Check elimination: Learn from if(CompareMap(x)) on true branch.

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 109 matching lines @@
   HCheckTable* Copy(HBasicBlock* succ, 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());
     }
+    if (succ->predecessors()->length() == 1) {
+      HControlInstruction* end = succ->predecessors()->at(0)->end();
+      if (end->IsCompareMap() && end->SuccessorAt(0) == succ) {
+        // 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());
+        } else {
+          MapSet list = new(phase_->zone()) UniqueSet<Map>();
+          list->Add(cmp->map(), phase_->zone());
+          entry->maps_ = list;
+        }
+      }
+      // TODO(titzer): is it worthwhile to learn on false branch too?
+    }
     return copy;
   }
 
   // Global analysis: Merge this state with the other incoming state.
   HCheckTable* Merge(HBasicBlock* succ, HCheckTable* that, Zone* zone) {
     if (that->size_ == 0) {
       // If the other state is empty, simply reset.
       size_ = 0;
       cursor_ = 0;
       return this;
@@ skipping 370 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