--- a/hotspot/src/share/vm/opto/cfgnode.cpp	Wed Jan 25 07:03:26 2017 +0100
+++ b/hotspot/src/share/vm/opto/cfgnode.cpp	Wed Jan 25 07:05:38 2017 +0100
@@ -2097,6 +2097,7 @@
   uint ideal_reg = _type->ideal_reg();
   assert( ideal_reg != Node::NotAMachineReg, "invalid type at Phi" );
   if( ideal_reg == 0 ) return RegMask::Empty;
+  assert(ideal_reg != Op_RegFlags, "flags register is not spillable");
   return *(Compile::current()->matcher()->idealreg2spillmask[ideal_reg]);
 }
 

--- a/hotspot/src/share/vm/opto/coalesce.cpp	Wed Jan 25 07:03:26 2017 +0100
+++ b/hotspot/src/share/vm/opto/coalesce.cpp	Wed Jan 25 07:05:38 2017 +0100
@@ -292,7 +292,14 @@
               // Copy any flags as well
               _phc.clone_projs(pred, pred->end_idx(), m, copy, _phc._lrg_map);
             } else {
-              const RegMask *rm = C->matcher()->idealreg2spillmask[m->ideal_reg()];
+              int ireg = m->ideal_reg();
+              if (ireg == 0 || ireg == Op_RegFlags) {
+                assert(false, "attempted to spill a non-spillable item: %d: %s, ireg = %d, spill_type: %s",
+                       m->_idx, m->Name(), ireg, MachSpillCopyNode::spill_type(MachSpillCopyNode::PhiInput));
+                C->record_method_not_compilable("attempted to spill a non-spillable item");
+                return;
+              }
+              const RegMask *rm = C->matcher()->idealreg2spillmask[ireg];
               copy = new MachSpillCopyNode(MachSpillCopyNode::PhiInput, m, *rm, *rm);
               // Find a good place to insert.  Kinda tricky, use a subroutine
               insert_copy_with_overlap(pred,copy,phi_name,src_name);
@@ -326,7 +333,14 @@
               b->insert_node(copy, l++);
               l += _phc.clone_projs(b, l, m, copy, _phc._lrg_map);
             } else {
-              const RegMask *rm = C->matcher()->idealreg2spillmask[m->ideal_reg()];
+              int ireg = m->ideal_reg();
+              if (ireg == 0 || ireg == Op_RegFlags) {
+                assert(false, "attempted to spill a non-spillable item: %d: %s, ireg = %d, spill_type: %s",
+                       m->_idx, m->Name(), ireg, MachSpillCopyNode::spill_type(MachSpillCopyNode::TwoAddress));
+                C->record_method_not_compilable("attempted to spill a non-spillable item");
+                return;
+              }
+              const RegMask *rm = C->matcher()->idealreg2spillmask[ireg];
               copy = new MachSpillCopyNode(MachSpillCopyNode::TwoAddress, m, *rm, *rm);
               // Insert the copy in the basic block, just before us
               b->insert_node(copy, l++);
@@ -373,7 +387,14 @@
               if( k < b->_num_succs )
                 continue;     // Live out; do not pre-split
               // Split the lrg at this use
-              const RegMask *rm = C->matcher()->idealreg2spillmask[inp->ideal_reg()];
+              int ireg = inp->ideal_reg();
+              if (ireg == 0 || ireg == Op_RegFlags) {
+                assert(false, "attempted to spill a non-spillable item: %d: %s, ireg = %d, spill_type: %s",
+                       inp->_idx, inp->Name(), ireg, MachSpillCopyNode::spill_type(MachSpillCopyNode::DebugUse));
+                C->record_method_not_compilable("attempted to spill a non-spillable item");
+                return;
+              }
+              const RegMask *rm = C->matcher()->idealreg2spillmask[ireg];
               Node* copy = new MachSpillCopyNode(MachSpillCopyNode::DebugUse, inp, *rm, *rm);
               // Insert the copy in the use-def chain
               n->set_req(inpidx, copy );

--- a/hotspot/src/share/vm/opto/machnode.cpp	Wed Jan 25 07:03:26 2017 +0100
+++ b/hotspot/src/share/vm/opto/machnode.cpp	Wed Jan 25 07:05:38 2017 +0100
@@ -643,6 +643,7 @@
   }
 
   // Values outside the domain represent debug info
+  assert(in(idx)->ideal_reg() != Op_RegFlags, "flags register is not spillable");
   return *Compile::current()->matcher()->idealreg2spillmask[in(idx)->ideal_reg()];
 }
 

