Use stability to only conditionally flush information from the CheckMaps table.

src/hydrogen-check-elimination.cc

 // Copyright 2013 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 32 matching lines @@
 
 namespace v8 {
 namespace internal {
 
 typedef UniqueSet<Map>* MapSet;
 
 struct HCheckTableEntry {
   HValue* object_;  // The object being approximated. NULL => invalid entry.
   HInstruction* check_;  // The last check instruction.
   MapSet maps_;          // The set of known maps for the object.
+  bool is_stable_;
 };
 
 
 // The main data structure used during check elimination, which stores a
 // set of known maps for each object.
 class HCheckTable : public ZoneObject {
  public:
   static const int kMaxTrackedObjects = 10;
 
   explicit HCheckTable(HCheckEliminationPhase* phase)
@@ skipping 33 matching lines @@
       case HValue::kCheckMapValue: {
         ReduceCheckMapValue(HCheckMapValue::cast(instr));
         break;
       }
       case HValue::kCheckHeapObject: {
         ReduceCheckHeapObject(HCheckHeapObject::cast(instr));
         break;
       }
       default: {
         // If the instruction changes maps uncontrollably, drop everything.
-        if (instr->CheckChangesFlag(kMaps) ||
-            instr->CheckChangesFlag(kOsrEntries)) {
-          Kill();
+        if (instr->CheckChangesFlag(kOsrEntries)) {
+          Reset();
+        } else if (instr->CheckChangesFlag(kMaps)) {
+          KillUnstableEntries();
         }
       }
       // Improvements possible:
       // - eliminate redundant HCheckSmi, HCheckInstanceType instructions
       // - track which values have been HCheckHeapObject'd
     }
 
     return this;
   }
 
@@ skipping 28 matching lines @@
     HCheckTable* copy = new(phase_->zone()) HCheckTable(phase_);
     for (int i = 0; i < size_; i++) {
       HCheckTableEntry* old_entry = &entries_[i];
       HCheckTableEntry* new_entry = &copy->entries_[i];
       new_entry->object_ = old_entry->object_;
       new_entry->maps_ = old_entry->maps_->Copy(phase_->zone());
       // Keep the check if the existing check's block dominates the successor.
       if (old_entry->check_ != NULL &&
           old_entry->check_->block()->Dominates(succ)) {
         new_entry->check_ = old_entry->check_;
+        new_entry->is_stable_ = old_entry->is_stable_;
       } else {
         // Leave it NULL till we meet a new check instruction for this object
         // in the control flow.
         new_entry->check_ = NULL;
       }
     }
     copy->cursor_ = cursor_;
     copy->size_ = size_;
 
     // Create entries for succ block's phis.
     if (succ->phis()->length() > 0) {
       int pred_index = succ->PredecessorIndexOf(from_block);
       for (int phi_index = 0;
            phi_index < succ->phis()->length();
            ++phi_index) {
         HPhi* phi = succ->phis()->at(phi_index);
         HValue* phi_operand = phi->OperandAt(pred_index);
 
         HCheckTableEntry* pred_entry = copy->Find(phi_operand);
         if (pred_entry != NULL) {
           // Create an entry for a phi in the table.
-          copy->Insert(phi, NULL, pred_entry->maps_->Copy(phase_->zone()));
+          copy->Insert(phi, NULL, pred_entry->maps_->Copy(phase_->zone()),
+                       pred_entry->is_stable_);
         }
       }
     }
 
