[turbofan] Clarify comment about Parameter indexing.

src/compiler/ast-graph-builder.cc

 // 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.
 
 #include "src/compiler/ast-graph-builder.h"
 
 #include "src/compiler.h"
 #include "src/compiler/ast-loop-assignment-analyzer.h"
 #include "src/compiler/control-builders.h"
 #include "src/compiler/js-type-feedback.h"
@@ skipping 664 matching lines @@
   int param_num = 0;
   if (builder->info()->is_this_defined()) {
     const Operator* op = common()->Parameter(param_num++, "%this");
     Node* receiver = builder->graph()->NewNode(op, builder->graph()->start());
     values()->push_back(receiver);
   } else {
     values()->push_back(builder->jsgraph()->UndefinedConstant());
   }
 
   // Bind all parameter variables. The parameter indices are shifted by 1
-  // (receiver is parameter index -1 but environment index 0).
+  // (receiver is variable index -1 but {Parameter} node index 0 and located at
+  // index 0 in the environment).
   for (int i = 0; i < scope->num_parameters(); ++i) {
     const char* debug_name = GetDebugParameterName(graph()->zone(), scope, i);
     const Operator* op = common()->Parameter(param_num++, debug_name);
     Node* parameter = builder->graph()->NewNode(op, builder->graph()->start());
     values()->push_back(parameter);
   }
 
   // Bind all local variables to undefined.
   Node* undefined_constant = builder->jsgraph()->UndefinedConstant();
   values()->insert(values()->end(), locals_count(), undefined_constant);
@@ skipping 18 matching lines @@
   values_.insert(values_.begin(), copy->values_.begin(), copy->values_.end());
   contexts_.reserve(copy->contexts_.size());
   contexts_.insert(contexts_.begin(), copy->contexts_.begin(),
                    copy->contexts_.end());
 }
 
 
 void AstGraphBuilder::Environment::Bind(Variable* variable, Node* node) {
   DCHECK(variable->IsStackAllocated());
   if (variable->IsParameter()) {
-    // The parameter indices are shifted by 1 (receiver is parameter
-    // index -1 but environment index 0).
+    // The parameter indices are shifted by 1 (receiver is variable
+    // index -1 but located at index 0 in the environment).
     values()->at(variable->index() + 1) = node;
   } else {
     DCHECK(variable->IsStackLocal());
     values()->at(variable->index() + parameters_count_) = node;
     DCHECK(IsLivenessBlockConsistent());
     if (liveness_block() != nullptr) {
       liveness_block()->Bind(variable->index());
     }
   }
 }
 
 
 Node* AstGraphBuilder::Environment::Lookup(Variable* variable) {
   DCHECK(variable->IsStackAllocated());
   if (variable->IsParameter()) {
-    // The parameter indices are shifted by 1 (receiver is parameter
-    // index -1 but environment index 0).
+    // The parameter indices are shifted by 1 (receiver is variable
+    // index -1 but located at index 0 in the environment).
     return values()->at(variable->index() + 1);
   } else {
     DCHECK(variable->IsStackLocal());
     DCHECK(IsLivenessBlockConsistent());
     if (liveness_block() != nullptr) {
       liveness_block()->Lookup(variable->index());
     }
     return values()->at(variable->index() + parameters_count_);
   }
 }
@@ skipping 3513 matching lines @@
     // Phi does not exist yet, introduce one.
     value = NewPhi(inputs, value, control);
     value->ReplaceInput(inputs - 1, other);
   }
   return value;
 }
 
 }  // namespace compiler
 }  // namespace internal
 }  // namespace v8