8210389: C2: assert(n->outcnt() != 0 || C->top() == n || n->is_Proj()) failed: No dead instructions after post-alloc

Reviewed-by: kvn, thartmann

src/hotspot/share/opto/compile.cpp

@@ -3379,6 +3379,32 @@ void Compile::final_graph_reshaping_impl( Node *n, Final_Reshape_Counts &frc) {
       n->set_req(MemBarNode::Precedent, top());
     }
     break;
+  case Op_MemBarAcquire: {
+    if (n->as_MemBar()->trailing_load() && n->req() > MemBarNode::Precedent) {
+      // At parse time, the trailing MemBarAcquire for a volatile load
+      // is created with an edge to the load. After optimizations,
+      // that input may be a chain of Phis. If those phis have no
+      // other use, then the MemBarAcquire keeps them alive and
+      // register allocation can be confused.
+      ResourceMark rm;
+      Unique_Node_List wq;
+      wq.push(n->in(MemBarNode::Precedent));
+      n->set_req(MemBarNode::Precedent, top());
+      while (wq.size() > 0) {
+        Node* m = wq.pop();
+        if (m->outcnt() == 0) {
+          for (uint j = 0; j < m->req(); j++) {
+            Node* in = m->in(j);
+            if (in != NULL) {
+              wq.push(in);
+            }
+          }
+          m->disconnect_inputs(NULL, this);
+        }
+      }
+    }
+    break;
+  }
   case Op_RangeCheck: {
     RangeCheckNode* rc = n->as_RangeCheck();
     Node* iff = new IfNode(rc->in(0), rc->in(1), rc->_prob, rc->_fcnt);

test/hotspot/jtreg/compiler/regalloc/VolatileLoadMemBarsOnlyUses.java

@@ -0,0 +1,80 @@
+/*
+ * Copyright (c) 2018, Red Hat, 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 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 8210389
+ * @summary C2: assert(n->outcnt() != 0 || C->top() == n || n->is_Proj()) failed: No dead instructions after post-alloc
+ *
+ * @run main/othervm -Xcomp -XX:CompileOnly=VolatileLoadMemBarsOnlyUses VolatileLoadMemBarsOnlyUses
+ *
+ */
+
+public class VolatileLoadMemBarsOnlyUses {
+
+    public static final int N = 400;
+    public static long instanceCount=-94L;
+    public static volatile byte byFld=-108;
+
+    public int mainTest(String[] strArr1) {
+
+        int i17=9, i19=1, i20=63, i21=-32916, i22=0, iArr[]=new int[N];
+        boolean b1=false;
+        double d3=76.18241;
+
+        for (int i : iArr) {
+            for (i17 = 2; i17 < 63; i17++) {
+                if (b1) break;
+                byFld += (byte)(0.131F + (i17 * i17));
+            }
+            for (i19 = 1; 63 > i19; ++i19) {
+                for (i21 = 1; i21 < 2; i21++) {
+                    d3 = i22;
+                    if (b1) continue;
+                    i20 = i21;
+                }
+                d3 -= byFld;
+                instanceCount = 46725L;
+            }
+            switch ((((i22 >>> 1) % 4) * 5) + 91) {
+            case 98:
+                break;
+            case 110:
+                break;
+            case 105:
+                break;
+            case 103:
+                break;
+            default:
+            }
+        }
+
+        return i20;
+    }
+    public static void main(String[] strArr) {
+        VolatileLoadMemBarsOnlyUses _instance = new VolatileLoadMemBarsOnlyUses();
+        for (int i = 0; i < 10; i++ ) {
+            _instance.mainTest(strArr);
+        }
+    }
+}