public DalvInsn expandedPrefix(BitSet compatRegs) {
    RegisterSpecList regs = registers;
    boolean firstBit = compatRegs.get(0);

    if (hasResult()) compatRegs.set(0);

    regs = regs.subset(compatRegs);

    if (hasResult()) compatRegs.set(0, firstBit);

    if (regs.size() == 0) return null;

    return new HighRegisterPrefix(position, regs);
  }
Gets the instruction prefix required, if any, to use in an expanded version of this instance.
Will not generate moves for registers marked compatible to the format by the given BitSet.

@param compatRegs {@code non-null;} set of compatible registers
@return {@code null-ok;} the prefix, if any
@see #expandedVersion
android/android_framework/dex-maker/src/main/java/com/android/dx/dex/code/DalvInsn.java