Invalid register Must be between v0 and v15, inclusive

Question

I had below smali code that worked well with no error.I have add an statement after the move-result to log v18 at logcat, but now I have an error

    invoke-static {v9}, Lcom/mycompany/myapp/MyClass1;->getMystring((Ljava/lang/Object;)Ljava/lang/String;
    move-result-object v18
    #I added below line 
    invoke-static {v18}, Lcom/mycompany/myapp/MyClass2;->logMessage(Ljava/lang/String;)V

but I have error now:

    Invalid register: v18. Must be between v0 and v15, inclusive

What should I use instead of v18 to remove the error in the last line

Answer 1

invoke-static is one of the instructions that can address only the first 16 registers (v0 to v15 as said in the error message). There are a few options, all of them involve finding a register that you can use for yourself.

1. Replace v18 in move-result-object with another register. You might need to update its uses in other places accordingly.

2. Copy value from v18 into another register using move-object vX, v18 where vX is the new register and use invoke-static with vX.

3. If you can't find a register that would be safe to override, you can temporary copy it to a register with a higher index using move-object/16 vYY, vX and then restore it with move-object/from16 vX, vYY. You'll need to increase the number of registers the function uses.

Note that registers aren't shared between methods, so you need to care only about preserving the behavior of the current method.