[interpreter] Simplify graph builder control flow simulation.

src/compiler/bytecode-graph-builder.h

Index: src/compiler/bytecode-graph-builder.h
diff --git a/src/compiler/bytecode-graph-builder.h b/src/compiler/bytecode-graph-builder.h
index 7932b8722827a0112b1063ed1c7d1a5e6e64c42f..a25f3a2037d5002906a4232100b46b5311adfc5b 100644
--- a/src/compiler/bytecode-graph-builder.h
+++ b/src/compiler/bytecode-graph-builder.h
@@ -147,17 +147,15 @@ class BytecodeGraphBuilder {
   void BuildForInNext();
 
   // Control flow plumbing.
-  void BuildJump(int source_offset, int target_offset);
   void BuildJump();
   void BuildConditionalJump(Node* condition);
   void BuildJumpIfEqual(Node* comperand);
   void BuildJumpIfToBooleanEqual(Node* boolean_comperand);
 
-  // Constructing merge and loop headers.
-  void MergeEnvironmentsOfBackwardBranches(int source_offset,
-                                           int target_offset);
-  void MergeEnvironmentsOfForwardBranches(int source_offset);
-  void BuildLoopHeaderForBackwardBranches(int source_offset);
+  // Simulates control flow by forward-propagating environments.
+  void MergeIntoSuccessorEnvironment(int target_offset);
+  void BuildLoopHeaderEnvironment(int current_offset);
+  void SwitchToMergeEnvironment(int current_offset);
 
   // Simulates entry and exit of exception handlers.
   void EnterAndExitExceptionHandlers(int current_offset);
@@ -233,14 +231,11 @@ class BytecodeGraphBuilder {
   BytecodeBranchAnalysis* branch_analysis_;  // TODO(mstarzinger): Make const.
   Environment* environment_;
 
-  // Merge environments are snapshots of the environment at a particular
-  // bytecode offset to be merged into a later environment.
+  // Merge environments are snapshots of the environment at points where the
+  // control flow merges. This models a forward data flow propagation of all
+  // values from all predecessors of the merge in question.
   ZoneMap<int, Environment*> merge_environments_;
 
-  // Loop header environments are environments created for bytecodes
-  // where it is known there are back branches, ie a loop header.
-  ZoneMap<int, Environment*> loop_header_environments_;
-
   // Exception handlers currently entered by the iteration.
   ZoneStack<ExceptionHandler> exception_handlers_;
   int current_exception_handler_;