[PATCH/RFC] Pre-increment preparation pass

[Hal Finkel]

----- Original Message -----
> From: "Nadav Rotem" <nrotem at apple.com>
> To: "Hal Finkel" <hfinkel at anl.gov>
> Cc: "l >> Commit Messages and Patches for LLVM" <llvm-commits at cs.uiuc.edu>
> Sent: Thursday, January 31, 2013 12:59:11 AM
> Subject: Re: [PATCH/RFC] Pre-increment preparation pass
> 
> Hi Hal,
> 
> Isn't this something that needs to go into LSR ?

This was also my initial thought, but it was not obvious to me how. I wrote in my initial e-mail, "Should this really be part of LSR somehow?", and I'd certainly welcome advice in this regard.

Thanks again,
Hal

> 
> Thanks,
> Nadav
> 
> On Jan 29, 2013, at 1:31 PM, Hal Finkel <hfinkel at anl.gov> wrote:
> 
> > Hello again,
> > 
> > When targeting PPC A2 cores, use of pre-increment loads and stores
> > is very important for performance. The fundamental problem with
> > the generation of pre-increment instructions is that most loops
> > are not naturally written to take advantage of them. For example,
> > a loop written as:
> > for (int i = 0; i < N; ++i) {
> >  x[i] = y[i]
> > }
> > needs to be transformed to look more like this:
> > T *a = x[-1], *b = y[-1];
> > for (int i = 0; i < N; ++i) {
> >  *++a = *++b;
> > }
> > 
> > I've attached a pass that performs this transformation. For
> > unrolled loops (or other situations with multiple accesses to the
> > same array), the lowest-offset use is transformed into the
> > pre-increment access and the others are updated to base their
> > addressing off of the pre-increment access. I schedule this pass
> > after LSR is run.
> > 
> > This seems to work pretty well, but I don't know if this is the
> > best way of doing what I'd like. Should this really be part of LSR
> > somehow?
> > 
> > In case you're curious, for inner loops (where this really
> > matters), the induction variable is often moved into the special
> > loop counter register, and so removing the dependence of the
> > addressing on the induction variable is also a good thing.
> > 
> > Thanks again,
> > Hal
> > 
> > --
> > Hal Finkel
> > Postdoctoral Appointee
> > Leadership Computing Facility
> > Argonne National Laboratory
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>