8237837: Shenandoah: assert(mem == __null) failed: only one safepoint

Reviewed-by: rkennke

src/hotspot/share/gc/shenandoah/c2/shenandoahSupport.cpp

@@ -2014,21 +2014,22 @@ void ShenandoahBarrierC2Support::optimize_after_expansion(VectorSet &visited, No
       if (loop != phase->ltree_root() &&
           loop->_child == NULL &&
           !loop->_irreducible) {
-        LoopNode* head = loop->_head->as_Loop();
-        if ((!head->is_CountedLoop() || head->as_CountedLoop()->is_main_loop() || head->as_CountedLoop()->is_normal_loop()) &&
+        Node* head = loop->_head;
+        if (head->is_Loop() &&
+            (!head->is_CountedLoop() || head->as_CountedLoop()->is_main_loop() || head->as_CountedLoop()->is_normal_loop()) &&
             !seen.test_set(head->_idx)) {
           IfNode* iff = find_unswitching_candidate(loop, phase);
           if (iff != NULL) {
             Node* bol = iff->in(1);
-            if (head->is_strip_mined()) {
-              head->verify_strip_mined(0);
+            if (head->as_Loop()->is_strip_mined()) {
+              head->as_Loop()->verify_strip_mined(0);
             }
             move_heap_stable_test_out_of_loop(iff, phase);
 
             AutoNodeBudget node_budget(phase);
 
             if (loop->policy_unswitching(phase)) {
-              if (head->is_strip_mined()) {
+              if (head->as_Loop()->is_strip_mined()) {
                 OuterStripMinedLoopNode* outer = head->as_CountedLoop()->outer_loop();
                 hide_strip_mined_loop(outer, head->as_CountedLoop(), phase);
               }
@@ -2296,7 +2297,12 @@ void MemoryGraphFixer::collect_memory_nodes() {
         if (in_opc == Op_Return || in_opc == Op_Rethrow) {
           mem = in->in(TypeFunc::Memory);
         } else if (in_opc == Op_Halt) {
-          if (!in->in(0)->is_Region()) {
+          if (in->in(0)->is_Region()) {
+            Node* r = in->in(0);
+            for (uint j = 1; j < r->req(); j++) {
+              assert(r->in(j)->Opcode() != Op_NeverBranch, "");
+            }
+          } else {
             Node* proj = in->in(0);
             assert(proj->is_Proj(), "");
             Node* in = proj->in(0);
@@ -2308,25 +2314,37 @@ void MemoryGraphFixer::collect_memory_nodes() {
               assert(call->is_Call(), "");
               mem = call->in(TypeFunc::Memory);
             } else if (in->Opcode() == Op_NeverBranch) {
-              ResourceMark rm;
-              Unique_Node_List wq;
-              wq.push(in);
-              wq.push(in->as_Multi()->proj_out(0));
-              for (uint j = 1; j < wq.size(); j++) {
-                Node* c = wq.at(j);
-                assert(!c->is_Root(), "shouldn't leave loop");
-                if (c->is_SafePoint()) {
-                  assert(mem == NULL, "only one safepoint");
+              Node* head = in->in(0);
+              assert(head->is_Region() && head->req() == 3, "unexpected infinite loop graph shape");
+              assert(_phase->is_dominator(head, head->in(1)) || _phase->is_dominator(head, head->in(2)), "no back branch?");
+              Node* tail = _phase->is_dominator(head, head->in(1)) ? head->in(1) : head->in(2);
+              Node* c = tail;
+              while (c != head) {
+                if (c->is_SafePoint() && !c->is_CallLeaf()) {
                   mem = c->in(TypeFunc::Memory);
                 }
-                for (DUIterator_Fast kmax, k = c->fast_outs(kmax); k < kmax; k++) {
-                  Node* u = c->fast_out(k);
-                  if (u->is_CFG()) {
-                    wq.push(u);
+                c = _phase->idom(c);
+              }
+              assert(mem != NULL, "should have found safepoint");
+
+              Node* phi_mem = NULL;
+              for (DUIterator_Fast jmax, j = head->fast_outs(jmax); j < jmax; j++) {
+                Node* u = head->fast_out(j);
+                if (u->is_Phi() && u->bottom_type() == Type::MEMORY) {
+                  if (_phase->C->get_alias_index(u->adr_type()) == _alias) {
+                    assert(phi_mem == NULL || phi_mem->adr_type() == TypePtr::BOTTOM, "");
+                    phi_mem = u;
+                  } else if (u->adr_type() == TypePtr::BOTTOM) {
+                    assert(phi_mem == NULL || _phase->C->get_alias_index(phi_mem->adr_type()) == _alias, "");
+                    if (phi_mem == NULL) {
+                      phi_mem = u;
+                    }
                   }
                 }
               }
-              assert(mem != NULL, "should have found safepoint");
+              if (phi_mem != NULL) {
+                mem = phi_mem;
+              }
             }
           }
         } else {

test/hotspot/jtreg/gc/shenandoah/compiler/BarrierInInfiniteLoop.java

@@ -0,0 +1,51 @@
+/*
+ * Copyright (c) 2020, 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 8237837
+ * @summary  Shenandoah: assert(mem == __null) failed: only one safepoint
+ * @key gc
+ * @requires vm.flavor == "server"
+ * @requires vm.gc.Shenandoah & !vm.graal.enabled
+ *
+ * @run main/othervm -XX:+UnlockExperimentalVMOptions -XX:+UseShenandoahGC -Xcomp -XX:CompileOnly=BarrierInInfiniteLoop::test -XX:CompileCommand=quiet BarrierInInfiniteLoop
+ *
+ */
+
+public class BarrierInInfiniteLoop {
+    private static Object field1 = new Object();
+    private static Object field2 = new Object();
+
+    public static void main(String[] args) {
+        test(false);
+    }
+
+    private static void test(boolean flag) {
+        if (flag) {
+            for (;;) {
+                field1 = field2;
+            }
+        }
+    }
+}