[turbofan] Fix select lowering.

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(kMachInt32), p0, p1, p2);
 
   Capture<Node*> branch;
   Capture<Node*> merge;
   {
-    Reduction const r =
-        Reduce(graph()->NewNode(common()->Select(kMachInt32), p0, p1, p2));
+    Reduction const r = Reduce(s0);
     ASSERT_TRUE(r.Changed());
     EXPECT_THAT(
         r.replacement(),
         IsPhi(
             kMachInt32, 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(kMachInt32), p0, p3, p4));
     ASSERT_TRUE(r.Changed());
     EXPECT_THAT(r.replacement(), IsPhi(kMachInt32, 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(kMachInt32), p0, s0, p0));
+    ASSERT_TRUE(r.Changed());
+    EXPECT_THAT(r.replacement(),
+                IsPhi(kMachInt32, s0, p0, Not(CaptureEq(&merge))));
+  }
 }
 
 }  // namespace compiler
 }  // namespace internal
 }  // namespace v8