[LLVMdev] Unnecessary moves after sign-extension in 2-address target

[Dan Gohman]

On Apr 19, 2009, at 6:15 PM, Greg McGary wrote:
>
> Because sextb_r and sextw_r have destination tied to source operands,
> TwoAddressInstructionPass thinks it needs a copy.  However, since the
> sext kills its source, the copy is unnecessary.  Why does this happen?
> Is TwoAddressInstructionPass relying on a later pass to notice this  
> and
> transform it again?

Yes, the later pass is the coalescer. It would be worthwhile to
understand why it is not coalescing the copies.

Dan