8296545: C2 Blackholes should allow load optimizations

Reviewed-by: kvn, vlivanov

Author: shipilev

src/hotspot/share/opto/cfgnode.cpp

@@ -39,6 +39,7 @@
 #include "opto/narrowptrnode.hpp"
 #include "opto/mulnode.hpp"
 #include "opto/phaseX.hpp"
+#include "opto/regalloc.hpp"
 #include "opto/regmask.hpp"
 #include "opto/runtime.hpp"
 #include "opto/subnode.hpp"
@@ -2814,3 +2815,25 @@ void NeverBranchNode::format( PhaseRegAlloc *ra_, outputStream *st) const {
   st->print("%s", Name());
 }
 #endif
+
+#ifndef PRODUCT
+void BlackholeNode::format(PhaseRegAlloc* ra, outputStream* st) const {
+  st->print("blackhole ");
+  bool first = true;
+  for (uint i = 0; i < req(); i++) {
+    Node* n = in(i);
+    if (n != NULL && OptoReg::is_valid(ra->get_reg_first(n))) {
+      if (first) {
+        first = false;
+      } else {
+        st->print(", ");
+      }
+      char buf[128];
+      ra->dump_register(n, buf);
+      st->print("%s", buf);
+    }
+  }
+  st->cr();
+}
+#endif
+

src/hotspot/share/opto/cfgnode.hpp

@@ -47,6 +47,7 @@ class       PCTableNode;
 class         JumpNode;
 class         CatchNode;
 class       NeverBranchNode;
+class     BlackholeNode;
 class   ProjNode;
 class     CProjNode;
 class       IfTrueNode;
@@ -604,4 +605,28 @@ class NeverBranchNode : public MultiBranchNode {
 #endif
 };
 
+//------------------------------BlackholeNode----------------------------
+// Blackhole all arguments. This node would survive through the compiler
+// the effects on its arguments, and would be finally matched to nothing.
+class BlackholeNode : public MultiNode {
+public:
+  BlackholeNode(Node* ctrl) : MultiNode(1) {
+    init_req(TypeFunc::Control, ctrl);
+  }
+  virtual int   Opcode() const;
+  virtual uint ideal_reg() const { return 0; } // not matched in the AD file
+  virtual const Type* bottom_type() const { return TypeTuple::MEMBAR; }
+
+  const RegMask &in_RegMask(uint idx) const {
+    // Fake the incoming arguments mask for blackholes: accept all registers
+    // and all stack slots. This would avoid any redundant register moves
+    // for blackhole inputs.
+    return RegMask::All;
+  }
+#ifndef PRODUCT
+  virtual void format(PhaseRegAlloc* ra, outputStream* st) const;
+#endif
+};
+
+
 #endif // SHARE_OPTO_CFGNODE_HPP

src/hotspot/share/opto/library_call.cpp

@@ -7841,8 +7841,15 @@ bool LibraryCallKit::inline_blackhole() {
   assert(callee()->is_empty(), "Should have been checked before: only empty methods here");
   assert(callee()->holder()->is_loaded(), "Should have been checked before: only methods for loaded classes here");
 
+  // Blackhole node pinches only the control, not memory. This allows
+  // the blackhole to be pinned in the loop that computes blackholed
+  // values, but have no other side effects, like breaking the optimizations
+  // across the blackhole.
+
+  Node* bh = _gvn.transform(new BlackholeNode(control()));
+  set_control(_gvn.transform(new ProjNode(bh, TypeFunc::Control)));
+
   // Bind call arguments as blackhole arguments to keep them alive
-  Node* bh = insert_mem_bar(Op_Blackhole);
   uint nargs = callee()->arg_size();
   for (uint i = 0; i < nargs; i++) {
     bh->add_req(argument(i));

src/hotspot/share/opto/memnode.cpp

@@ -3261,7 +3261,6 @@ MemBarNode* MemBarNode::make(Compile* C, int opcode, int atp, Node* pn) {
   case Op_MemBarCPUOrder:    return new MemBarCPUOrderNode(C, atp, pn);
   case Op_OnSpinWait:        return new OnSpinWaitNode(C, atp, pn);
   case Op_Initialize:        return new InitializeNode(C, atp, pn);
-  case Op_Blackhole:         return new BlackholeNode(C, atp, pn);
   default: ShouldNotReachHere(); return NULL;
   }
 }
@@ -3501,26 +3500,6 @@ MemBarNode* MemBarNode::leading_membar() const {
   return mb;
 }
 
-#ifndef PRODUCT
-void BlackholeNode::format(PhaseRegAlloc* ra, outputStream* st) const {
-  st->print("blackhole ");
-  bool first = true;
-  for (uint i = 0; i < req(); i++) {
-    Node* n = in(i);
-    if (n != NULL && OptoReg::is_valid(ra->get_reg_first(n))) {
-      if (first) {
-        first = false;
-      } else {
-        st->print(", ");
-      }
-      char buf[128];
-      ra->dump_register(n, buf);
-      st->print("%s", buf);
-    }
-  }
-  st->cr();
-}
-#endif
 
