[turbofan] New Register Allocator Validator

src/compiler/register-allocator-verifier.cc

Index: src/compiler/register-allocator-verifier.cc
diff --git a/src/compiler/register-allocator-verifier.cc b/src/compiler/register-allocator-verifier.cc
index f2160f52ce7a788530be617526b25f5a7d286740..eddcb3657a6a373ec56067f68aa4482b29d5ea6a 100644
--- a/src/compiler/register-allocator-verifier.cc
+++ b/src/compiler/register-allocator-verifier.cc
@@ -44,39 +44,17 @@ void VerifyAllocatedGaps(const Instruction* instr) {
 
 }  // namespace
 
-
-void RegisterAllocatorVerifier::VerifyInput(
-    const OperandConstraint& constraint) {
-  CHECK_NE(kSameAsFirst, constraint.type_);
-  if (constraint.type_ != kImmediate && constraint.type_ != kExplicit) {
-    CHECK_NE(InstructionOperand::kInvalidVirtualRegister,
-             constraint.virtual_register_);
-  }
-}
-
-
-void RegisterAllocatorVerifier::VerifyTemp(
-    const OperandConstraint& constraint) {
-  CHECK_NE(kSameAsFirst, constraint.type_);
-  CHECK_NE(kImmediate, constraint.type_);
-  CHECK_NE(kExplicit, constraint.type_);
-  CHECK_NE(kConstant, constraint.type_);
-}
-
-
-void RegisterAllocatorVerifier::VerifyOutput(
-    const OperandConstraint& constraint) {
-  CHECK_NE(kImmediate, constraint.type_);
-  CHECK_NE(kExplicit, constraint.type_);
-  CHECK_NE(InstructionOperand::kInvalidVirtualRegister,
-           constraint.virtual_register_);
-}
-
-
 RegisterAllocatorVerifier::RegisterAllocatorVerifier(
     Zone* zone, const RegisterConfiguration* config,
     const InstructionSequence* sequence)
-    : zone_(zone), config_(config), sequence_(sequence), constraints_(zone) {
+    : zone_(zone),
+      config_(config),
+      sequence_(sequence),
+      constraints_(zone),
+      assessments_(zone),
+      outstanding_assessments_(zone),
+      worklist_(zone),
+      seen_(zone) {
   constraints_.reserve(sequence->instructions().size());
   // TODO(dcarney): model unique constraints.
   // Construct OperandConstraints for all InstructionOperands, eliminating
@@ -111,6 +89,30 @@ RegisterAllocatorVerifier::RegisterAllocatorVerifier(
   }
 }
 
+void RegisterAllocatorVerifier::VerifyInput(
+    const OperandConstraint& constraint) {
+  CHECK_NE(kSameAsFirst, constraint.type_);
+  if (constraint.type_ != kImmediate && constraint.type_ != kExplicit) {
+    CHECK_NE(InstructionOperand::kInvalidVirtualRegister,
+             constraint.virtual_register_);
+  }
+}
+
+void RegisterAllocatorVerifier::VerifyTemp(
+    const OperandConstraint& constraint) {
+  CHECK_NE(kSameAsFirst, constraint.type_);
+  CHECK_NE(kImmediate, constraint.type_);
+  CHECK_NE(kExplicit, constraint.type_);
+  CHECK_NE(kConstant, constraint.type_);
+}
+
+void RegisterAllocatorVerifier::VerifyOutput(
+    const OperandConstraint& constraint) {
+  CHECK_NE(kImmediate, constraint.type_);
+  CHECK_NE(kExplicit, constraint.type_);
+  CHECK_NE(InstructionOperand::kInvalidVirtualRegister,
+           constraint.virtual_register_);
+}
 
 void RegisterAllocatorVerifier::VerifyAssignment() {
   CHECK(sequence()->instructions().size() == constraints()->size());
@@ -138,7 +140,6 @@ void RegisterAllocatorVerifier::VerifyAssignment() {
   }
 }
 
-
 void RegisterAllocatorVerifier::BuildConstraint(const InstructionOperand* op,
                                                 OperandConstraint* constraint) {
   constraint->value_ = kMinInt;
@@ -204,7 +205,6 @@ void RegisterAllocatorVerifier::BuildConstraint(const InstructionOperand* op,
   }
 }
 
