[turbofan] Implement simplistic checkpoint reducer.

This adds a very simplistic reduction of {CheckPoint} nodes via the new
{CheckpointElimination}, eliminating redundant check points that appear
in an effect chain of operations that are all {kNoWrite}. Such a chain
allows an arbitrary check point to be chosen. The current approach will
end up choosing the first one for all deopts in the chain.

R=bmeurer@chromium.org
TEST=unittests/CheckpointEliminationTest.CheckPointChain
BUG=v8:5021

Review-Url: https://codereview.chromium.org/2022913003
Cr-Commit-Position: refs/heads/master@{#36634}

BUILD.gn

@@ -840,6 +840,8 @@ v8_source_set("v8_base") {
     "src/compiler/bytecode-graph-builder.cc",
     "src/compiler/bytecode-graph-builder.h",
     "src/compiler/c-linkage.cc",
+    "src/compiler/checkpoint-elimination.cc",
+    "src/compiler/checkpoint-elimination.h",
     "src/compiler/coalesced-live-ranges.cc",
     "src/compiler/coalesced-live-ranges.h",
     "src/compiler/code-assembler.cc",

src/compiler/checkpoint-elimination.cc

+// Copyright 2016 the V8 project authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "src/compiler/checkpoint-elimination.h"
+
+#include "src/compiler/node-properties.h"
+
+namespace v8 {
+namespace internal {
+namespace compiler {
+
+CheckpointElimination::CheckpointElimination(Editor* editor)
+    : AdvancedReducer(editor) {}
+
+namespace {
+
+// The given checkpoint is redundant if it is effect-wise dominated by another
+// checkpoint and there is no observable write in between. For now we consider
+// a linear effect chain only instead of true effect-wise dominance.
+bool IsRedundantCheckPoint(Node* node) {
+  Node* effect = NodeProperties::GetEffectInput(node);
+  while (effect->op()->HasProperty(Operator::kNoWrite) &&
+         effect->op()->EffectInputCount() == 1) {
+    if (effect->opcode() == IrOpcode::kCheckPoint) return true;
+    effect = NodeProperties::GetEffectInput(effect);
+  }
+  return false;
+}
+
+}  // namespace
+
+Reduction CheckpointElimination::Reduce(Node* node) {
+  if (node->opcode() != IrOpcode::kCheckPoint) return NoChange();
+  if (IsRedundantCheckPoint(node)) {
+    return Replace(NodeProperties::GetEffectInput(node));
+  }
+  return NoChange();
+}
+
+}  // namespace compiler
+}  // namespace internal
+}  // namespace v8

src/compiler/checkpoint-elimination.h

+// Copyright 2016 the V8 project authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef V8_COMPILER_CHECKPOINT_ELIMINATION_H_
+#define V8_COMPILER_CHECKPOINT_ELIMINATION_H_
+
+#include "src/compiler/graph-reducer.h"
+
+namespace v8 {
+namespace internal {
+namespace compiler {
+
+// Performs elimination of redundant checkpoints within the graph.
+class CheckpointElimination final : public AdvancedReducer {
+ public:
+  explicit CheckpointElimination(Editor* editor);
+  ~CheckpointElimination() final {}
+
+  Reduction Reduce(Node* node) final;
+};
+
+}  // namespace compiler
+}  // namespace internal
+}  // namespace v8
+
+#endif  // V8_COMPILER_CHECKPOINT_ELIMINATION_H_