 //===========================InitializeNode====================================
 // SUMMARY:

src/hotspot/share/opto/memnode.hpp

@@ -1296,26 +1296,6 @@ class OnSpinWaitNode: public MemBarNode {
   virtual int Opcode() const;
 };
 
-//------------------------------BlackholeNode----------------------------
-// Blackhole all arguments. This node would survive through the compiler
-// the effects on its arguments, and would be finally matched to nothing.
-class BlackholeNode : public MemBarNode {
-public:
-  BlackholeNode(Compile* C, int alias_idx, Node* precedent)
-    : MemBarNode(C, alias_idx, precedent) {}
-  virtual int   Opcode() const;
-  virtual uint ideal_reg() const { return 0; } // not matched in the AD file
-  const RegMask &in_RegMask(uint idx) const {
-    // Fake the incoming arguments mask for blackholes: accept all registers
-    // and all stack slots. This would avoid any redundant register moves
-    // for blackhole inputs.
-    return RegMask::All;
-  }
-#ifndef PRODUCT
-  virtual void format(PhaseRegAlloc* ra, outputStream* st) const;
-#endif
-};
-
 // Isolation of object setup after an AllocateNode and before next safepoint.
 // (See comment in memnode.cpp near InitializeNode::InitializeNode for semantics.)
 class InitializeNode: public MemBarNode {

src/hotspot/share/runtime/vmStructs.cpp

@@ -1565,7 +1565,7 @@
   declare_c2_type(MemBarVolatileNode, MemBarNode)                         \
   declare_c2_type(MemBarCPUOrderNode, MemBarNode)                         \
   declare_c2_type(OnSpinWaitNode, MemBarNode)                             \
-  declare_c2_type(BlackholeNode, MemBarNode)                              \
+  declare_c2_type(BlackholeNode, MultiNode)                               \
   declare_c2_type(InitializeNode, MemBarNode)                             \
   declare_c2_type(ThreadLocalNode, Node)                                  \
   declare_c2_type(Opaque1Node, Node)                                      \

test/hotspot/jtreg/compiler/c2/irTests/blackhole/BlackholeLoadOptoTest.java

@@ -0,0 +1,111 @@
+/*
+ * Copyright (c) 2022, 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 8296545
+ * @requires vm.compiler2.enabled
+ * @summary Blackholes should allow load optimizations
+ * @library /test/lib /
+ * @run driver compiler.c2.irTests.blackhole.BlackholeLoadOptoTest
+ */
+
+package compiler.c2.irTests.blackhole;
+
+import compiler.lib.ir_framework.*;
+import jdk.test.lib.Asserts;
+
+public class BlackholeLoadOptoTest {
+
+    public static void main(String[] args) {
+        TestFramework.runWithFlags(
+            "-XX:+UnlockExperimentalVMOptions",
+            "-XX:CompileThreshold=100",
+            "-XX:-TieredCompilation",
+            "-XX:CompileCommand=blackhole,compiler.c2.irTests.blackhole.BlackholeLoadOptoTest::blackhole",
+            "-XX:CompileCommand=dontinline,compiler.c2.irTests.blackhole.BlackholeLoadOptoTest::dontinline"
+        );
+    }
+
+    static int x, y;
+
+
+    /*
+     * Negative test: check that dangling expressions are eliminated
+     */
+
+    @Test
+    @IR(failOn = {IRNode.LOAD_I, IRNode.MUL_I})
+    static void testNothing() {
+        int r1 = x * y;
+        int r2 = x * y;
+    }
+
+    @Run(test = "testNothing")
+    static void runNothing() {
+        testNothing();
+    }
+
+    /*
+     * Auxiliary test: check that dontinline method does break optimizations
+     */
+
+    @Test
+    @IR(counts = {IRNode.LOAD_I, "4"})
+    @IR(counts = {IRNode.MUL_I, "2"})
+    static void testDontline() {
+        int r1 = x * y;
+        dontinline(r1);
+        int r2 = x * y;
+        dontinline(r2);
+    }
+
+    static void dontinline(int x) {}
+
+    @Run(test = "testDontline")
+    static void runDontinline() {
+        testDontline();
+    }
+
+    /*
+     * Positive test: check that blackhole does not break optimizations
+     */
+
+    @Test
+    @IR(counts = {IRNode.LOAD_I, "2"})
+    @IR(counts = {IRNode.MUL_I, "1"})
+    static void testBlackholed() {
+        int r1 = x * y;
+        blackhole(r1);
+        int r2 = x * y;
+        blackhole(r2);
+    }
+
+    static void blackhole(int x) {}
+
+    @Run(test = "testBlackholed")
+    static void runBlackholed() {
+        testBlackholed();
+    }
+
+}