount;
+                TestReplaceEquivPhis.iArrFld[i] = 0;
+                iFld += TestReplaceEquivPhis.instanceCount;
+                TestReplaceEquivPhis.iArrFld[i] = 0;
+                bArr[j] = false;
+                TestReplaceEquivPhis.instanceCount = 1;
+
+                for (int k = 1; k < 3; k++) {
+                    // do nothing
+                }
+            }
+        }
+        return v;
+    }
+
+    public static void main(String[] args) {
+            TestReplaceEquivPhis obj = new TestReplaceEquivPhis();
+            for (int i = 0; i < 5; i++) {
+                int result = obj.test();
+                if (result != 2) {
+                    throw new RuntimeException("Test failed.");
+                }
+            }
+            System.out.println("Test passed.");
+    }
+
+}
-- 
2.19.0