[turbofan] Remove some clever-but-wrong bits from select lowering.

BUG=chromium:600593
LOG=n
R=bmeurer@chromium.org

Review URL: https://codereview.chromium.org/1870763003

Cr-Commit-Position: refs/heads/master@{#35347}

src/compiler/select-lowering.cc

@@ -15,10 +15,7 @@ namespace internal {
 namespace compiler {
 
 SelectLowering::SelectLowering(Graph* graph, CommonOperatorBuilder* common)
-    : common_(common),
-      graph_(graph),
-      merges_(Merges::key_compare(), Merges::allocator_type(graph->zone())) {}
-
+    : common_(common), graph_(graph) {}
 
 SelectLowering::~SelectLowering() {}
 
@@ -30,58 +27,16 @@ Reduction SelectLowering::Reduce(Node* node) {
   Node* cond = node->InputAt(0);
   Node* vthen = node->InputAt(1);
   Node* velse = node->InputAt(2);
-  Node* merge = nullptr;
 
-  // Check if we already have a diamond for this condition.
-  auto range = merges_.equal_range(cond);
-  for (auto i = range.first;; ++i) {
-    if (i == range.second) {
-      // Create a new diamond for this condition and remember its merge node.
+  // Create a diamond and a phi.
   Diamond d(graph(), common(), cond, p.hint());
-      merges_.insert(std::make_pair(cond, d.merge));
-      merge = d.merge;
-      break;
-    }
-
-    // If the diamond is reachable from the Select, merging them would result in
-    // an unschedulable graph, so we cannot reuse the diamond in that case.
-    merge = i->second;
-    if (!ReachableFrom(merge, node)) {
-      break;
-    }
-  }
-
-  // Create a Phi hanging off the previously determined merge.
   node->ReplaceInput(0, vthen);
   node->ReplaceInput(1, velse);
-  node->ReplaceInput(2, merge);
+  node->ReplaceInput(2, d.merge);
   NodeProperties::ChangeOp(node, common()->Phi(p.representation(), 2));
   return Changed(node);
 }
 
-
-bool SelectLowering::ReachableFrom(Node* const sink, Node* const source) {
-  // TODO(turbofan): This is probably horribly expensive, and it should be moved
-  // into node.h or somewhere else?!
-  Zone zone(graph()->zone()->allocator());
-  std::queue<Node*, NodeDeque> queue((NodeDeque(&zone)));
-  BoolVector visited(graph()->NodeCount(), false, &zone);
-  queue.push(source);
-  visited[source->id()] = true;
-  while (!queue.empty()) {
-    Node* current = queue.front();
-    if (current == sink) return true;
-    queue.pop();
-    for (auto input : current->inputs()) {
-      if (!visited[input->id()]) {
-        queue.push(input);
-        visited[input->id()] = true;
-      }
-    }
-  }
-  return false;
-}
-
 }  // namespace compiler
 }  // namespace internal
 }  // namespace v8

src/compiler/select-lowering.h

@@ -5,10 +5,7 @@
 #ifndef V8_COMPILER_SELECT_LOWERING_H_
 #define V8_COMPILER_SELECT_LOWERING_H_
 
-#include <map>
-
 #include "src/compiler/graph-reducer.h"
-#include "src/zone-allocator.h"
 
 namespace v8 {
 namespace internal {
@@ -28,17 +25,11 @@ class SelectLowering final : public Reducer {
   Reduction Reduce(Node* node) override;
 
  private:
-  typedef std::multimap<Node*, Node*, std::less<Node*>,
-                        zone_allocator<std::pair<Node* const, Node*>>> Merges;
-
-  bool ReachableFrom(Node* const sink, Node* const source);
-
   CommonOperatorBuilder* common() const { return common_; }
   Graph* graph() const { return graph_; }
 
   CommonOperatorBuilder* common_;
   Graph* graph_;
-  Merges merges_;
 };
 
 }  // namespace compiler

test/mjsunit/compiler/regress-600593.js

+// 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.
+
+// Flags: --allow-natives-syntax
+
+"use strict"
+
+function f(c) {
+  if (c) { throw new Error(); }
+  throw new Error();
+};
+
+function Error()  {
+  return arguments.length;
+}
+
+assertThrows(function() { f(true); });
+assertThrows(function() { f(false); });
+%OptimizeFunctionOnNextCall(f);
+assertThrows(function() { f(true); });

test/unittests/compiler/select-lowering-unittest.cc

-// Copyright 2014 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/select-lowering.h"
-#include "test/unittests/compiler/graph-unittest.h"
-#include "test/unittests/compiler/node-test-utils.h"
-#include "testing/gmock-support.h"
-
-using testing::AllOf;
-using testing::Capture;
-using testing::CaptureEq;
-using testing::Not;
-
-namespace v8 {
-namespace internal {
-namespace compiler {
-
-class SelectLoweringTest : public GraphTest {
- public:
-  SelectLoweringTest() : GraphTest(5), lowering_(graph(), common()) {}
-
- protected:
-  Reduction Reduce(Node* node) { return lowering_.Reduce(node); }
-
- private:
-  SelectLowering lowering_;
-};
-
-
-TEST_F(SelectLoweringTest, SelectWithSameConditions) {
-  Node* const p0 = Parameter(0);
-  Node* const p1 = Parameter(1);
-  Node* const p2 = Parameter(2);
-  Node* const p3 = Parameter(3);
-  Node* const p4 = Parameter(4);
-  Node* const s0 = graph()->NewNode(
-      common()->Select(MachineRepresentation::kWord32), p0, p1, p2);
-
-  Capture<Node*> branch;
-  Capture<Node*> merge;
-  {
-    Reduction const r = Reduce(s0);
-    ASSERT_TRUE(r.Changed());
-    EXPECT_THAT(
-        r.replacement(),
-        IsPhi(
-            MachineRepresentation::kWord32, p1, p2,
-            AllOf(CaptureEq(&merge),
-                  IsMerge(IsIfTrue(CaptureEq(&branch)),
-                          IsIfFalse(AllOf(CaptureEq(&branch),
-                                          IsBranch(p0, graph()->start())))))));
-  }
-  {
-    Reduction const r = Reduce(graph()->NewNode(
-        common()->Select(MachineRepresentation::kWord32), p0, p3, p4));
-    ASSERT_TRUE(r.Changed());
-    EXPECT_THAT(r.replacement(), IsPhi(MachineRepresentation::kWord32, p3, p4,
-                                       CaptureEq(&merge)));
-  }
-  {
-    // We must not reuse the diamond if it is reachable from either else/then
-    // values of the Select, because the resulting graph can not be scheduled.
-    Reduction const r = Reduce(graph()->NewNode(
-        common()->Select(MachineRepresentation::kWord32), p0, s0, p0));
-    ASSERT_TRUE(r.Changed());
-    EXPECT_THAT(r.replacement(), IsPhi(MachineRepresentation::kWord32, s0, p0,
-                                       Not(CaptureEq(&merge))));
-  }
-}
-
-}  // namespace compiler
-}  // namespace internal
-}  // namespace v8

test/unittests/unittests.gyp

@@ -83,7 +83,6 @@
         'compiler/opcodes-unittest.cc',
         'compiler/register-allocator-unittest.cc',
         'compiler/schedule-unittest.cc',
-        'compiler/select-lowering-unittest.cc',
         'compiler/scheduler-unittest.cc',
         'compiler/scheduler-rpo-unittest.cc',
         'compiler/simplified-operator-reducer-unittest.cc',