-
 void RegisterAllocatorVerifier::CheckConstraint(
     const InstructionOperand* op, const OperandConstraint* constraint) {
   switch (constraint->type_) {
@@ -264,457 +264,217 @@ void RegisterAllocatorVerifier::CheckConstraint(
   }
 }
 
-namespace {
-
-typedef RpoNumber Rpo;
-
-static const int kInvalidVreg = InstructionOperand::kInvalidVirtualRegister;
-
-struct PhiData : public ZoneObject {
-  PhiData(Rpo definition_rpo, const PhiInstruction* phi, int first_pred_vreg,
-          const PhiData* first_pred_phi, Zone* zone)
-      : definition_rpo(definition_rpo),
-        virtual_register(phi->virtual_register()),
-        first_pred_vreg(first_pred_vreg),
-        first_pred_phi(first_pred_phi),
-        operands(zone) {
-    operands.reserve(phi->operands().size());
-    operands.insert(operands.begin(), phi->operands().begin(),
-                    phi->operands().end());
-  }
-  const Rpo definition_rpo;
-  const int virtual_register;
-  const int first_pred_vreg;
-  const PhiData* first_pred_phi;
-  IntVector operands;
-};
-
-class PhiMap : public ZoneMap<int, PhiData*>, public ZoneObject {
- public:
-  explicit PhiMap(Zone* zone) : ZoneMap<int, PhiData*>(zone) {}
-};
-
-struct OperandLess {
-  bool operator()(const InstructionOperand* a,
-                  const InstructionOperand* b) const {
-    return a->CompareCanonicalized(*b);
-  }
-};
-
-class OperandMap : public ZoneObject {
- public:
-  struct MapValue : public ZoneObject {
-    MapValue()
-        : incoming(nullptr),
-          define_vreg(kInvalidVreg),
-          use_vreg(kInvalidVreg),
-          succ_vreg(kInvalidVreg) {}
-    MapValue* incoming;  // value from first predecessor block.
-    int define_vreg;     // valid if this value was defined in this block.
-    int use_vreg;        // valid if this value was used in this block.
-    int succ_vreg;       // valid if propagated back from successor block.
-  };
-
-  class Map
-      : public ZoneMap<const InstructionOperand*, MapValue*, OperandLess> {
-   public:
-    explicit Map(Zone* zone)
-        : ZoneMap<const InstructionOperand*, MapValue*, OperandLess>(zone) {}
-
-    // Remove all entries with keys not in other.
-    void Intersect(const Map& other) {
-      if (this->empty()) return;
-      auto it = this->begin();
-      OperandLess less;
-      for (const std::pair<const InstructionOperand*, MapValue*>& o : other) {
-        while (less(it->first, o.first)) {
-          this->erase(it++);
-          if (it == this->end()) return;
-        }
-        if (it->first->EqualsCanonicalized(*o.first)) {
-          ++it;
-          if (it == this->end()) return;
-        } else {
-          CHECK(less(o.first, it->first));
-        }
-      }
-    }
-  };
-
-  explicit OperandMap(Zone* zone) : map_(zone) {}
-
-  Map& map() { return map_; }
+void BlockAssessments::PerformMoves(const Instruction* instruction) {
+  const ParallelMove* first =
+      instruction->GetParallelMove(Instruction::GapPosition::START);
+  PerformParallelMoves(first);
+  const ParallelMove* last =
+      instruction->GetParallelMove(Instruction::GapPosition::END);
+  PerformParallelMoves(last);
+}
 
