[generators] Add some explanation on forcing context allocation.

src/full-codegen/s390/full-codegen-s390.cc

 // Copyright 2015 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_S390
 
 #include "src/ast/scopes.h"
 #include "src/code-factory.h"
 #include "src/code-stubs.h"
 #include "src/codegen.h"
@@ skipping 1807 matching lines @@
                       kLRHasBeenSaved, kDontSaveFPRegs);
 
   // Load suspended function and context.
   __ LoadP(cp, FieldMemOperand(r3, JSGeneratorObject::kContextOffset));
   __ LoadP(r6, FieldMemOperand(r3, JSGeneratorObject::kFunctionOffset));
 
   // Load receiver and store as the first argument.
   __ LoadP(r4, FieldMemOperand(r3, JSGeneratorObject::kReceiverOffset));
   __ push(r4);
 
-  // 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(r5, FieldMemOperand(r6, JSFunction::kSharedFunctionInfoOffset));
   __ LoadW(
       r5, FieldMemOperand(r5, SharedFunctionInfo::kFormalParameterCountOffset));
   __ LoadRoot(r4, Heap::kTheHoleValueRootIndex);
   Label argument_loop, push_frame;
 #if V8_TARGET_ARCH_S390X
   __ CmpP(r5, Operand::Zero());
   __ beq(&push_frame, Label::kNear);
 #else
   __ SmiUntag(r5);
@@ skipping 2145 matching lines @@
   DCHECK(kOSRBranchInstruction == br_instr);
 
   DCHECK(interrupt_address ==
          isolate->builtins()->OnStackReplacement()->entry());
   return ON_STACK_REPLACEMENT;
 }
 
 }  // namespace internal
 }  // namespace v8
 #endif  // V8_TARGET_ARCH_S390