[LLVMdev] Instruction Cleanup Questions

[Hal Finkel]

On Thu, 07 Jun 2012 14:10:09 -0700
Owen Anderson <resistor at mac.com> wrote:

> 
> On Jun 7, 2012, at 1:42 PM, Hal Finkel <hfinkel at anl.gov> wrote:
> > 
> > On PPC, normal moves are encoded as OR instructions where the two
> > operands being ORed together are the same. These self moves, as it
> > turns out, come from things like this:
> > 
> > %vreg18<def> = OR8To4 %vreg16, %vreg16; GPRC:%vreg18 G8RC:%vreg16
> > 
> > This is generated from the pattern:
> > 
> > def : Pat<(i32 (trunc G8RC:$in)),
> >          (OR8To4 G8RC:$in, G8RC:$in)>;
> > 
> > So, as far as RA is concerned, this is a "real" operation (a binary
> > OR which truncates the result to 32-bits (from 64-bit inputs)). In
> > effect, however, this is just a self copy. 
> > 
> > How can I fix this?
> 
> What is this pattern trying to achieve?  Is the OR actually necessary
> at all, or can you use an EXTRACT_SUBREG instead?

I can, thanks!

 -Hal

> 
> --Owen



-- 
Hal Finkel
Postdoctoral Appointee
Leadership Computing Facility
Argonne National Laboratory