-  void RunParallelMoves(Zone* zone, const ParallelMove* moves) {
-    // Compute outgoing mappings.
-    Map to_insert(zone);
-    for (const MoveOperands* move : *moves) {
-      if (move->IsEliminated()) continue;
-      auto cur = map().find(&move->source());
-      CHECK(cur != map().end());
-      auto res =
-          to_insert.insert(std::make_pair(&move->destination(), cur->second));
-      // Ensure injectivity of moves.
-      CHECK(res.second);
-    }
-    // Drop current mappings.
-    for (const MoveOperands* move : *moves) {
-      if (move->IsEliminated()) continue;
-      auto cur = map().find(&move->destination());
-      if (cur != map().end()) map().erase(cur);
-    }
-    // Insert new values.
-    map().insert(to_insert.begin(), to_insert.end());
+void BlockAssessments::PerformParallelMoves(const ParallelMove* moves) {
+  if (moves == nullptr) return;
+
+  CHECK(map_for_moves_.empty());
+  for (MoveOperands* move : *moves) {
+    if (move->IsEliminated() || move->IsRedundant()) continue;
+    auto it = map_.find(move->source());
+    // The RHS of a parallel move should have been already assessed.
+    CHECK(it != map_.end());
+    // The LHS of a parallel move should not have been assigned in this
+    // parallel move.
+    CHECK(map_for_moves_.find(move->destination()) == map_for_moves_.end());
+    // Copy the assessment to the destination.
+    map_for_moves_[move->destination()] = it->second;
   }
-
-  void RunGaps(Zone* zone, const Instruction* instr) {
-    for (int i = Instruction::FIRST_GAP_POSITION;
-         i <= Instruction::LAST_GAP_POSITION; i++) {
-      Instruction::GapPosition inner_pos =
-          static_cast<Instruction::GapPosition>(i);
-      const ParallelMove* move = instr->GetParallelMove(inner_pos);
-      if (move == nullptr) continue;
-      RunParallelMoves(zone, move);
-    }
+  for (auto pair : map_for_moves_) {
+    map_[pair.first] = pair.second;
   }
+  map_for_moves_.clear();
+}
 
-  void Drop(const InstructionOperand* op) {
-    auto it = map().find(op);
-    if (it != map().end()) map().erase(it);
+void BlockAssessments::DropRegisters() {
+  for (auto iterator = map().begin(), end = map().end(); iterator != end;) {
+    auto current = iterator;
+    ++iterator;
+    InstructionOperand op = current->first;
+    if (op.IsAnyRegister()) map().erase(current);
   }
+}
 
-  void DropRegisters(const RegisterConfiguration* config) {
-    // TODO(dcarney): sort map by kind and drop range.
-    for (auto it = map().begin(); it != map().end();) {
-      const InstructionOperand* op = it->first;
-      if (op->IsRegister() || op->IsDoubleRegister()) {
-        map().erase(it++);
-      } else {
-        ++it;
+BlockAssessments* RegisterAllocatorVerifier::CreateForBlock(
+    const InstructionBlock* block) {
+  RpoNumber current_block_id = block->rpo_number();
+
+  BlockAssessments* ret = new (zone()) BlockAssessments(zone());
+  if (block->PredecessorCount() == 0) {
+    // TODO(mtrofin): the following check should hold, however, in certain
+    // unit tests it is invalidated by the last block. Investigate and
+    // normalize the CFG.
+    // CHECK(current_block_id.ToInt() == 0);
+    // The phi size test below is because we can, technically, have phi
+    // instructions with one argument. Some tests expose that, too.
+  } else if (block->PredecessorCount() == 1 && block->phis().size() == 0) {
+    const BlockAssessments* prev_block = assessments_[block->predecessors()[0]];
+    ret->CopyFrom(prev_block);
+  } else {
+    for (RpoNumber pred_id : block->predecessors()) {
+      // For every operand coming from any of the predecessors, create an
+      // Unfinalized assessment.
+      auto iterator = assessments_.find(pred_id);
+      if (iterator == assessments_.end()) {
+        // This block is the head of a loop, and this predecessor is the
+        // loopback
+        // arc.
+        // Validate this is a loop case, otherwise the CFG is malformed.
+        CHECK(pred_id >= current_block_id);
+        CHECK(block->IsLoopHeader());
+        continue;
+      }
+      const BlockAssessments* pred_assessments = iterator->second;
+      CHECK_NOT_NULL(pred_assessments);
+      for (auto pair : pred_assessments->map()) {
+        InstructionOperand operand = pair.first;
+        if (ret->map().find(operand) == ret->map().end()) {
+          ret->map().insert(std::make_pair(
+              operand, new (zone()) PendingAssessment(block, operand)));
+        }
       }
     }
   }
+  return ret;
+}
 
