[turbofan] fine grained in-block move optimization

src/compiler/register-allocator.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/base/adapters.h"
 #include "src/compiler/linkage.h"
 #include "src/compiler/register-allocator.h"
 #include "src/string-stream.h"
 
 namespace v8 {
@@ skipping 3321 matching lines @@
         FindResult result;
         const InstructionBlock* pred_block = code()->InstructionBlockAt(pred);
         if (!array->FindConnectableSubranges(block, pred_block, &result)) {
           continue;
         }
         InstructionOperand pred_op = result.pred_cover_->GetAssignedOperand();
         InstructionOperand cur_op = result.cur_cover_->GetAssignedOperand();
         if (pred_op.Equals(cur_op)) continue;
         if (!pred_op.IsAnyRegister() && cur_op.IsAnyRegister()) {
           // We're doing a reload.
+          // We don't need to, if:
+          // 1) there's no register use in this block, and
+          // 2) the range ends before the block does, and
+          // 3) we don't have a successor, or the successor is spilled.
+          LifetimePosition block_start =
+              LifetimePosition::GapFromInstructionIndex(block->code_start());
+          LifetimePosition block_end =
+              LifetimePosition::GapFromInstructionIndex(block->code_end());
           const LiveRange* current = result.cur_cover_;
+          const LiveRange* successor = current->next();
+          if (current->End() < block_end &&
+              (successor == nullptr || successor->spilled())) {
+            // verify point 1: no register use. We can go to the end of the
+            // range, since it's all within the block.
 
+            bool uses_reg = false;
+            for (const UsePosition* use = current->NextUsePosition(block_start);
+                 use != nullptr; use = use->next()) {
+              if (use->operand()->IsAnyRegister()) {
+                uses_reg = true;
+                break;
+              }
+            }
+            if (!uses_reg) continue;
+          }
           if (current->TopLevel()->IsSpilledOnlyInDeferredBlocks() &&
               pred_block->IsDeferred()) {
             // The spill location should be defined in pred_block, so add
             // pred_block to the list of blocks requiring a spill operand.
             current->TopLevel()->GetListOfBlocksRequiringSpillOperands()->Add(
                 pred_block->rpo_number().ToInt());
           }
         }
         int move_loc = ResolveControlFlow(block, cur_op, pred_block, pred_op);
         USE(move_loc);
@@ skipping 198 matching lines @@
                            spill_operand);
       }
     }
   }
 }
 
 
 }  // namespace compiler
 }  // namespace internal
 }  // namespace v8