[turbofan] Deferred block spilling heuristic - first step.

src/compiler/register-allocator.h

 // 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.
 
 #ifndef V8_REGISTER_ALLOCATOR_H_
 #define V8_REGISTER_ALLOCATOR_H_
 
 #include "src/compiler/instruction.h"
 #include "src/ostreams.h"
 #include "src/zone-containers.h"
@@ skipping 312 matching lines @@
   }
 
   // Returns use position in this live range that follows both start
   // and last processed use position.
   UsePosition* NextUsePosition(LifetimePosition start) const;
 
   // Returns use position for which register is required in this live
   // range and which follows both start and last processed use position
   UsePosition* NextRegisterPosition(LifetimePosition start) const;
 
+  // Returns the first use position requiring stack slot, or nullptr.
+  UsePosition* NextStackPosition(LifetimePosition start) const;
+
   // Returns use position for which register is beneficial in this live
   // range and which follows both start and last processed use position
   UsePosition* NextUsePositionRegisterIsBeneficial(
       LifetimePosition start) const;
 
   // Returns use position for which register is beneficial in this live
   // range and which precedes start.
   UsePosition* PreviousUsePositionRegisterIsBeneficial(
       LifetimePosition start) const;
 
@@ skipping 47 matching lines @@
   bool HasSpillRange() const { return spill_type() == SpillType::kSpillRange; }
   AllocatedOperand GetSpillRangeOperand() const;
 
   void SpillAtDefinition(Zone* zone, int gap_index,
                          InstructionOperand* operand);
   void SetSpillOperand(InstructionOperand* operand);
   void SetSpillRange(SpillRange* spill_range);
   void CommitSpillsAtDefinition(InstructionSequence* sequence,
                                 const InstructionOperand& operand,
                                 bool might_be_duplicated);
+  // This must be applied on top level ranges.
+  // If only one of the children(1) of this range is spilled, and the block is
+  // deferred(2), then we will spill in that block rather than at definition,
+  // to avoid the penalty of the spill operation.
+  // (1) if the top level were spilled, it means it is defined on the stack,
+  // so we wouldn't have had spilled it at definition in the first place. If
+  // more than one child is spilled, we would also need to change how we handle
+  // traced references, and we'd need to ensure the first spill dominates the
+  // rest.
+  // (2) if the block is not deferred, it may be on a hot path (or loop), case
+  // in which it may be worse to spill in it.
+  bool TryCommitSpillInDeferredBlock(InstructionSequence* code);
 
   void SetSpillStartIndex(int start) {
     spill_start_index_ = Min(start, spill_start_index_);
   }
 
   bool ShouldBeAllocatedBefore(const LiveRange* other) const;
   bool CanCover(LifetimePosition position) const;
   bool Covers(LifetimePosition position) const;
   LifetimePosition FirstIntersection(LiveRange* other) const;
 
@@ skipping 16 matching lines @@
 
   SpillAtDefinitionList* spills_at_definition() const {
     return spills_at_definition_;
   }
 
   // Used solely by the Greedy Allocator:
   unsigned GetSize();
   float weight() const { return weight_; }
   void set_weight(float weight) { weight_ = weight; }
 
+  bool IsSpilledInSingleDeferredBlock() const {
+    return spilled_in_deferred_block_;
+  }
+
   static const int kInvalidSize = -1;
   static const float kInvalidWeight;
   static const float kMaxWeight;
 
  private:
   void set_spill_type(SpillType value) {
     bits_ = SpillTypeField::update(bits_, value);
   }
 
   void set_spilled(bool value) { bits_ = SpilledField::update(bits_, value); }
@@ skipping 32 matching lines @@
   mutable UsePosition* current_hint_position_;
 
   // greedy: the number of LifetimePositions covered by this range. Used to
   // prioritize selecting live ranges for register assignment, as well as
   // in weight calculations.
   int size_;
 
   // greedy: a metric for resolving conflicts between ranges with an assigned
   // register and ranges that intersect them and need a register.
   float weight_;
+
+  // TODO(mtrofin): generalize spilling after definition, currently specialized
+  // just for spill in a single deferred block.
+  bool spilled_in_deferred_block_;
   DISALLOW_COPY_AND_ASSIGN(LiveRange);
 };
 
 
 struct PrintableLiveRange {
   const RegisterConfiguration* register_configuration_;
   const LiveRange* range_;
 };
 
 
@@ skipping 438 matching lines @@
   RegisterAllocationData* const data_;
 
   DISALLOW_COPY_AND_ASSIGN(LiveRangeConnector);
 };
 
 }  // namespace compiler
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_REGISTER_ALLOCATOR_H_