-  MapValue* Define(Zone* zone, const InstructionOperand* op,
-                   int virtual_register) {
-    MapValue* value = new (zone) MapValue();
-    value->define_vreg = virtual_register;
-    auto res = map().insert(std::make_pair(op, value));
-    if (!res.second) res.first->second = value;
-    return value;
-  }
-
-  void Use(const InstructionOperand* op, int use_vreg, bool initial_pass) {
-    auto it = map().find(op);
-    CHECK(it != map().end());
-    MapValue* v = it->second;
-    if (v->define_vreg != kInvalidVreg) {
-      CHECK_EQ(v->define_vreg, use_vreg);
-    }
-    // Already used this vreg in this block.
-    if (v->use_vreg != kInvalidVreg) {
-      CHECK_EQ(v->use_vreg, use_vreg);
-      return;
-    }
-    if (!initial_pass) {
-      // A value may be defined and used in this block or the use must have
-      // propagated up.
-      if (v->succ_vreg != kInvalidVreg) {
-        CHECK_EQ(v->succ_vreg, use_vreg);
-      } else {
-        CHECK_EQ(v->define_vreg, use_vreg);
+void RegisterAllocatorVerifier::ValidatePendingAssessment(
+    RpoNumber block_id, InstructionOperand op, PendingAssessment* assessment,
+    int virtual_register) {
+  // When validating a pending assessment, it is possible some of the
+  // assessments
+  // for the original operand (the one where the assessment was created for
+  // first) are also pending. To avoid recursion, we use a work list. To
+  // deal with cycles, we keep a set of seen nodes.
+  CHECK(worklist_.empty());
+  CHECK(seen_.empty());
+  worklist_.push(std::make_pair(assessment, virtual_register));
+  seen_.insert(block_id);
+
+  while (!worklist_.empty()) {
+    auto work = worklist_.front();
+    PendingAssessment* current_assessment = work.first;
+    int current_virtual_register = work.second;
+    InstructionOperand current_operand = current_assessment->operand();
+    worklist_.pop();
+
+    const InstructionBlock* origin = current_assessment->origin();
+    CHECK(origin->PredecessorCount() > 1 || origin->phis().size() > 0);
+
+    // Check if the virtual register is a phi first, instead of relying on
+    // the incoming assessments. In particular, this handles the case
+    // v1 = phi v0 v0, which structurally is identical to v0 having been
+    // defined at the top of a diamond, and arriving at the node joining the
+    // diamond's branches.
+    const PhiInstruction* phi = nullptr;
+    for (const PhiInstruction* candidate : origin->phis()) {
+      if (candidate->virtual_register() == current_virtual_register) {
+        phi = candidate;
+        break;
       }
-      // Mark the use.
-      it->second->use_vreg = use_vreg;
-      return;
     }
-    // Go up block list and ensure the correct definition is reached.
-    for (; v != nullptr; v = v->incoming) {
-      // Value unused in block.
-      if (v->define_vreg == kInvalidVreg && v->use_vreg == kInvalidVreg) {
-        continue;
-      }
-      // Found correct definition or use.
-      CHECK(v->define_vreg == use_vreg || v->use_vreg == use_vreg);
-      // Mark the use.
-      it->second->use_vreg = use_vreg;
-      return;
-    }
-    // Use of a non-phi value without definition.
-    CHECK(false);
-  }
 
