[turbofan] put gaps before instructions

src/compiler/instruction.h

Index: src/compiler/instruction.h
diff --git a/src/compiler/instruction.h b/src/compiler/instruction.h
index 1740dfbf25b970054417d75c533ba95e8ea65366..19d50a530aa0b33a474b85efe81ea1dac1a68f68 100644
--- a/src/compiler/instruction.h
+++ b/src/compiler/instruction.h
@@ -495,9 +495,7 @@ class Instruction : public ZoneObject {
   bool NeedsPointerMap() const { return IsCall(); }
   bool HasPointerMap() const { return pointer_map_ != NULL; }
 
-  bool IsGapMoves() const {
-    return opcode() == kGapInstruction || opcode() == kBlockStartInstruction;
-  }
+  bool IsGapMoves() const { return opcode() == kGapInstruction; }
   bool IsBlockStart() const { return opcode() == kBlockStartInstruction; }
   bool IsSourcePosition() const {
     return opcode() == kSourcePositionInstruction;
@@ -601,7 +599,7 @@ class GapInstruction : public Instruction {
 
   static GapInstruction* New(Zone* zone) {
     void* buffer = zone->New(sizeof(GapInstruction));
-    return new (buffer) GapInstruction(kGapInstruction);
+    return new (buffer) GapInstruction();
   }
 
   static GapInstruction* cast(Instruction* instr) {
@@ -614,24 +612,22 @@ class GapInstruction : public Instruction {
     return static_cast<const GapInstruction*>(instr);
   }
 
- protected:
-  explicit GapInstruction(InstructionCode opcode) : Instruction(opcode) {
+ private:
+  GapInstruction() : Instruction(kGapInstruction) {
     parallel_moves_[BEFORE] = NULL;
     parallel_moves_[START] = NULL;
     parallel_moves_[END] = NULL;
     parallel_moves_[AFTER] = NULL;
   }
 
- private:
   friend std::ostream& operator<<(std::ostream& os,
                                   const PrintableInstruction& instr);
   ParallelMove* parallel_moves_[LAST_INNER_POSITION + 1];
 };
 
 
-// This special kind of gap move instruction represents the beginning of a
-// block of code.
-class BlockStartInstruction FINAL : public GapInstruction {
+// This is a placeholder instruction marking the start of a block.
+class BlockStartInstruction FINAL : public Instruction {
  public:
   static BlockStartInstruction* New(Zone* zone) {
     void* buffer = zone->New(sizeof(BlockStartInstruction));
@@ -649,7 +645,7 @@ class BlockStartInstruction FINAL : public GapInstruction {
   }
 
  private:
-  BlockStartInstruction() : GapInstruction(kBlockStartInstruction) {}
+  BlockStartInstruction() : Instruction(kBlockStartInstruction) {}
 };
 
 
@@ -957,7 +953,7 @@ class InstructionSequence FINAL : public ZoneObject {
 
   void AddGapMove(int index, InstructionOperand* from, InstructionOperand* to);
 
-  BlockStartInstruction* GetBlockStart(BasicBlock::RpoNumber rpo) const;
+  const BlockStartInstruction* GetBlockStart(BasicBlock::RpoNumber rpo) const;
 
   typedef InstructionDeque::const_iterator const_iterator;
   const_iterator begin() const { return instructions_.begin(); }