     // Branch-sensitive analysis for certain comparisons may add more facts
     // to the state for the successor on the true branch.
     bool learned = false;
     if (succ->predecessors()->length() == 1) {
       HControlInstruction* end = succ->predecessors()->at(0)->end();
       bool is_true_branch = end->SuccessorAt(0) == succ;
       if (end->IsCompareMap()) {
         HCompareMap* cmp = HCompareMap::cast(end);
         HValue* object = cmp->value()->ActualValue();
         HCheckTableEntry* entry = copy->Find(object);
         if (is_true_branch) {
           // Learn on the true branch of if(CompareMap(x)).
           if (entry == NULL) {
-            copy->Insert(object, cmp, cmp->map());
+            copy->Insert(object, cmp, cmp->map(), cmp->is_stable());
           } else {
             MapSet list = new(phase_->zone()) UniqueSet<Map>();
             list->Add(cmp->map(), phase_->zone());
             entry->maps_ = list;
             entry->check_ = cmp;
+            entry->is_stable_ = cmp->is_stable();
           }
         } else {
           // Learn on the false branch of if(CompareMap(x)).
           if (entry != NULL) {
             entry->maps_->Remove(cmp->map());
           }
         }
         learned = true;
       } else if (is_true_branch && end->IsCompareObjectEqAndBranch()) {
         // Learn on the true branch of if(CmpObjectEq(x, y)).
         HCompareObjectEqAndBranch* cmp =
           HCompareObjectEqAndBranch::cast(end);
         HValue* left = cmp->left()->ActualValue();
         HValue* right = cmp->right()->ActualValue();
         HCheckTableEntry* le = copy->Find(left);
         HCheckTableEntry* re = copy->Find(right);
         if (le == NULL) {
           if (re != NULL) {
-            copy->Insert(left, NULL, re->maps_->Copy(zone));
+            copy->Insert(left, NULL, re->maps_->Copy(zone), re->is_stable_);
           }
         } else if (re == NULL) {
-          copy->Insert(right, NULL, le->maps_->Copy(zone));
+          copy->Insert(right, NULL, le->maps_->Copy(zone), le->is_stable_);
         } else {
           MapSet intersect = le->maps_->Intersect(re->maps_, zone);
           le->maps_ = intersect;
           re->maps_ = intersect->Copy(zone);
         }
         learned = true;
       }
       // Learning on false branches requires storing negative facts.
     }
 
     if (FLAG_trace_check_elimination) {
       PrintF("B%d checkmaps-table %s from B%d:\n",
              succ->block_id(),
              learned ? "learned" : "copied",
              from_block->block_id());
       copy->Print();
     }
 
     return copy;
   }
 
   // Merge this state with the other incoming state.
   HCheckTable* Merge(HBasicBlock* succ, HCheckTable* that,
                      HBasicBlock* pred_block, Zone* zone) {
     if (that->size_ == 0) {
       // If the other state is empty, simply reset.
-      size_ = 0;
-      cursor_ = 0;
+      Reset();
     } else {
       int pred_index = succ->PredecessorIndexOf(pred_block);
       bool compact = false;
       for (int i = 0; i < size_; i++) {
         HCheckTableEntry* this_entry = &entries_[i];
         HCheckTableEntry* that_entry;
         if (this_entry->object_->IsPhi() &&
             this_entry->object_->block() == succ) {
           HPhi* phi = HPhi::cast(this_entry->object_);
           HValue* phi_operand = phi->OperandAt(pred_index);
           that_entry = that->Find(phi_operand);
 
         } else {
           that_entry = that->Find(this_entry->object_);
         }
 
         if (that_entry == NULL) {
           this_entry->object_ = NULL;
           compact = true;
         } else {
           this_entry->maps_ =
               this_entry->maps_->Union(that_entry->maps_, phase_->zone());
+          this_entry->is_stable_ =
+              this_entry->is_stable_ && that_entry->is_stable_;
           if (this_entry->check_ != that_entry->check_) {
             this_entry->check_ = NULL;
           }
           ASSERT(this_entry->maps_->size() > 0);
         }
       }
       if (compact) Compact();
     }
 
     if (FLAG_trace_check_elimination) {
@@ skipping 60 matching lines @@
           }
 
           if (FLAG_trace_check_elimination) {
             Print();
           }
           INC_STAT(narrowed_);
         }
       }
     } else {
       // No entry; insert a new one.
-      Insert(object, instr, instr->map_set().Copy(phase_->zone()));
+      Insert(object, instr, instr->map_set().Copy(phase_->zone()),
+             instr->is_stable());
     }
   }
 
   void ReduceCheckValue(HCheckValue* instr) {
     // Canonicalize HCheckValues; they might have their values load-eliminated.
     HValue* value = instr->Canonicalize();
     if (value == NULL) {
       instr->DeleteAndReplaceWith(instr->value());
       INC_STAT(removed_);
     } else if (value != instr) {
@@ skipping 41 matching lines @@
           INC_STAT(removed_);
         } else {
           // Only one map survives the check.
           maps->Clear();
           maps->Add(map, phase_->zone());
           entry->check_ = instr;
         }
       }
     } else {
       // No prior information.
-      Insert(object, instr, map);
+      // TODO(verwaest): Tag map constants with stability.
+      Insert(object, instr, map, false);
     }
   }
 