-  void UsePhi(const InstructionOperand* op, const PhiData* phi,
-              bool initial_pass) {
-    auto it = map().find(op);
-    CHECK(it != map().end());
-    MapValue* v = it->second;
-    int use_vreg = phi->virtual_register;
-    // Phis are not defined.
-    CHECK_EQ(kInvalidVreg, v->define_vreg);
-    // Already used this vreg in this block.
-    if (v->use_vreg != kInvalidVreg) {
-      CHECK_EQ(v->use_vreg, use_vreg);
-      return;
-    }
-    if (!initial_pass) {
-      // A used phi must have propagated its use to a predecessor.
-      CHECK_EQ(v->succ_vreg, use_vreg);
-      // Mark the use.
-      v->use_vreg = use_vreg;
-      return;
-    }
-    // Go up the block list starting at the first predecessor and ensure this
-    // phi has a correct use or definition.
-    for (v = v->incoming; v != nullptr; v = v->incoming) {
-      // Value unused in block.
-      if (v->define_vreg == kInvalidVreg && v->use_vreg == kInvalidVreg) {
-        continue;
-      }
-      // Found correct definition or use.
-      if (v->define_vreg != kInvalidVreg) {
-        CHECK(v->define_vreg == phi->first_pred_vreg);
-      } else if (v->use_vreg != phi->first_pred_vreg) {
-        // Walk the phi chain, hunting for a matching phi use.
-        const PhiData* p = phi;
-        for (; p != nullptr; p = p->first_pred_phi) {
-          if (p->virtual_register == v->use_vreg) break;
+    int op_index = 0;
+    for (RpoNumber pred : origin->predecessors()) {
+      int expected =
+          phi != nullptr ? phi->operands()[op_index] : current_virtual_register;
+
+      ++op_index;
+      auto pred_assignment = assessments_.find(pred);
+      if (pred_assignment == assessments_.end()) {
+        CHECK(origin->IsLoopHeader());
+        auto todo_iter = outstanding_assessments_.find(pred);
+        DelayedAssessments* set = nullptr;
+        if (todo_iter == outstanding_assessments_.end()) {
+          set = new (zone()) DelayedAssessments(zone());
+          outstanding_assessments_.insert(std::make_pair(pred, set));
+        } else {
+          set = todo_iter->second;
         }
-        CHECK(p);
+        set->AddDelayedAssessment(current_operand, expected);
+        continue;
       }
-      // Mark the use.
-      it->second->use_vreg = use_vreg;
-      return;
-    }
-    // Use of a phi value without definition.
-    UNREACHABLE();
-  }
-
- private:
-  Map map_;
-  DISALLOW_COPY_AND_ASSIGN(OperandMap);
-};
 
-}  // namespace
+      const BlockAssessments* pred_assessments = pred_assignment->second;
+      auto found_contribution = pred_assessments->map().find(current_operand);
+      CHECK(found_contribution != pred_assessments->map().end());
+      Assessment* contribution = found_contribution->second;
 
