[compiler] Drop the CompareNilIC.

src/crankshaft/hydrogen-instructions.h

 // Copyright 2012 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_CRANKSHAFT_HYDROGEN_INSTRUCTIONS_H_
 #define V8_CRANKSHAFT_HYDROGEN_INSTRUCTIONS_H_
 
 #include <cstring>
 #include <iosfwd>
 
@@ skipping 4411 matching lines @@
   DECLARE_INSTRUCTION_FACTORY_P1(HIsUndetectableAndBranch, HValue*);
   DECLARE_INSTRUCTION_FACTORY_P3(HIsUndetectableAndBranch, HValue*,
                                  HBasicBlock*, HBasicBlock*);
 
   Representation RequiredInputRepresentation(int index) override {
     return Representation::Tagged();
   }
 
   bool KnownSuccessorBlock(HBasicBlock** block) override;
 
+  static const int kNoKnownSuccessorIndex = -1;
+  int known_successor_index() const { return known_successor_index_; }
+  void set_known_successor_index(int known_successor_index) {
+    known_successor_index_ = known_successor_index;
+  }
+
   DECLARE_CONCRETE_INSTRUCTION(IsUndetectableAndBranch)
 
  private:
   HIsUndetectableAndBranch(HValue* value,
                            HBasicBlock* true_target = NULL,
                            HBasicBlock* false_target = NULL)
-      : HUnaryControlInstruction(value, true_target, false_target) {}
+      : HUnaryControlInstruction(value, true_target, false_target),
+        known_successor_index_(kNoKnownSuccessorIndex) {}
+
+  int known_successor_index_;

[danno, 2016/02/24 08:25:40]

Maybe I'm missing something, but it doesn't look like known_successor_index_ is
ever set to anything other than  kNoKnownSuccessorIndex, and is
set_known_successor_index isn't called in your patch nor is it a virtual method
that could be called somewhere else. It looks like your logic to make the
successor decision is completely in KnownSuccessorBlock based on the Types. Is
all that known_successor_index stuff then even necessary?

[Benedikt Meurer, 2016/02/24 08:41:18]

Right, I tried to add support for HIsUndetectableAndBranch to check elimination,
but undid that change because there was no benefit; forgot to remove the
known_successor_index_ later.

 };
 
 
 class HStringCompareAndBranch final : public HTemplateControlInstruction<2, 3> {
  public:
   DECLARE_INSTRUCTION_WITH_CONTEXT_FACTORY_P3(HStringCompareAndBranch,
                                               HValue*,
                                               HValue*,
                                               Token::Value);
 
@@ skipping 3219 matching lines @@
 
 
 
 #undef DECLARE_INSTRUCTION
 #undef DECLARE_CONCRETE_INSTRUCTION
 
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_CRANKSHAFT_HYDROGEN_INSTRUCTIONS_H_