Reland "VM: Handle null-comparisons in the flow graph type propagation"

runtime/vm/intrinsifier.cc

Index: runtime/vm/intrinsifier.cc
diff --git a/runtime/vm/intrinsifier.cc b/runtime/vm/intrinsifier.cc
index 9c6d9eef36ee37b4b50dd56d17d2dfc69e98fcc3..3b915134ac574dd327aff43230438ae834bf1b46 100644
--- a/runtime/vm/intrinsifier.cc
+++ b/runtime/vm/intrinsifier.cc
@@ -285,7 +285,14 @@ static intptr_t CidForRepresentation(Representation rep) {
 class BlockBuilder : public ValueObject {
  public:
   BlockBuilder(FlowGraph* flow_graph, TargetEntryInstr* entry)
-      : flow_graph_(flow_graph), entry_(entry), current_(entry) {}
+      : flow_graph_(flow_graph),
+        entry_(entry),
+        current_(entry),
+        empty_env_(new Environment(0,

[rmacnak, 2017/03/31 22:28:58]

Maybe fall_through_env_ ?

+                                   0,
+                                   Thread::kNoDeoptId,
+                                   flow_graph->parsed_function(),
+                                   NULL)) {}
 
   Definition* AddToInitialDefinitions(Definition* def) {
     def->set_ssa_temp_index(flow_graph_->alloc_ssa_temp_index());
@@ -295,11 +302,17 @@ class BlockBuilder : public ValueObject {
 
   Definition* AddDefinition(Definition* def) {
     def->set_ssa_temp_index(flow_graph_->alloc_ssa_temp_index());
-    current_ = current_->AppendInstruction(def);
+    AddInstruction(def);
     return def;
   }
 
   Instruction* AddInstruction(Instruction* instr) {
+    if (instr->ComputeCanDeoptimize()) {
+      // Since we use the presence of an environment to determine if an
+      // instructions can deoptimize, we need an empty enviroment for
+      // instructions that "deoptimize" to the intrinsic fall-through code.
+      instr->SetEnvironment(empty_env_);
+    }
     current_ = current_->AppendInstruction(instr);
     return instr;
   }
@@ -360,6 +373,7 @@ class BlockBuilder : public ValueObject {
   FlowGraph* flow_graph_;
   BlockEntryInstr* entry_;
   Instruction* current_;
+  Environment* empty_env_;
 };