-
-class RegisterAllocatorVerifier::BlockMaps {
- public:
-  BlockMaps(Zone* zone, const InstructionSequence* sequence)
-      : zone_(zone),
-        sequence_(sequence),
-        phi_map_guard_(sequence->VirtualRegisterCount(), zone),
-        phi_map_(zone),
-        incoming_maps_(zone),
-        outgoing_maps_(zone) {
-    InitializePhis();
-    InitializeOperandMaps();
-  }
-
-  bool IsPhi(int virtual_register) {
-    return phi_map_guard_.Contains(virtual_register);
-  }
-
-  const PhiData* GetPhi(int virtual_register) {
-    auto it = phi_map_.find(virtual_register);
-    CHECK(it != phi_map_.end());
-    return it->second;
-  }
-
-  OperandMap* InitializeIncoming(size_t block_index, bool initial_pass) {
-    return initial_pass ? InitializeFromFirstPredecessor(block_index)
-                        : InitializeFromIntersection(block_index);
-  }
-
-  void PropagateUsesBackwards() {
-    typedef std::set<size_t, std::greater<size_t>, zone_allocator<size_t>>
-        BlockIds;
-    BlockIds block_ids((BlockIds::key_compare()),
-                       zone_allocator<size_t>(zone()));
-    // First ensure that incoming contains only keys in all predecessors.
-    for (const InstructionBlock* block : sequence()->instruction_blocks()) {
-      size_t index = block->rpo_number().ToSize();
-      block_ids.insert(index);
-      OperandMap::Map& succ_map = incoming_maps_[index]->map();
-      for (size_t i = 0; i < block->PredecessorCount(); ++i) {
-        RpoNumber pred_rpo = block->predecessors()[i];
-        succ_map.Intersect(outgoing_maps_[pred_rpo.ToSize()]->map());
-      }
-    }
-    // Back propagation fixpoint.
-    while (!block_ids.empty()) {
-      // Pop highest block_id.
-      auto block_id_it = block_ids.begin();
-      const size_t succ_index = *block_id_it;
-      block_ids.erase(block_id_it);
-      // Propagate uses back to their definition blocks using succ_vreg.
-      const InstructionBlock* block =
-          sequence()->instruction_blocks()[succ_index];
-      OperandMap::Map& succ_map = incoming_maps_[succ_index]->map();
-      for (size_t i = 0; i < block->PredecessorCount(); ++i) {
-        for (auto& succ_val : succ_map) {
-          // An incoming map contains no defines.
-          CHECK_EQ(kInvalidVreg, succ_val.second->define_vreg);
-          // Compute succ_vreg.
-          int succ_vreg = succ_val.second->succ_vreg;
-          if (succ_vreg == kInvalidVreg) {
-            succ_vreg = succ_val.second->use_vreg;
-            // Initialize succ_vreg in back propagation chain.
-            succ_val.second->succ_vreg = succ_vreg;
-          }
-          if (succ_vreg == kInvalidVreg) continue;
-          // May need to transition phi.
-          if (IsPhi(succ_vreg)) {
-            const PhiData* phi = GetPhi(succ_vreg);
-            if (phi->definition_rpo.ToSize() == succ_index) {
-              // phi definition block, transition to pred value.
-              succ_vreg = phi->operands[i];
-            }
-          }
-          // Push succ_vreg up to all predecessors.
-          RpoNumber pred_rpo = block->predecessors()[i];
-          OperandMap::Map& pred_map = outgoing_maps_[pred_rpo.ToSize()]->map();
-          auto& pred_val = *pred_map.find(succ_val.first);
-          if (pred_val.second->use_vreg != kInvalidVreg) {
-            CHECK_EQ(succ_vreg, pred_val.second->use_vreg);
-          }
-          if (pred_val.second->define_vreg != kInvalidVreg) {
-            CHECK_EQ(succ_vreg, pred_val.second->define_vreg);
-          }
-          if (pred_val.second->succ_vreg != kInvalidVreg) {
-            if (succ_vreg != pred_val.second->succ_vreg) {
-              // When a block introduces 2 identical phis A and B, and both are
-              // operands to other phis C and D, and we optimized the moves
-              // defining A or B such that they now appear in the block defining
-              // A and B, the back propagation will get confused when visiting
-              // upwards from C and D. The operand in the block defining A and B
-              // will be attributed to C (or D, depending which of these is
-              // visited first).
-              CHECK(IsPhi(pred_val.second->succ_vreg));
-              CHECK(IsPhi(succ_vreg));
-              const PhiData* current_phi = GetPhi(succ_vreg);
-              const PhiData* assigned_phi = GetPhi(pred_val.second->succ_vreg);
-              CHECK_EQ(current_phi->operands.size(),
-                       assigned_phi->operands.size());
-              CHECK_EQ(current_phi->definition_rpo,
-                       assigned_phi->definition_rpo);
-              for (size_t i = 0; i < current_phi->operands.size(); ++i) {
-                CHECK_EQ(current_phi->operands[i], assigned_phi->operands[i]);
-              }
-            }
-          } else {
-            pred_val.second->succ_vreg = succ_vreg;
-            block_ids.insert(pred_rpo.ToSize());
+      switch (contribution->kind()) {
+        case Final:
+          CHECK_EQ(FinalAssessment::cast(contribution)->virtual_register(),
+                   expected);
+          break;
+        case Pending: {
+          // This happens if we have a diamond feeding into another one, and
+          // the inner one never being used - other than for carrying the value.
+          PendingAssessment* next = PendingAssessment::cast(contribution);
+          if (seen_.find(pred) == seen_.end()) {
+            worklist_.push({next, expected});
+            seen_.insert(pred);
           }
+          // Note that we do not want to finalize pending assessments at the
+          // beginning of a block - which is the information we'd have
+          // available here. This is because this operand may be reused to
+          // define
+          // duplicate phis.
+          break;
         }
       }
     }
-    // Clear uses and back links for second pass.
-    for (OperandMap* operand_map : incoming_maps_) {
-      for (auto& succ_val : operand_map->map()) {
-        succ_val.second->incoming = nullptr;
-        succ_val.second->use_vreg = kInvalidVreg;
-      }
-    }
-  }
-
- private:
-  OperandMap* InitializeFromFirstPredecessor(size_t block_index) {
-    OperandMap* to_init = outgoing_maps_[block_index];
-    CHECK(to_init->map().empty());
-    const InstructionBlock* block =
-        sequence()->instruction_blocks()[block_index];
-    if (block->predecessors().empty()) return to_init;
-    size_t predecessor_index = block->predecessors()[0].ToSize();
-    // Ensure not a backedge.
-    CHECK(predecessor_index < block->rpo_number().ToSize());
-    OperandMap* incoming = outgoing_maps_[predecessor_index];
-    // Copy map and replace values.
-    to_init->map() = incoming->map();
-    for (auto& it : to_init->map()) {
-      OperandMap::MapValue* incoming = it.second;
-      it.second = new (zone()) OperandMap::MapValue();
-      it.second->incoming = incoming;
-    }
-    // Copy to incoming map for second pass.
-    incoming_maps_[block_index]->map() = to_init->map();
-    return to_init;
-  }
-
-  OperandMap* InitializeFromIntersection(size_t block_index) {
-    return incoming_maps_[block_index];
-  }
-
-  void InitializeOperandMaps() {
-    size_t block_count = sequence()->instruction_blocks().size();
-    incoming_maps_.reserve(block_count);
-    outgoing_maps_.reserve(block_count);
-    for (size_t i = 0; i < block_count; ++i) {
-      incoming_maps_.push_back(new (zone()) OperandMap(zone()));
-      outgoing_maps_.push_back(new (zone()) OperandMap(zone()));
-    }
   }
+  seen_.clear();
+  CHECK(worklist_.empty());
+}
 
