More check elimination improvements including partial learning on false branches of CompareMap and b

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 98 matching lines @@
         }
       }
       // Improvements possible:
       // - eliminate redundant HCheckSmi, HCheckInstanceType instructions
       // - track which values have been HCheckHeapObject'd
     }
 
     return this;
   }
 
-  // Global analysis: Copy state to successor block.
+  // Support for global analysis with HFlowEngine: Merge given state with
+  // the other incoming state.
+  static HCheckTable* Merge(HCheckTable* succ_state, HBasicBlock* succ_block,
+                            HCheckTable* pred_state, HBasicBlock* pred_block,
+                            Zone* zone) {
+    if (pred_state == NULL || pred_block->IsUnreachable()) {
+      return succ_state;
+    }
+    if (succ_state == NULL) {
+      return pred_state->Copy(succ_block, pred_block, zone);
+    } else {
+      return succ_state->Merge(succ_block, pred_state, pred_block, zone);
+    }
+  }
+
+  // Support for global analysis with HFlowEngine: Given state merged with all
+  // the other incoming states, prepare it for use.
+  static HCheckTable* Finish(HCheckTable* state, HBasicBlock* block,
+                             Zone* zone) {
+    if (state == NULL) {
+      block->MarkUnreachable();
+    }
+    return state;
+  }
+
+ private:
+  // Copy state to successor block.
   HCheckTable* Copy(HBasicBlock* succ, HBasicBlock* from_block, Zone* zone) {
     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];
-      // TODO(titzer): keep the check if this block dominates the successor?
       new_entry->object_ = old_entry->object_;
-      new_entry->check_ = NULL;
       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_;
+      } 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()));
         }
       }
     }
 
     // Branch-sensitive analysis for certain comparisons may add more facts
     // to the state for the successor on the true branch.
     bool learned = false;
-    HControlInstruction* end = succ->predecessors()->at(0)->end();
-    if (succ->predecessors()->length() == 1 && end->SuccessorAt(0) == succ) {
+    if (succ->predecessors()->length() == 1) {
+      HControlInstruction* end = succ->predecessors()->at(0)->end();
+      bool is_true_branch = end->SuccessorAt(0) == succ;
       if (end->IsCompareMap()) {
-        // Learn on the true branch of if(CompareMap(x)).
         HCompareMap* cmp = HCompareMap::cast(end);
         HValue* object = cmp->value()->ActualValue();
         HCheckTableEntry* entry = copy->Find(object);
-        if (entry == NULL) {
-          copy->Insert(object, cmp->map());
+        if (is_true_branch) {
+          // Learn on the true branch of if(CompareMap(x)).
+          if (entry == NULL) {
+            copy->Insert(object, cmp, cmp->map());
+          } else {
+            MapSet list = new(phase_->zone()) UniqueSet<Map>();
+            list->Add(cmp->map(), phase_->zone());
+            entry->maps_ = list;
+            entry->check_ = cmp;
+          }
         } else {
-          MapSet list = new(phase_->zone()) UniqueSet<Map>();
-          list->Add(cmp->map(), phase_->zone());
-          entry->maps_ = list;
+          // Learn on the false branch of if(CompareMap(x)).
+          if (entry != NULL) {
+            entry->maps_->Remove(cmp->map());
+          }
         }
         learned = true;
-      } else if (end->IsCompareObjectEqAndBranch()) {
+      } 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));
@@ skipping 14 matching lines @@
       PrintF("B%d checkmaps-table %s from B%d:\n",
              succ->block_id(),
              learned ? "learned" : "copied",
              from_block->block_id());
       copy->Print();
     }
 
     return copy;
   }
 
-  // Global analysis: Merge this state with the other incoming state.
+  // Merge this state with the other incoming state.
   HCheckTable* Merge(HBasicBlock* succ, HCheckTable* that,
                      HBasicBlock* pred_block, Zone* zone) {
-    if (pred_block->IsReachable()) {
-      if (that->size_ == 0) {
-        // If the other state is empty, simply reset.
-        size_ = 0;
-        cursor_ = 0;
-      } 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);
+    if (that->size_ == 0) {
+      // If the other state is empty, simply reset.
+      size_ = 0;
+      cursor_ = 0;
+    } 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_);
+        } 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());
+          if (this_entry->check_ != that_entry->check_) {
+            this_entry->check_ = NULL;
           }
+          ASSERT(this_entry->maps_->size() > 0);
+        }
+      }
+      if (compact) Compact();
+    }
 
-          if (that_entry == NULL) {
-            this_entry->object_ = NULL;
-            compact = true;
-          } else {
-            this_entry->maps_ =
-                this_entry->maps_->Union(that_entry->maps_, phase_->zone());
-            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) {
       PrintF("B%d checkmaps-table merged with B%d table:\n",
              succ->block_id(), pred_block->block_id());
       Print();
     }
     return this;
   }
 
   void ReduceCheckMaps(HCheckMaps* instr) {
     HValue* object = instr->value()->ActualValue();
@@ skipping 88 matching lines @@
     instr->DeleteAndReplaceWith(constant);
     INC_STAT(loads_);
   }
 
   void ReduceCheckMapValue(HCheckMapValue* instr) {
     if (!instr->map()->IsConstant()) return;  // Nothing to learn.
 
