[LLVMdev] Loop localize global variables

[Hal Finkel]

----- Original Message -----
> From: "Charlie Turner" <charlesturner7c5 at gmail.com>
> To: sundeepk at codeaurora.org
> Cc: "LLVM Developers Mailing List" <llvmdev at cs.uiuc.edu>
> Sent: Tuesday, July 21, 2015 9:22:04 AM
> Subject: Re: [LLVMdev] Loop localize global variables
> 
> Hi Sundeep,
> 
> I am also interested in the load-store lifting transformation.

I am as well, LICM should certainly be taught to speculatively load/store to conditionally-accessed dereferenceable addresses when that is likely to be profitable.

> 
> For static globals as-in your example, the transformation in general
> would rely on a better static global aliasing information that is
> currently in review http://reviews.llvm.org/D10059

This will help, yes, but the transformation is quite useful regardless.

> 
> For non-static globals, one problem with loop-based analysis alone is
> that in a popular embedded benchmark suite, you get serious gains if
> you can localize the globals in code like
> 
> int *G1;
> int *G2;
> 
> foo () {
>   G1 = malloc(...);
>   for (...) {
>        // Lots of stuff with G1 and G2 worth localizing
>   }
> }
> 
> That malloc is very important to consider when doing an alias query
> because now the aliasing infrastructure knows G1, G2 don't alias, and
> you won't see it from a loop pass. If you were to try and modify LICM
> to localize the globals for example, it would have to assume G1 and
> G2
> MayAlias.

Why?

 -Hal

> I believe this implies we must use a FunctionPass, and I
> have a prototype that catches cases like the above, as well as the
> simpler ones. I can't commit myself however to when that will be
> ready, so I'm just sharing what I've found out, maybe it's helpful.
> 
> I'd be interested to hear your thoughts / approach in greater detail.
> 
> Thanks!
> --Charlie.
> 
> On 21 July 2015 at 06:10,  <sundeepk at codeaurora.org> wrote:
> > typo corrected
> >
> >   lcl_var = gbl_var;
> >   for () {
> >      ...access lcl_var...
> >   }
> >   gbl_var = lcl_var;
> >
> >
> >> Hello all,
> >>
> >> I am writing to get some feedback on an optimization that I would
> >> like to
> >> upstream. The basic idea is to localize global variables inside
> >> loops so
> >> that it can be allocated into registers. For example, transform
> >> the
> >> following sequence
> >>
> >> static int gbl_var;
> >> void foo() {
> >>
> >>   for () {
> >>      ...access gbl_var...
> >>   }
> >>
> >> }
> >>
> >> into something like
> >>
> >> static int gbl_var;
> >> void foo() {
> >>   int lcl_var;
> >>
> >>   lcl_var = gbl_var;
> >>   for () {
> >>      ...access clc_var...
> >>   }
> >>   gbl_var = lcl_var;
> >>
> >> }
> >>
> >> This transformation helps a couple of EEMBC benchmarks on both
> >> Aarch64 and
> >> Hexagon backends. I was wondering if there is interest to get this
> >> optimization upstreamed or if there is a better way of doing this.
> >>
> >> Thanks,
> >> Sundeep
> >>
> >> Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum,
> >> hosted by The Linux Foundation
> >>
> >
> >
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-- 
Hal Finkel
Assistant Computational Scientist
Leadership Computing Facility
Argonne National Laboratory