-  void InitializePhis() {
-    const size_t block_count = sequence()->instruction_blocks().size();
-    for (size_t block_index = 0; block_index < block_count; ++block_index) {
-      const InstructionBlock* block =
-          sequence()->instruction_blocks()[block_index];
-      for (const PhiInstruction* phi : block->phis()) {
-        int first_pred_vreg = phi->operands()[0];
-        const PhiData* first_pred_phi = nullptr;
-        if (IsPhi(first_pred_vreg)) {
-          first_pred_phi = GetPhi(first_pred_vreg);
-          first_pred_vreg = first_pred_phi->first_pred_vreg;
-        }
-        CHECK(!IsPhi(first_pred_vreg));
-        PhiData* phi_data = new (zone()) PhiData(
-            block->rpo_number(), phi, first_pred_vreg, first_pred_phi, zone());
-        auto res =
-            phi_map_.insert(std::make_pair(phi->virtual_register(), phi_data));
-        CHECK(res.second);
-        phi_map_guard_.Add(phi->virtual_register());
-      }
+void RegisterAllocatorVerifier::ValidateUse(
+    RpoNumber block_id, BlockAssessments* current_assessments,
+    InstructionOperand op, int virtual_register) {
+  auto iterator = current_assessments->map().find(op);
+  // We should have seen this operand before.
+  CHECK(iterator != current_assessments->map().end());
+  Assessment* assessment = iterator->second;
+
+  switch (assessment->kind()) {
+    case Final:
+      // The virtual registers should match.
+      CHECK_EQ(FinalAssessment::cast(assessment)->virtual_register(),
+               virtual_register);
+      break;
+    case Pending: {
+      ValidatePendingAssessment(
+          block_id, op, PendingAssessment::cast(assessment), virtual_register);
+      // If everything checks out, we may make the assessment.
+      current_assessments->map()[op] =
+          new (zone()) FinalAssessment(virtual_register);
+      break;
     }
   }
-
-  typedef ZoneVector<OperandMap*> OperandMaps;
-  typedef ZoneVector<PhiData*> PhiVector;
-
-  Zone* zone() const { return zone_; }
-  const InstructionSequence* sequence() const { return sequence_; }
-
-  Zone* const zone_;
-  const InstructionSequence* const sequence_;
-  BitVector phi_map_guard_;
-  PhiMap phi_map_;
-  OperandMaps incoming_maps_;
-  OperandMaps outgoing_maps_;
-};
-
-
-void RegisterAllocatorVerifier::VerifyGapMoves() {
-  BlockMaps block_maps(zone(), sequence());
-  VerifyGapMoves(&block_maps, true);
-  block_maps.PropagateUsesBackwards();
-  VerifyGapMoves(&block_maps, false);
 }
 
