[generators] Add some explanation on forcing context allocation.

src/full-codegen/x64/full-codegen-x64.cc

 // Copyright 2012 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.
 
 #if V8_TARGET_ARCH_X64
 
 #include "src/ast/scopes.h"
 #include "src/code-factory.h"
 #include "src/code-stubs.h"
 #include "src/codegen.h"
@@ skipping 1830 matching lines @@
   __ RecordWriteField(rbx, JSGeneratorObject::kInputOffset, rcx, rdx,
                       kDontSaveFPRegs);
 
   // Load suspended function and context.
   __ movp(rsi, FieldOperand(rbx, JSGeneratorObject::kContextOffset));
   __ movp(rdi, FieldOperand(rbx, JSGeneratorObject::kFunctionOffset));
 
   // Push receiver.
   __ Push(FieldOperand(rbx, JSGeneratorObject::kReceiverOffset));
 
-  // Push holes for arguments to generator function.
+  // Push holes for arguments to generator function. Since the parser forced
+  // context allocation for any variables in generators, the actual argument
+  // values have already been copied into the context and these dummy values
+  // will never be used.
   __ movp(rdx, FieldOperand(rdi, JSFunction::kSharedFunctionInfoOffset));
   __ LoadSharedFunctionInfoSpecialField(rdx, rdx,
       SharedFunctionInfo::kFormalParameterCountOffset);
   __ LoadRoot(rcx, Heap::kTheHoleValueRootIndex);
   Label push_argument_holes, push_frame;
   __ bind(&push_argument_holes);
   __ subp(rdx, Immediate(1));
   __ j(carry, &push_frame);
   __ Push(rcx);
   __ jmp(&push_argument_holes);
@@ skipping 2120 matching lines @@
   DCHECK_EQ(
       isolate->builtins()->OnStackReplacement()->entry(),
       Assembler::target_address_at(call_target_address, unoptimized_code));
   return ON_STACK_REPLACEMENT;
 }
 
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_TARGET_ARCH_X64