[compiler] Drop the CompareNilIC.

src/crankshaft/hydrogen-instructions.h

Index: src/crankshaft/hydrogen-instructions.h
diff --git a/src/crankshaft/hydrogen-instructions.h b/src/crankshaft/hydrogen-instructions.h
index 22ed052ba39eccaf4500e8366d553765416b09e1..11381a2194161ef7522f9fccda27e69deb500947 100644
--- a/src/crankshaft/hydrogen-instructions.h
+++ b/src/crankshaft/hydrogen-instructions.h
@@ -4429,13 +4429,22 @@ class HIsUndetectableAndBranch final : public HUnaryControlInstruction {
 
   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.

 };