-
-// Compute and verify outgoing values for every block.
-void RegisterAllocatorVerifier::VerifyGapMoves(BlockMaps* block_maps,
-                                               bool initial_pass) {
+void RegisterAllocatorVerifier::VerifyGapMoves() {
+  CHECK(assessments_.empty());
+  CHECK(outstanding_assessments_.empty());
   const size_t block_count = sequence()->instruction_blocks().size();
   for (size_t block_index = 0; block_index < block_count; ++block_index) {
-    OperandMap* current =
-        block_maps->InitializeIncoming(block_index, initial_pass);
     const InstructionBlock* block =
         sequence()->instruction_blocks()[block_index];
+    BlockAssessments* block_assessments = CreateForBlock(block);
+
     for (int instr_index = block->code_start(); instr_index < block->code_end();
          ++instr_index) {
       const InstructionConstraint& instr_constraint = constraints_[instr_index];
       const Instruction* instr = instr_constraint.instruction_;
-      current->RunGaps(zone(), instr);
+      block_assessments->PerformMoves(instr);
+
       const OperandConstraint* op_constraints =
           instr_constraint.operand_constraints_;
       size_t count = 0;
@@ -724,24 +484,19 @@ void RegisterAllocatorVerifier::VerifyGapMoves(BlockMaps* block_maps,
           continue;
         }
         int virtual_register = op_constraints[count].virtual_register_;
-        const InstructionOperand* op = instr->InputAt(i);
-        if (!block_maps->IsPhi(virtual_register)) {
-          current->Use(op, virtual_register, initial_pass);
-        } else {
-          const PhiData* phi = block_maps->GetPhi(virtual_register);
-          current->UsePhi(op, phi, initial_pass);
-        }
+        InstructionOperand op = *instr->InputAt(i);
+        ValidateUse(block->rpo_number(), block_assessments, op,
+                    virtual_register);
       }
       for (size_t i = 0; i < instr->TempCount(); ++i, ++count) {
-        current->Drop(instr->TempAt(i));
+        block_assessments->Drop(*instr->TempAt(i));
       }
       if (instr->IsCall()) {
-        current->DropRegisters(config());
+        block_assessments->DropRegisters();
       }
       for (size_t i = 0; i < instr->OutputCount(); ++i, ++count) {
         int virtual_register = op_constraints[count].virtual_register_;
-        OperandMap::MapValue* value =
-            current->Define(zone(), instr->OutputAt(i), virtual_register);
+        block_assessments->AddDefinition(*instr->OutputAt(i), virtual_register);
         if (op_constraints[count].type_ == kRegisterAndSlot) {
           const AllocatedOperand* reg_op =
               AllocatedOperand::cast(instr->OutputAt(i));
@@ -749,13 +504,35 @@ void RegisterAllocatorVerifier::VerifyGapMoves(BlockMaps* block_maps,
           const AllocatedOperand* stack_op = AllocatedOperand::New(
               zone(), LocationOperand::LocationKind::STACK_SLOT, rep,
               op_constraints[i].spilled_slot_);
-          auto insert_result =
-              current->map().insert(std::make_pair(stack_op, value));
-          DCHECK(insert_result.second);
-          USE(insert_result);
+          block_assessments->AddDefinition(*stack_op, virtual_register);
         }
       }
     }
+    // Now commit the assessments for this block. If there are any delayed
+    // assessments, ValidatePendingAssessment should see this block, too.
+    assessments_[block->rpo_number()] = block_assessments;
+
+    auto todo_iter = outstanding_assessments_.find(block->rpo_number());
+    if (todo_iter == outstanding_assessments_.end()) continue;
+    DelayedAssessments* todo = todo_iter->second;
+    for (auto pair : todo->map()) {
+      InstructionOperand op = pair.first;
+      int vreg = pair.second;
+      auto found_op = block_assessments->map().find(op);
+      CHECK(found_op != block_assessments->map().end());
+      switch (found_op->second->kind()) {
+        case Final:
+          CHECK(FinalAssessment::cast(found_op->second)->virtual_register() ==
+                vreg);
+          break;
+        case Pending:
+          ValidatePendingAssessment(block->rpo_number(), op,
+                                    PendingAssessment::cast(found_op->second),
+                                    vreg);
+          block_assessments->map()[op] = new (zone()) FinalAssessment(vreg);
+          break;
+      }
+    }
   }
 }