A CheckClass instruction that compares against a null class only can be optimized to check instance…

runtime/vm/intermediate_language.cc

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #include "vm/intermediate_language.h"
 
 #include "vm/bit_vector.h"
 #include "vm/constant_propagator.h"
 #include "vm/cpu.h"
 #include "vm/dart_entry.h"
@@ skipping 173 matching lines @@
 }
 
 
 EffectSet CheckClassIdInstr::Dependencies() const {
   // Externalization of strings via the API can change the class-id.
   return !CheckClassInstr::IsImmutableClassId(cid_) ?
       EffectSet::Externalization() : EffectSet::None();
 }
 
 
-bool CheckClassInstr::IsNullCheck() const {
+bool CheckClassInstr::DeoptIfNull() const {
   if (unary_checks().NumberOfChecks() != 1) {
     return false;
   }
   CompileType* in_type = value()->Type();
   const intptr_t cid = unary_checks().GetCidAt(0);
   // Performance check: use CheckSmiInstr instead.
   ASSERT(cid != kSmiCid);
   return in_type->is_nullable() && (in_type->ToNullableCid() == cid);
 }
 
 
+// Null object is a singleton of null-class (except for some sentinel,
+// transitional temporaries). Instead of checking against the null class only
+// we can check against null instance instead.
+bool CheckClassInstr::DeoptIfNotNull() const {
+  if (unary_checks().NumberOfChecks() != 1) {
+    return false;
+  }
+  const intptr_t cid = unary_checks().GetCidAt(0);
+  return cid == kNullCid;
+}
+
+
+
 bool CheckClassInstr::IsDenseSwitch() const {
   if (unary_checks().GetReceiverClassIdAt(0) == kSmiCid) return false;
   if (cids_.length() > 2 &&
       cids_[cids_.length() - 1] - cids_[0] < kBitsPerWord) {
     return true;
   }
   return false;
 }
 
 
@@ skipping 3371 matching lines @@
     case Token::kTRUNCDIV: return 0;
     case Token::kMOD: return 1;
     default: UNIMPLEMENTED(); return -1;
   }
 }
 
 
 #undef __
 
 }  // namespace dart