   void ReduceCheckHeapObject(HCheckHeapObject* instr) {
     if (FindMaps(instr->value()->ActualValue()) != NULL) {
       // If the object has known maps, it's definitely a heap object.
       instr->DeleteAndReplaceWith(instr->value());
       INC_STAT(removed_cho_);
     }
   }
 
   void ReduceStoreNamedField(HStoreNamedField* instr) {
     HValue* object = instr->object()->ActualValue();
     if (instr->has_transition()) {
       // This store transitions the object to a new map.
       Kill(object);
-      Insert(object, NULL, MapConstant(instr->transition()));
+      Insert(object, NULL, MapConstant(instr->transition()),
+             instr->is_stable());
     } else if (IsMapAccess(instr->access())) {
       // This is a store directly to the map field of the object.
       Kill(object);
       if (!instr->value()->IsConstant()) return;
-      Insert(object, NULL, MapConstant(instr->value()));
+      // TODO(verwaest): Tag with stability.
+      Insert(object, NULL, MapConstant(instr->value()), false);
     } else {
       // If the instruction changes maps, it should be handled above.
       CHECK(!instr->CheckChangesFlag(kMaps));
     }
   }
 
   void ReduceCompareMap(HCompareMap* instr) {
     MapSet maps = FindMaps(instr->value()->ActualValue());
     if (maps == NULL) return;
 
@@ skipping 29 matching lines @@
       // If the object has the original map, it will be transitioned.
       maps->Remove(instr->original_map());
       maps->Add(instr->transitioned_map(), phase_->zone());
     } else {
       // Object does not have the given map, thus the transition is redundant.
       instr->DeleteAndReplaceWith(instr->object());
       INC_STAT(transitions_);
     }
   }
 
-  // Kill everything in the table.
-  void Kill() {
+  // Reset the table.
+  void Reset() {
     size_ = 0;
     cursor_ = 0;
   }
 
+  // Kill everything in the table.
+  void KillUnstableEntries() {
+    bool compact = false;
+    for (int i = 0; i < size_; i++) {
+      HCheckTableEntry* entry = &entries_[i];
+      ASSERT(entry->object_ != NULL);
+      if (!entry->is_stable_) {
+        entry->object_ = NULL;
+        compact = true;
+      }
+    }
+    if (compact) Compact();
+  }
+
   // Kill everything in the table that may alias {object}.
   void Kill(HValue* object) {
     bool compact = false;
     for (int i = 0; i < size_; i++) {
       HCheckTableEntry* entry = &entries_[i];
       ASSERT(entry->object_ != NULL);
       if (phase_->aliasing_->MayAlias(entry->object_, object)) {
         entry->object_ = NULL;
         compact = true;
       }
@@ skipping 61 matching lines @@
       if (phase_->aliasing_->MustAlias(entry->object_, object)) return entry;
     }
     return NULL;
   }
 
   MapSet FindMaps(HValue* object) {
     HCheckTableEntry* entry = Find(object);
     return entry == NULL ? NULL : entry->maps_;
   }
 
-  void Insert(HValue* object, HInstruction* check, Unique<Map> map) {
+  void Insert(HValue* object,
+              HInstruction* check,
+              Unique<Map> map,
+              bool is_stable) {
     MapSet list = new(phase_->zone()) UniqueSet<Map>();
     list->Add(map, phase_->zone());
-    Insert(object, check, list);
+    Insert(object, check, list, is_stable);
   }
 
-  void Insert(HValue* object, HInstruction* check, MapSet maps) {
+  void Insert(HValue* object,
+              HInstruction* check,
+              MapSet maps,
+              bool is_stable) {
     HCheckTableEntry* entry = &entries_[cursor_++];
     entry->object_ = object;
     entry->check_ = check;
     entry->maps_ = maps;
+    entry->is_stable_ = is_stable;
     // If the table becomes full, wrap around and overwrite older entries.
     if (cursor_ == kMaxTrackedObjects) cursor_ = 0;
     if (size_ < kMaxTrackedObjects) size_++;
   }
 
   bool IsMapAccess(HObjectAccess access) {
     return access.IsInobject() && access.offset() == JSObject::kMapOffset;
   }
 
   Unique<Map> MapConstant(HValue* value) {
     return Unique<Map>::cast(HConstant::cast(value)->GetUnique());
   }
 
   friend class HCheckMapsEffects;
   friend class HCheckEliminationPhase;
 
