[generators] Add some explanation on forcing context allocation.

src/full-codegen/arm64/full-codegen-arm64.cc

 // Copyright 2013 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_ARM64
 
 #include "src/ast/scopes.h"
 #include "src/code-factory.h"
 #include "src/code-stubs.h"
 #include "src/codegen.h"
@@ skipping 3826 matching lines @@
   __ Ldr(cp, FieldMemOperand(generator_object,
                              JSGeneratorObject::kContextOffset));
   __ Ldr(function, FieldMemOperand(generator_object,
                                    JSGeneratorObject::kFunctionOffset));
 
   // Load receiver and store as the first argument.
   __ Ldr(x10, FieldMemOperand(generator_object,
                               JSGeneratorObject::kReceiverOffset));
   __ Push(x10);
 
-  // Push holes for the rest of the arguments to the 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.
   __ Ldr(x10, FieldMemOperand(function, JSFunction::kSharedFunctionInfoOffset));
 
   // The number of arguments is stored as an int32_t, and -1 is a marker
   // (SharedFunctionInfo::kDontAdaptArgumentsSentinel), so we need sign
   // extension to correctly handle it. However, in this case, we operate on
   // 32-bit W registers, so extension isn't required.
   __ Ldr(w10, FieldMemOperand(x10,
                               SharedFunctionInfo::kFormalParameterCountOffset));
   __ LoadRoot(the_hole, Heap::kTheHoleValueRootIndex);
   __ PushMultipleTimes(the_hole, w10);
@@ skipping 310 matching lines @@
   }
 
   return INTERRUPT;
 }
 
 
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_TARGET_ARCH_ARM64