[generators] Add some explanation on forcing context allocation.

src/full-codegen/mips/full-codegen-mips.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_MIPS
 
 // Note on Mips implementation:
 //
 // The result_register() for mips is the 'v0' register, which is defined
 // by the ABI to contain function return values. However, the first
@@ skipping 1885 matching lines @@
                       kRAHasBeenSaved, kDontSaveFPRegs);
 
   // Load suspended function and context.
   __ lw(cp, FieldMemOperand(a1, JSGeneratorObject::kContextOffset));
   __ lw(t0, FieldMemOperand(a1, JSGeneratorObject::kFunctionOffset));
 
   // Load receiver and store as the first argument.
   __ lw(a2, FieldMemOperand(a1, JSGeneratorObject::kReceiverOffset));
   __ push(a2);
 
-  // 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.
   __ lw(a3, FieldMemOperand(t0, JSFunction::kSharedFunctionInfoOffset));
   __ lw(a3,
         FieldMemOperand(a3, SharedFunctionInfo::kFormalParameterCountOffset));
   __ LoadRoot(a2, Heap::kTheHoleValueRootIndex);
   Label push_argument_holes, push_frame;
   __ bind(&push_argument_holes);
   __ Subu(a3, a3, Operand(Smi::FromInt(1)));
   __ Branch(&push_frame, lt, a3, Operand(zero_reg));
   __ push(a2);
   __ jmp(&push_argument_holes);
@@ skipping 2197 matching lines @@
          reinterpret_cast<uint32_t>(
              isolate->builtins()->OnStackReplacement()->entry()));
   return ON_STACK_REPLACEMENT;
 }
 
 
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_TARGET_ARCH_MIPS