--- a/hotspot/src/share/vm/opto/matcher.cpp	Wed Jan 25 07:03:26 2017 +0100
+++ b/hotspot/src/share/vm/opto/matcher.cpp	Wed Jan 25 07:05:38 2017 +0100
@@ -85,6 +85,7 @@
   idealreg2spillmask  [Op_VecX] = NULL;
   idealreg2spillmask  [Op_VecY] = NULL;
   idealreg2spillmask  [Op_VecZ] = NULL;
+  idealreg2spillmask  [Op_RegFlags] = NULL;
 
   idealreg2debugmask  [Op_RegI] = NULL;
   idealreg2debugmask  [Op_RegN] = NULL;
@@ -97,6 +98,7 @@
   idealreg2debugmask  [Op_VecX] = NULL;
   idealreg2debugmask  [Op_VecY] = NULL;
   idealreg2debugmask  [Op_VecZ] = NULL;
+  idealreg2debugmask  [Op_RegFlags] = NULL;
 
   idealreg2mhdebugmask[Op_RegI] = NULL;
   idealreg2mhdebugmask[Op_RegN] = NULL;
@@ -109,6 +111,7 @@
   idealreg2mhdebugmask[Op_VecX] = NULL;
   idealreg2mhdebugmask[Op_VecY] = NULL;
   idealreg2mhdebugmask[Op_VecZ] = NULL;
+  idealreg2mhdebugmask[Op_RegFlags] = NULL;
 
   debug_only(_mem_node = NULL;)   // Ideal memory node consumed by mach node
 }

--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/hotspot/test/compiler/gcbarriers/TestMembarDependencies.java	Wed Jan 25 07:05:38 2017 +0100
@@ -0,0 +1,101 @@
+/*
+ * Copyright (c) 2017, Oracle and/or its affiliates. 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 TestMembarDependencies
+ * @bug 8172850
+ * @summary Tests correct scheduling of memory loads around MembarVolatile emitted by GC barriers.
+ * @library /test/lib /
+ * @modules java.base/jdk.internal.misc
+ *          java.management
+ * @run driver compiler.membars.TestMembarDependencies
+ */
+
+package compiler.membars;
+
+import jdk.test.lib.process.OutputAnalyzer;
+import jdk.test.lib.process.ProcessTools;
+
+public class TestMembarDependencies {
+    private static TestMembarDependencies f1;
+    private static TestMembarDependencies f2;
+
+    public static void main(String args[]) throws Exception {
+        if (args.length == 0) {
+            // For debugging, add "-XX:+TraceOptoPipelining"
+            OutputAnalyzer oa = ProcessTools.executeTestJvm("-XX:+IgnoreUnrecognizedVMOptions",
+                "-XX:-TieredCompilation", "-XX:-BackgroundCompilation", "-XX:+PrintOpto",
+                "-XX:CompileCommand=compileonly,compiler.membars.TestMembarDependencies::test*",
+                "-XX:CompileCommand=dontinline,compiler.membars.TestMembarDependencies::test_m1",
+                TestMembarDependencies.class.getName(), "run");
+            // C2 should not crash or bail out from compilation
+            oa.shouldHaveExitValue(0);
+            oa.shouldNotMatch("Bailout: Recompile without subsuming loads");
+            System.out.println(oa.getOutput());
+        } else {
+            f2 = new TestMembarDependencies();
+            // Trigger compilation of test1 and test2
+            for (int i = 0; i < 10_000; ++i) {
+              f2.test1(f2);
+              f2.test2(f2);
+            }
+        }
+    }
+
+    public void test_m1() { }
+    public void test_m2() { }
+
+    public void test1(TestMembarDependencies obj) {
+        // Try/catch/finally is used to create a CFG block without a test + jmpCon
+        // allowing GCM to schedule the testN_mem_reg0 instruction into that block.
+        try {
+            // Method call defines memory state that is then
+            // used by subsequent instructions/blocks (see below).
+            test_m1();
+        } catch (Exception e) {
+
+        } finally {
+            // Oop write to field emits a GC post-barrier with a MembarVolatile
+            // which has a wide memory effect (kills all memory). This creates an
+            // anti-dependency on all surrounding memory loads.
+            f1 = obj;
+        }
+        // The empty method m2 is inlined but the null check of f2 remains. It is encoded
+        // as CmpN(LoadN(MEM), NULL) where MEM is the memory after the call to test_m1().
+        // This is matched to testN_mem_reg0 on x86 which is scheduled before the barrier
+        // in the try/catch block due to the anti-dependency on the MembarVolatile.
+        // C2 crashes in the register allocator when trying to spill the flag register
+        // to keep the result of the testN instruction live from the try/catch block
+        // until it is here.
+        f2.test_m2();
+    }
+
+    public void test2(TestMembarDependencies obj) {
+        // Same as test1 but without try/catch/finally.
+        // This causes C2 to bail out in block local scheduling because testN_mem_reg0 is
+        // scheduled into a block that already contains another test + jmpCon instruction.
+        test_m1();
+        f1 = obj;
+        f2.test_m2();
+    }
+}