[turbofan] Deferred block spilling heuristic - first step.

src/compiler/register-allocator.h

Index: src/compiler/register-allocator.h
diff --git a/src/compiler/register-allocator.h b/src/compiler/register-allocator.h
index cd3a5f6ca5b1cb7aff1b38ee9e52917f52be2622..7d838aa3b69b7a64c6fb3b373a69ce21d8864713 100644
--- a/src/compiler/register-allocator.h
+++ b/src/compiler/register-allocator.h
@@ -334,6 +334,9 @@ class LiveRange final : public ZoneObject {
   // 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* NextSlotPosition(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(
@@ -401,6 +404,19 @@ class LiveRange final : public ZoneObject {
   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,
+                                     const InstructionOperand& spill_operand);
 
   void SetSpillStartIndex(int start) {
     spill_start_index_ = Min(start, spill_start_index_);
@@ -433,10 +449,16 @@ class LiveRange final : public ZoneObject {
   }
 
   // Used solely by the Greedy Allocator:
-  unsigned GetSize();
+  unsigned size() const { return size_; }
+  bool IsSizeValid() const { return size() != kInvalidSize; }
+  void UpdateSize();
   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;
@@ -489,6 +511,10 @@ class LiveRange final : public ZoneObject {
   // 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);
 };