llvm dev - Jul 2015 - [LLVMdev] Loop localize global variables

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

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
>

Hi Sundeep,

I am also interested in the load-store lifting transformation.

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

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. 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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