[LLVMdev] Loop localize global variables

[Charlie Turner]

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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