     HValue* object = instr->value()->ActualValue();
     // Match a HCheckMapValue(object, HConstant(map))
     Unique<Map> map = MapConstant(instr->map());
-    MapSet maps = FindMaps(object);
-    if (maps != NULL) {
+
+    HCheckTableEntry* entry = Find(object);
+    if (entry != NULL) {
+      MapSet maps = entry->maps_;
       if (maps->Contains(map)) {
         if (maps->size() == 1) {
           // Object is known to have exactly this map.
-          instr->DeleteAndReplaceWith(NULL);
+          if (entry->check_ != NULL) {
+            instr->DeleteAndReplaceWith(entry->check_);
+          } else {
+            // Mark check as dead but leave it in the graph as a checkpoint for
+            // subsequent checks.
+            instr->SetFlag(HValue::kIsDead);
+            entry->check_ = instr;
+          }
           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, map);
+      Insert(object, instr, map);
     }
   }
 
   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, MapConstant(instr->transition()));
+      Insert(object, NULL, MapConstant(instr->transition()));
     } 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, MapConstant(instr->value()));
+      Insert(object, NULL, MapConstant(instr->value()));
     } 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;
+
+    int succ;
     if (maps->Contains(instr->map())) {
-      if (maps->size() == 1) {
-        TRACE(("Marking redundant CompareMap #%d at B%d as true\n",
-            instr->id(), instr->block()->block_id()));
-        instr->set_known_successor_index(0);
-        INC_STAT(compares_true_);
+      if (maps->size() != 1) {
+        TRACE(("CompareMap #%d for #%d at B%d can't be eliminated: "
+               "ambiguous set of maps\n", instr->id(), instr->value()->id(),
+               instr->block()->block_id()));
+        return;
       }
+      succ = 0;
+      INC_STAT(compares_true_);
     } else {
-      TRACE(("Marking redundant CompareMap #%d at B%d as false\n",
-          instr->id(), instr->block()->block_id()));
-      instr->set_known_successor_index(1);
+      succ = 1;
       INC_STAT(compares_false_);
     }
+
+    TRACE(("Marking redundant CompareMap #%d for #%d at B%d as %s\n",
+        instr->id(), instr->value()->id(), instr->block()->block_id(),
+        succ == 0 ? "true" : "false"));
+    instr->set_known_successor_index(succ);
+
+    int unreachable_succ = 1 - succ;
+    instr->block()->MarkSuccEdgeUnreachable(unreachable_succ);
   }
 
   void ReduceTransitionElementsKind(HTransitionElementsKind* instr) {
     MapSet maps = FindMaps(instr->object()->ActualValue());
     // Can only learn more about an object that already has a known set of maps.
     if (maps == NULL) return;
     if (maps->Contains(instr->original_map())) {
       // If the object has the original map, it will be transitioned.
       maps->Remove(instr->original_map());
       maps->Add(instr->transitioned_map(), phase_->zone());
@@ skipping 55 matching lines @@
     }
 
     cursor_ = size_;  // Move cursor to end.
   }
 
   void Print() {
     for (int i = 0; i < size_; i++) {
       HCheckTableEntry* entry = &entries_[i];
       ASSERT(entry->object_ != NULL);
       PrintF("  checkmaps-table @%d: %s #%d ", i,
-             entry->object_->IsPhi() ? "phi" : "object",
-             entry->object_->id());
+             entry->object_->IsPhi() ? "phi" : "object", entry->object_->id());
       if (entry->check_ != NULL) {
         PrintF("check #%d ", entry->check_->id());
       }
       MapSet list = entry->maps_;
       PrintF("%d maps { ", list->size());
       for (int j = 0; j < list->size(); j++) {
         if (j > 0) PrintF(", ");
         PrintF("%" V8PRIxPTR, list->at(j).Hashcode());
       }
       PrintF(" }\n");
     }
   }
 
- private:
   HCheckTableEntry* Find(HValue* object) {
     for (int i = size_ - 1; i >= 0; i--) {
       // Search from most-recently-inserted to least-recently-inserted.
       HCheckTableEntry* entry = &entries_[i];
       ASSERT(entry->object_ != NULL);
       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, Unique<Map> map) {
+  void Insert(HValue* object, HInstruction* check, Unique<Map> map) {
     MapSet list = new(phase_->zone()) UniqueSet<Map>();
     list->Add(map, phase_->zone());
-    Insert(object, NULL, list);
+    Insert(object, check, list);
   }
 
-  void Insert(HValue* object, HCheckMaps* check, MapSet maps) {
+  void Insert(HValue* object, HInstruction* check, MapSet maps) {
     HCheckTableEntry* entry = &entries_[cursor_++];
     entry->object_ = object;
     entry->check_ = check;
     entry->maps_ = maps;
     // 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
@@ skipping 90 matching lines @@
   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