diff --git a/src/share/vm/opto/ifg.cpp b/src/share/vm/opto/ifg.cpp
index 9d260cbec671c80f36e0d5b168e7539a4084a3ca..bd10efdc4787c7d1c92186d2f0f21171471e723a 100644
--- a/src/share/vm/opto/ifg.cpp
+++ b/src/share/vm/opto/ifg.cpp
@@ -1,5 +1,5 @@
 /*
- * Copyright 1998-2009 Sun Microsystems, Inc.  All Rights Reserved.
+ * Copyright 1998-2010 Sun Microsystems, Inc.  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
@@ -736,7 +736,28 @@ uint PhaseChaitin::build_ifg_physical( ResourceArea *a ) {
         // the flags and assumes it's dead.  This keeps the (useless)
         // flag-setting behavior alive while also keeping the (useful)
         // memory update effect.
-        for( uint k = ((n->Opcode() == Op_SCMemProj) ? 0:1); k < n->req(); k++ ) {
+        uint begin = 1;
+        uint end = n->req();
+        if (n->Opcode() == Op_SCMemProj) {
+          begin = 0;
+        } else if (n->is_Mach()) {
+          switch (n->as_Mach()->ideal_Opcode()) {
+            case Op_MemBarAcquire:
+            case Op_MemBarVolatile:
+              if (n->len() >= MemBarNode::Precedent + 1 &&
+                  n->in(MemBarNode::Precedent) != NULL &&
+                  n->in(MemBarNode::Precedent)->outcnt() == 1) {
+                // This membar node is the single user of it's input
+                // so the input won't be considered live and this node
+                // would get deleted during copy elimination so force
+                // it to be live.
+                end = MemBarNode::Precedent + 1;
+              }
+              break;
+          }
+        }
+
+        for( uint k = begin; k < end; k++ ) {
           Node *def = n->in(k);
           uint x = n2lidx(def);
           if( !x ) continue;
diff --git a/test/compiler/6792161/Test6792161.java b/test/compiler/6792161/Test6792161.java
new file mode 100644
index 0000000000000000000000000000000000000000..b6c9b579d8246c0e98b275ce1dac1ad40e25888e
--- /dev/null
+++ b/test/compiler/6792161/Test6792161.java
@@ -0,0 +1,49 @@
+/*
+ * Copyright 2010 Sun Microsystems, Inc.  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 Sun Microsystems, Inc., 4150 Network Circle, Santa Clara,
+ * CA 95054 USA or visit www.sun.com if you need additional information or
+ * have any questions.
+ *
+ */
+
+/**
+ * @test
+ * @bug 6792161
+ * @summary assert("No dead instructions after post-alloc")
+ *
+ * @run main/othervm -Xcomp -XX:MaxInlineSize=120 Test6792161
+ */
+
+import java.lang.reflect.Constructor;
+public class Test6792161 {
+    static Constructor test(Class cls) throws Exception {
+        Class[] args= { String.class };
+        try {
+            return cls.getConstructor(args);
+        } catch (NoSuchMethodException e) {}
+        return cls.getConstructor(new Class[0]);
+    }
+    public static void main(final String[] args) throws Exception {
+        try {
+            for (int i = 0; i < 100000; i++) {
+                Constructor ctor = test(Class.forName("Test6792161"));
+            }
+        } catch (NoSuchMethodException e) {}
+    }
+}