8325372: Shenandoah: SIGSEGV crash in unnecessary_acquire due to LoadStore split through phi

Backport-of: 5d414da50459b7a1e6f0f537ff3b318854b2c427

Author: Dan Lutker

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

@@ -1730,11 +1730,26 @@ bool ShenandoahBarrierC2Support::identical_backtoback_ifs(Node* n, PhaseIdealLoo
   return true;
 }
 
+bool ShenandoahBarrierC2Support::merge_point_safe(Node* region) {
+  for (DUIterator_Fast imax, i = region->fast_outs(imax); i < imax; i++) {
+    Node* n = region->fast_out(i);
+    if (n->is_LoadStore()) {
+      // Splitting a LoadStore node through phi, causes it to lose its SCMemProj: the split if code doesn't have support
+      // for a LoadStore at the region the if is split through because that's not expected to happen (LoadStore nodes
+      // should be between barrier nodes). It does however happen with Shenandoah though because barriers can get
+      // expanded around a LoadStore node.
+      return false;
+    }
+  }
+  return true;
+}
+
+
 void ShenandoahBarrierC2Support::merge_back_to_back_tests(Node* n, PhaseIdealLoop* phase) {
   assert(is_heap_stable_test(n), "no other tests");
   if (identical_backtoback_ifs(n, phase)) {
     Node* n_ctrl = n->in(0);
-    if (phase->can_split_if(n_ctrl)) {
+    if (phase->can_split_if(n_ctrl) && merge_point_safe(n_ctrl)) {
       IfNode* dom_if = phase->idom(n_ctrl)->as_If();
       if (is_heap_stable_test(n)) {
         Node* gc_state_load = n->in(1)->in(1)->in(1)->in(1);

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

@@ -65,6 +65,7 @@ class ShenandoahBarrierC2Support : public AllStatic {
   static void test_in_cset(Node*& ctrl, Node*& not_cset_ctrl, Node* val, Node* raw_mem, PhaseIdealLoop* phase);
   static void move_gc_state_test_out_of_loop(IfNode* iff, PhaseIdealLoop* phase);
   static void merge_back_to_back_tests(Node* n, PhaseIdealLoop* phase);
+  static bool merge_point_safe(Node* region);
   static bool identical_backtoback_ifs(Node *n, PhaseIdealLoop* phase);
   static void fix_ctrl(Node* barrier, Node* region, const MemoryGraphFixer& fixer, Unique_Node_List& uses, Unique_Node_List& uses_to_ignore, uint last, PhaseIdealLoop* phase);
   static IfNode* find_unswitching_candidate(const IdealLoopTree *loop, PhaseIdealLoop* phase);

src/hotspot/share/opto/memnode.cpp

@@ -3353,6 +3353,7 @@ Node *MemBarNode::Ideal(PhaseGVN *phase, bool can_reshape) {
           my_mem = load_node;
         } else {
           assert(my_mem->unique_out() == this, "sanity");
+          assert(!trailing_load_store(), "load store node can't be eliminated");
           del_req(Precedent);
           phase->is_IterGVN()->_worklist.push(my_mem); // remove dead node later
           my_mem = nullptr;

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

@@ -0,0 +1,82 @@
+/*
+ * Copyright (c) 2024, 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 8325372
+ * @summary fusion of heap stable test causes GetAndSet node to be removed
+ * @requires vm.gc.Shenandoah
+ * @modules java.base/jdk.internal.misc:+open
+ *
+ * @run main/othervm -XX:+UseShenandoahGC -XX:-BackgroundCompilation TestUnsafeLoadStoreMergedHeapStableTests
+ */
+
+import jdk.internal.misc.Unsafe;
+
+import java.lang.reflect.Field;
+
+public class TestUnsafeLoadStoreMergedHeapStableTests {
+
+    static final jdk.internal.misc.Unsafe UNSAFE = Unsafe.getUnsafe();
+    static long F_OFFSET;
+
+    static class A {
+        Object f;
+    }
+
+    static {
+        try {
+            Field fField = A.class.getDeclaredField("f");
+            F_OFFSET = UNSAFE.objectFieldOffset(fField);
+        } catch (Exception e) {
+            throw new RuntimeException(e);
+        }
+    }
+
+    static Object testHelper(boolean flag, Object o, long offset, Object x) {
+        if (flag) {
+            return UNSAFE.getAndSetObject(o, offset, x);
+        }
+        return null;
+    }
+
+    static Object field;
+
+
+    static Object test1(boolean flag, Object o, long offset) {
+        return testHelper(flag, null, offset, field);
+    }
+
+    static Object test2(Object o, long offset) {
+        return UNSAFE.getAndSetObject(o, offset, field);
+    }
+
+    static public void main(String[] args) {
+        A a = new A();
+        for (int i = 0; i < 20_000; i++) {
+            testHelper(true, a, F_OFFSET, null);
+            test1(false, a, F_OFFSET);
+            test2(a, F_OFFSET);
+        }
+    }
+}