src/compiler/pipeline.cc

@@ -14,6 +14,7 @@
 #include "src/compiler/basic-block-instrumentor.h"
 #include "src/compiler/branch-elimination.h"
 #include "src/compiler/bytecode-graph-builder.h"
+#include "src/compiler/checkpoint-elimination.h"
 #include "src/compiler/code-generator.h"
 #include "src/compiler/common-operator-reducer.h"
 #include "src/compiler/control-flow-optimizer.h"
@@ -891,6 +892,7 @@ struct TypedLoweringPhase {
             ? JSIntrinsicLowering::kDeoptimizationEnabled
             : JSIntrinsicLowering::kDeoptimizationDisabled);
     SimplifiedOperatorReducer simple_reducer(data->jsgraph());
+    CheckpointElimination checkpoint_elimination(&graph_reducer);
     CommonOperatorReducer common_reducer(&graph_reducer, data->graph(),
                                          data->common(), data->machine());
     AddReducer(data, &graph_reducer, &dead_code_elimination);
@@ -902,6 +904,7 @@ struct TypedLoweringPhase {
     AddReducer(data, &graph_reducer, &intrinsic_lowering);
     AddReducer(data, &graph_reducer, &load_elimination);
     AddReducer(data, &graph_reducer, &simple_reducer);
+    AddReducer(data, &graph_reducer, &checkpoint_elimination);
     AddReducer(data, &graph_reducer, &common_reducer);
     graph_reducer.ReduceGraph();
   }

src/v8.gyp

@@ -505,6 +505,8 @@
         'compiler/bytecode-graph-builder.cc',
         'compiler/bytecode-graph-builder.h',
         'compiler/c-linkage.cc',
+        'compiler/checkpoint-elimination.cc',
+        'compiler/checkpoint-elimination.h',
         'compiler/coalesced-live-ranges.cc',
         'compiler/coalesced-live-ranges.h',
         'compiler/code-generator-impl.h',

test/unittests/compiler/checkpoint-elimination-unittest.cc

+// Copyright 2016 the V8 project authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "src/compiler/checkpoint-elimination.h"
+#include "src/compiler/common-operator.h"
+#include "src/compiler/operator.h"
+#include "test/unittests/compiler/graph-reducer-unittest.h"
+#include "test/unittests/compiler/graph-unittest.h"
+#include "test/unittests/compiler/node-test-utils.h"
+
+using testing::StrictMock;
+
+namespace v8 {
+namespace internal {
+namespace compiler {
+
+class CheckpointEliminationTest : public GraphTest {
+ public:
+  CheckpointEliminationTest() : GraphTest() {}
+  ~CheckpointEliminationTest() override {}
+
+ protected:
+  Reduction Reduce(AdvancedReducer::Editor* editor, Node* node) {
+    CheckpointElimination reducer(editor);
+    return reducer.Reduce(node);
+  }
+
+  Reduction Reduce(Node* node) {
+    StrictMock<MockAdvancedReducerEditor> editor;
+    return Reduce(&editor, node);
+  }
+};
+
+namespace {
+
+const Operator kOpNoWrite(0, Operator::kNoWrite, "OpNoWrite", 0, 1, 0, 0, 1, 0);
+
+}  // namespace
+
+// -----------------------------------------------------------------------------
+// CheckPoint
+
+TEST_F(CheckpointEliminationTest, CheckPointChain) {
+  Node* const control = graph()->start();
+  Node* frame_state = EmptyFrameState();
+  Node* checkpoint1 = graph()->NewNode(common()->CheckPoint(), frame_state,
+                                       graph()->start(), control);
+  Node* effect_link = graph()->NewNode(&kOpNoWrite, checkpoint1);
+  Node* checkpoint2 = graph()->NewNode(common()->CheckPoint(), frame_state,
+                                       effect_link, control);
+  Reduction r = Reduce(checkpoint2);
+  ASSERT_TRUE(r.Changed());
+  EXPECT_EQ(effect_link, r.replacement());
+}
+
+}  // namespace compiler
+}  // namespace internal
+}  // namespace v8

test/unittests/unittests.gyp

@@ -41,6 +41,7 @@
         'cancelable-tasks-unittest.cc',
         'char-predicates-unittest.cc',
         'compiler/branch-elimination-unittest.cc',
+        'compiler/checkpoint-elimination-unittest.cc',
         'compiler/coalesced-live-ranges-unittest.cc',
         'compiler/common-operator-reducer-unittest.cc',
         'compiler/common-operator-unittest.cc',