[LLVMdev] Instruction Cleanup Questions

[Hal Finkel]

On Thu, 07 Jun 2012 10:34:03 +0200
Ivan Llopard <ivanllopard at gmail.com> wrote:

> Hi Hal,
> 
> On 07/06/2012 09:57, Chandler Carruth wrote:
> > On Wed, Jun 6, 2012 at 10:37 PM, Hal Finkel <hfinkel at anl.gov 
> > <mailto:hfinkel at anl.gov>> wrote:
> >
> >     I am working on cleaning up some PPC code generation. Two
> > questions:
> >
> >     1. Which pass is responsible for cleaning up self-moves:
> >       0x00000000100057c0 <+208>:   mr      r3,r3
> >
> 
> and the RA should eliminate trivial copies.

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?

Thanks again,
Hal

> 
> >
> >     2. Which pass is responsible for cleaning up unconditional
> > jumps that should be fall-throughs:
> >       0x0000000010005d88 <+1688>:  b       0x10005d8c
> > <._Z11sfoo+1692> 0x0000000010005d8c <+1692>:  ld r3,-32056(r2)
> >
> >
> > This should be handled by the MachineBlockPlacement (among others).
> > Do you have a reduced est case?
> >
> >
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-- 
Hal Finkel
Postdoctoral Appointee
Leadership Computing Facility
Argonne National Laboratory