[turbofan] Bring back long lost comment to AstGraphBuilder.

src/compiler/ast-graph-builder.h

 // Copyright 2014 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_AST_GRAPH_BUILDER_H_
 #define V8_COMPILER_AST_GRAPH_BUILDER_H_
 
 #include "src/ast.h"
 #include "src/compiler/js-graph.h"
 
@@ skipping 176 matching lines @@
   Node* MergeValue(Node* value, Node* other, Node* control);
 
   // The main node creation chokepoint. Adds context, frame state, effect,
   // and control dependencies depending on the operator.
   Node* MakeNode(const Operator* op, int value_input_count, Node** value_inputs,
                  bool incomplete);
 
   // Helper to indicate a node exits the function body.
   void UpdateControlDependencyToLeaveFunction(Node* exit);
 
-  //
+  // Builds deoptimization for a given node.
+  void PrepareFrameState(
+      Node* node, BailoutId ast_id,
+      OutputFrameStateCombine combine = OutputFrameStateCombine::Ignore());
+  void PrepareFrameStateAfterAndBefore(Node* node, BailoutId ast_id,
+                                       OutputFrameStateCombine combine,
+                                       Node* frame_state_before);
+
+  BitVector* GetVariablesAssignedInLoop(IterationStatement* stmt);
+
+  // Check if the given statement is an OSR entry.
+  // If so, record the stack height into the compilation and return {true}.
+  bool CheckOsrEntry(IterationStatement* stmt);
+
+  // Helper to wrap a Handle<T> into a Unique<T>.
+  template <class T>
+  Unique<T> MakeUnique(Handle<T> object) {
+    return Unique<T>::CreateUninitialized(object);
+  }
+
+  Node** EnsureInputBufferSize(int size);
+
+  // Named and keyed loads require a VectorSlotPair for successful lowering.
+  VectorSlotPair CreateVectorSlotPair(FeedbackVectorICSlot slot) const;
+
+  // ===========================================================================
   // The following build methods all generate graph fragments and return one
   // resulting node. The operand stack height remains the same, variables and
   // other dependencies tracked by the environment might be mutated though.
-  //
 
   // Builder to create a receiver check for sloppy mode.
   Node* BuildPatchReceiverToGlobalProxy(Node* receiver);
 
   // Builders to create local function and block contexts.
   Node* BuildLocalFunctionContext(Node* context, Node* closure);
   Node* BuildLocalBlockContext(Scope* scope);
 
   // Builder to create an arguments object if it is used.
   Node* BuildArgumentsObject(Variable* arguments);
@@ skipping 53 matching lines @@
   // Builders for conditional errors.
   Node* BuildThrowIfStaticPrototype(Node* name, BailoutId bailout_id);
 
   // Builders for non-local control flow.
   Node* BuildReturn(Node* return_value);
   Node* BuildThrow(Node* exception_value);
 
   // Builders for binary operations.
   Node* BuildBinaryOp(Node* left, Node* right, Token::Value op);
 
-  // Check if the given statement is an OSR entry.
-  // If so, record the stack height into the compilation and return {true}.
-  bool CheckOsrEntry(IterationStatement* stmt);
-
-  // Helper to wrap a Handle<T> into a Unique<T>.
-  template <class T>
-  Unique<T> MakeUnique(Handle<T> object) {
-    return Unique<T>::CreateUninitialized(object);
-  }
-
-  Node** EnsureInputBufferSize(int size);
-
-  // Named and keyed loads require a VectorSlotPair for successful lowering.
-  VectorSlotPair CreateVectorSlotPair(FeedbackVectorICSlot slot) const;
-
   // Process arguments to a call by popping {arity} elements off the operand
   // stack and build a call node using the given call operator.
   Node* ProcessArguments(const Operator* op, int arity);
 
+  // ===========================================================================
+  // The following visitation methods all recursively visit a subtree of the
+  // underlying AST and extent the graph. The operand stack is mutated in a way
+  // consistent with other compilers:
+  //  - Expressions pop operands and push result, depending on {AstContext}.
+  //  - Statements keep the operand stack balanced.
+
   // Visit statements.
   void VisitIfNotNull(Statement* stmt);
 
   // Visit expressions.
   void Visit(Expression* expr);
   void VisitForTest(Expression* expr);
   void VisitForEffect(Expression* expr);
   void VisitForValue(Expression* expr);
   void VisitForValueOrNull(Expression* expr);
   void VisitForValueOrTheHole(Expression* expr);
@@ skipping 17 matching lines @@
   void VisitArithmeticExpression(BinaryOperation* expr);
 
   // Dispatched from VisitForInStatement.
   void VisitForInAssignment(Expression* expr, Node* value,
                             BailoutId bailout_id);
   void VisitForInBody(ForInStatement* stmt);
 
   // Dispatched from VisitClassLiteral.
   void VisitClassLiteralContents(ClassLiteral* expr);
 
-  // Builds deoptimization for a given node.
-  void PrepareFrameState(
-      Node* node, BailoutId ast_id,
-      OutputFrameStateCombine combine = OutputFrameStateCombine::Ignore());
-  void PrepareFrameStateAfterAndBefore(Node* node, BailoutId ast_id,
-                                       OutputFrameStateCombine combine,
-                                       Node* frame_state_before);
-
-  BitVector* GetVariablesAssignedInLoop(IterationStatement* stmt);
-
   DEFINE_AST_VISITOR_SUBCLASS_MEMBERS();
   DISALLOW_COPY_AND_ASSIGN(AstGraphBuilder);
 };
 
 
 // The abstract execution environment for generated code consists of
 // parameter variables, local variables and the operand stack. The
 // environment will perform proper SSA-renaming of all tracked nodes
 // at split and merge points in the control flow. Internally all the
 // values are stored in one list using the following layout:
@@ skipping 142 matching lines @@
 
   // Prepare environment to be used as loop header.
   void PrepareForLoop(BitVector* assigned, bool is_osr = false);
 };
 
 }  // namespace compiler
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_COMPILER_AST_GRAPH_BUILDER_H_