   HCheckEliminationPhase* phase_;
   HCheckTableEntry entries_[kMaxTrackedObjects];
   int16_t cursor_;  // Must be <= kMaxTrackedObjects
   int16_t size_;    // Must be <= kMaxTrackedObjects
   // TODO(titzer): STATIC_ASSERT kMaxTrackedObjects < max(cursor_)
 };
 
 
 // Collects instructions that can cause effects that invalidate information
 // needed for check elimination.
 class HCheckMapsEffects : public ZoneObject {
  public:
   explicit HCheckMapsEffects(Zone* zone)
-    : maps_stored_(false),
-      stores_(5, zone) { }
+    : stores_(5, zone) { }
 
   inline bool Disabled() {
     return false;  // Effects are _not_ disabled.
   }
 
   // Process a possibly side-effecting instruction.
   void Process(HInstruction* instr, Zone* zone) {
-    switch (instr->opcode()) {
-      case HValue::kStoreNamedField: {
-        stores_.Add(HStoreNamedField::cast(instr), zone);
-        break;
-      }
-      case HValue::kOsrEntry: {
-        // Kill everything. Loads must not be hoisted past the OSR entry.
-        maps_stored_ = true;
-      }
-      default: {
-        maps_stored_ |= (instr->CheckChangesFlag(kMaps) |
-                         instr->CheckChangesFlag(kElementsKind));
-      }
+    if (instr->IsStoreNamedField()) {
+      stores_.Add(HStoreNamedField::cast(instr), zone);
+    } else {
+      flags_.Add(instr->ChangesFlags());
     }
   }
 
   // Apply these effects to the given check elimination table.
   void Apply(HCheckTable* table) {
-    if (maps_stored_) {
+    if (flags_.Contains(kOsrEntries)) {
+      table->Reset();
+      return;
+    }
+    if (flags_.Contains(kMaps) || flags_.Contains(kElementsKind)) {
       // Uncontrollable map modifications; kill everything.
-      table->Kill();
+      table->KillUnstableEntries();
       return;
     }
 
     // Kill maps for each store contained in these effects.
     for (int i = 0; i < stores_.length(); i++) {
       HStoreNamedField* s = stores_[i];
       if (table->IsMapAccess(s->access()) || s->has_transition()) {
         table->Kill(s->object()->ActualValue());
       }
     }
   }
 
   // Union these effects with the other effects.
   void Union(HCheckMapsEffects* that, Zone* zone) {
-    maps_stored_ |= that->maps_stored_;
+    flags_.Add(that->flags_);
     for (int i = 0; i < that->stores_.length(); i++) {
       stores_.Add(that->stores_[i], zone);
     }
   }
 
  private:
-  bool maps_stored_ : 1;
+  GVNFlagSet flags_;
   ZoneList<HStoreNamedField*> stores_;
 };
 
 
 // The main routine of the analysis phase. Use the HFlowEngine for either a
 // local or a global analysis.
 void HCheckEliminationPhase::Run() {
   HFlowEngine<HCheckTable, HCheckMapsEffects> engine(graph(), zone());
   HCheckTable* table = new(zone()) HCheckTable(this);
 
   if (GLOBAL) {
     // Perform a global analysis.
     engine.AnalyzeDominatedBlocks(graph()->blocks()->at(0), table);
   } else {
     // Perform only local analysis.
     for (int i = 0; i < graph()->blocks()->length(); i++) {
-      table->Kill();
+      table->Reset();
       engine.AnalyzeOneBlock(graph()->blocks()->at(i), table);
     }
   }
 
   if (FLAG_trace_check_elimination) PrintStats();
 }
 
 
 // Are we eliminated yet?
 void HCheckEliminationPhase::PrintStats() {
 #if DEBUG
   #define PRINT_STAT(x) if (x##_ > 0) PrintF(" %-16s = %2d\n", #x, x##_)
 #else
   #define PRINT_STAT(x)
 #endif
   PRINT_STAT(redundant);
   PRINT_STAT(removed);
   PRINT_STAT(removed_cho);
   PRINT_STAT(narrowed);
   PRINT_STAT(loads);
   PRINT_STAT(empty);
   PRINT_STAT(compares_true);
   PRINT_STAT(compares_false);
   PRINT_STAT(transitions);
 }
 
 } }  // namespace v8::internal