[generators] Add some explanation on forcing context allocation.

src/full-codegen/ppc/full-codegen-ppc.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.
 
 #if V8_TARGET_ARCH_PPC
 
 #include "src/ast/scopes.h"
 #include "src/code-factory.h"
 #include "src/code-stubs.h"
 #include "src/codegen.h"
@@ skipping 1852 matching lines @@
                       kLRHasBeenSaved, kDontSaveFPRegs);
 
   // Load suspended function and context.
   __ LoadP(cp, FieldMemOperand(r4, JSGeneratorObject::kContextOffset));
   __ LoadP(r7, FieldMemOperand(r4, JSGeneratorObject::kFunctionOffset));
 
   // Load receiver and store as the first argument.
   __ LoadP(r5, FieldMemOperand(r4, JSGeneratorObject::kReceiverOffset));
   __ push(r5);
 
-  // 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.
   __ LoadP(r6, FieldMemOperand(r7, JSFunction::kSharedFunctionInfoOffset));
   __ LoadWordArith(
       r6, FieldMemOperand(r6, SharedFunctionInfo::kFormalParameterCountOffset));
   __ LoadRoot(r5, Heap::kTheHoleValueRootIndex);
   Label argument_loop, push_frame;
 #if V8_TARGET_ARCH_PPC64
   __ cmpi(r6, Operand::Zero());
   __ beq(&push_frame);
 #else
   __ SmiUntag(r6, SetRC);
@@ skipping 2204 matching lines @@
 
   DCHECK(Assembler::IsCrSet(Assembler::instr_at(cmp_address)));
 
   DCHECK(interrupt_address ==
          isolate->builtins()->OnStackReplacement()->entry());
   return ON_STACK_REPLACEMENT;
 }
 }  // namespace internal
 }  // namespace v8
 #endif  // V8_TARGET_ARCH_PPC