A64: Handle a few TODOs.

src/a64/macro-assembler-a64-inl.h

 // Copyright 2013 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 1255 matching lines @@
   ASSERT(allow_macro_instructions_);
   ASSERT(!rd.IsZero());
   uxtw(rd, rn);
 }
 
 
 void MacroAssembler::BumpSystemStackPointer(const Operand& space) {
   ASSERT(!csp.Is(sp_));
   // TODO(jbramley): Several callers rely on this not using scratch registers,
   // so we use the assembler directly here. However, this means that large
-  // immediate values of 'space' cannot be handled cleanly. Once we implement
-  // our flexible scratch register idea, we could greatly simplify this
-  // function.
+  // immediate values of 'space' cannot be handled cleanly. (Only 24-bits
+  // immediates or values of 'space' that can be encoded in one instruction are
+  // accepted.) Once we implement our flexible scratch register idea, we could
+  // greatly simplify this function.
   InstructionAccurateScope scope(this);
   if ((space.IsImmediate()) && !is_uint12(space.immediate())) {
     // The subtract instruction supports a 12-bit immediate, shifted left by
     // zero or 12 bits. So, in two instructions, we can subtract any immediate
     // between zero and (1 << 24) - 1.
     int64_t imm = space.immediate();
     ASSERT(is_uint24(imm));
 
     int64_t imm_top_12_bits = imm >> 12;
     sub(csp, StackPointer(), imm_top_12_bits << 12);
@@ skipping 378 matching lines @@
   // characters are reserved for controlling features of the instrumentation.
   ASSERT(isprint(marker_name[0]) && isprint(marker_name[1]));
 
   InstructionAccurateScope scope(this, 1);
   movn(xzr, (marker_name[1] << 8) | marker_name[0]);
 }
 
 } }  // namespace v8::internal
 
 #endif  // V8_A64_MACRO_ASSEMBLER_A64_INL_H_