[LLVMdev] removing unnecessary moves with 2-address machine

[Bagel]

I am new to LLVM and am trying to write a backend for a simple 2-address 
machine.
For a very simple program:

define i16 @add2aa(i16 %a, i16 %b) nounwind  {
entry:
        %tmp3 = add i16 %b, %a          ; <i16> [#uses=1]
        ret i16 %tmp3
}

The arguments a and b are passed in r15 and r14.  The result is returned 
in r15.
The generated code is:

add2aa:
        add.w   r15,r14
        mov.w   r14,r15
        ret

If the legs of the add.w were reversed, the mov.w would not be 
necessary.  I have declared the add instruction to be "commutable".  
What do I have to do to cause the optimization?

thanks,
bagel