[turbofan] simplify gap ordering

src/compiler/instruction.cc

Index: src/compiler/instruction.cc
diff --git a/src/compiler/instruction.cc b/src/compiler/instruction.cc
index 827cacf665107f13b0f49315c02f713a8fa5277c..86550d0e1d3291538029afa4152d9d94a56fd178 100644
--- a/src/compiler/instruction.cc
+++ b/src/compiler/instruction.cc
@@ -287,7 +287,7 @@ std::ostream& operator<<(std::ostream& os,
 
   if (instr.IsGapMoves()) {
     const GapInstruction* gap = GapInstruction::cast(&instr);
-    os << (instr.IsBlockStart() ? " block-start" : "gap ");
+    os << "gap ";
     for (int i = GapInstruction::FIRST_INNER_POSITION;
          i <= GapInstruction::LAST_INNER_POSITION; i++) {
       os << "(";
@@ -443,10 +443,10 @@ InstructionSequence::InstructionSequence(Zone* instruction_zone,
 }
 
 
-BlockStartInstruction* InstructionSequence::GetBlockStart(
+GapInstruction* InstructionSequence::GetBlockStart(
     BasicBlock::RpoNumber rpo) const {
   const InstructionBlock* block = InstructionBlockAt(rpo);
-  return BlockStartInstruction::cast(InstructionAt(block->code_start()));
+  return GapInstruction::cast(InstructionAt(block->code_start()));
 }
 
 
@@ -456,26 +456,26 @@ void InstructionSequence::StartBlock(BasicBlock::RpoNumber rpo) {
   int code_start = static_cast<int>(instructions_.size());
   block->set_code_start(code_start);
   block_starts_.push_back(code_start);
-  BlockStartInstruction* block_start = BlockStartInstruction::New(zone());
-  AddInstruction(block_start);
 }
 
 
 void InstructionSequence::EndBlock(BasicBlock::RpoNumber rpo) {
   int end = static_cast<int>(instructions_.size());
   InstructionBlock* block = InstructionBlockAt(rpo);
+  if (block->code_start() == end) {  // Empty block.  Insert a nop.
+    AddInstruction(Instruction::New(zone(), kArchNop));
+    end = static_cast<int>(instructions_.size());
+  }
   DCHECK(block->code_start() >= 0 && block->code_start() < end);
   block->set_code_end(end);
 }
 
 
 int InstructionSequence::AddInstruction(Instruction* instr) {
-  // TODO(titzer): the order of these gaps is a holdover from Lithium.
   GapInstruction* gap = GapInstruction::New(zone());
-  if (instr->IsControl()) instructions_.push_back(gap);
+  instructions_.push_back(gap);
   int index = static_cast<int>(instructions_.size());
   instructions_.push_back(instr);
-  if (!instr->IsControl()) instructions_.push_back(gap);
   if (instr->NeedsPointerMap()) {
     DCHECK(instr->pointer_map() == NULL);
     PointerMap* pointer_map = new (zone()) PointerMap(zone());