llvm dev - Mar 2008 - [LLVMdev] removing unnecessary moves with 2-address machine

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

I'm using released 2.2.  I guess I need to check out the svn trunk to get
this
enhancement.

bagel

On Wed, 5 Mar 2008, Bagel wrote:
> 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?
Marking it commutable and describing the move as a copy instruction should 
be enough.  Are you using llvm mainline or 2.2?  Evan implemented this in 
mainline after 2.2 was released.

-Chris

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