[turbofan] simplify gap ordering

src/compiler/instruction.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/common-operator.h"
 #include "src/compiler/graph.h"
 #include "src/compiler/instruction.h"
 
 namespace v8 {
 namespace internal {
@@ skipping 269 matching lines @@
     if (i > 0) os << ", ";
     printable_op.op_ = instr.OutputAt(i);
     os << printable_op;
   }
 
   if (instr.OutputCount() > 1) os << ") = ";
   if (instr.OutputCount() == 1) 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 << "(";
       if (gap->parallel_moves_[i] != NULL) {
         PrintableParallelMove ppm = {printable.register_configuration_,
                                      gap->parallel_moves_[i]};
         os << ppm;
       }
       os << ") ";
     }
@@ skipping 135 matching lines @@
       instructions_(zone()),
       next_virtual_register_(0),
       pointer_maps_(zone()),
       doubles_(std::less<int>(), VirtualRegisterSet::allocator_type(zone())),
       references_(std::less<int>(), VirtualRegisterSet::allocator_type(zone())),
       deoptimization_entries_(zone()) {
   block_starts_.reserve(instruction_blocks_->size());
 }
 
 
-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()));
 }
 
 
 void InstructionSequence::StartBlock(BasicBlock::RpoNumber rpo) {
   DCHECK(block_starts_.size() == rpo.ToSize());
   InstructionBlock* block = InstructionBlockAt(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());
     pointer_map->set_instruction_position(index);
     instr->set_pointer_map(pointer_map);
     pointer_maps_.push_back(pointer_map);
   }
   return index;
 }
 
@@ skipping 190 matching lines @@
       os << " B" << succ_block->id();
     }
     os << "\n";
   }
   return os;
 }
 
 }  // namespace compiler
 }  // namespace internal
 }  // namespace v8