[interpreter] Simplify graph builder control flow simulation.

src/compiler/bytecode-graph-builder.h

 // Copyright 2015 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef V8_COMPILER_BYTECODE_GRAPH_BUILDER_H_
 #define V8_COMPILER_BYTECODE_GRAPH_BUILDER_H_
 
 #include "src/compiler.h"
 #include "src/compiler/bytecode-branch-analysis.h"
 #include "src/compiler/js-graph.h"
@@ skipping 129 matching lines @@
   void BuildCallRuntimeForPair();
   void BuildCallConstruct();
   void BuildBinaryOp(const Operator* op);
   void BuildCompareOp(const Operator* op);
   void BuildDelete();
   void BuildCastOperator(const Operator* js_op);
   void BuildForInPrepare();
   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);
 
   // Attaches a frame state to |node| for the entry to the function.
   void PrepareEntryFrameState(Node* node);
 
   // Growth increment for the temporary buffer used to construct input lists to
   // new nodes.
   static const int kInputBufferSizeIncrement = 64;
@@ skipping 55 matching lines @@
   Zone* local_zone_;
   CompilationInfo* info_;
   JSGraph* jsgraph_;
   Handle<BytecodeArray> bytecode_array_;
   Handle<HandlerTable> exception_handler_table_;
   const FrameStateFunctionInfo* frame_state_function_info_;
   const interpreter::BytecodeArrayIterator* bytecode_iterator_;
   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_;
 
   // Temporary storage for building node input lists.
   int input_buffer_size_;
   Node** input_buffer_;
 
   // Nodes representing values in the activation record.
   SetOncePointer<Node> function_context_;
   SetOncePointer<Node> function_closure_;
   SetOncePointer<Node> new_target_;
 
   // Optimization to cache loaded feedback vector.
   SetOncePointer<Node> feedback_vector_;
 
   // Control nodes that exit the function body.
   ZoneVector<Node*> exit_controls_;
 
   DISALLOW_COPY_AND_ASSIGN(BytecodeGraphBuilder);
 };
 
 }  // namespace compiler
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_COMPILER_BYTECODE_GRAPH_BUILDER_H_