[llvm-dev] Loop Unrolling Fail in Simple Vectorized loop

[Charith Mendis via llvm-dev]

So, I tried unrolling the following simple loop.

int unroll(unsigned * a, unsigned * b, unsigned *c, unsigned count){

  for(unsigned i=0; i<count; i++){

    a[i] = b[i]*c[i-1];

  }

  return 0;

}


Then, the unroller is able to unroll it by 2 even though it doesn't know
about the range of count. SCEV of backedge taken count is (-1 + %count)


But, when I change the increment to 4, as in


int unroll(unsigned * a, unsigned * b, unsigned *c, unsigned count){

  for(unsigned i=0; i<count; i+=4){

    a[i] = b[i]*c[i-1];

  }

  return 0;

}


 The unroller cannot compute the backedge taken count. Therefore, it seems
like the problem is not with the range of "count", can't the unroller
compute it as (- 1 + %count / 4)?

On Wed, Oct 12, 2016 at 11:28 PM, Charith Mendis <char.mendis1989 at gmail.com>
wrote:

> Thanks for the explanation. But I am a little confused with the following
> fact. Can't LLVM keep vectorizable_elements as a symbolic value and convert
> the loop to say;
>
> for(unsigned i = 0; i < vectorizable_elements  ; i += 2){
>     //main loop
> }
>
> for(unsigned i=0 ; i < vectorizable_elements % 2; i++){
>    //fix up
> }
>
> Why does it have to reason about the range of vectorizable_elements? Even
> if vectorizable_elements == SIZE_MAX the above decomposition would work?
>
> On Wed, Oct 12, 2016 at 8:25 PM, Friedman, Eli <efriedma at codeaurora.org>
> wrote:
>
>> On 10/12/2016 4:35 PM, Charith Mendis via llvm-dev wrote:
>>
>> Hi all,
>>
>> Attached herewith is a simple vectorized function with loops performing a
>> simple shuffle.
>>
>> I want all loops (inner and outer) to be unrolled by 2 and as such used
>> -unroll-count=2
>> The inner loops(with k as the induction variable and having constant trip
>> counts) unroll fully, but the outer loop with (j) fails to unroll.
>>
>> The llvm code is also attached with inner loops fully unrolled.
>>
>> To inspect further, I added the following to the PassManagerBuilder.cpp
>> to run some canonicalization routines and redo unrolling again. I have set
>> partial unrolling on + have a huge threshold + allows expensive loop trip
>> counts. Still it didn't unroll by 2.
>>
>> MPM.add(createLoopUnrollPass());
>>
>> MPM.add(createCFGSimplificationPass());
>>
>> MPM.add(createLoopSimplifyPass());
>>
>> MPM.add(createLoopRotatePass(SizeLevel == 2 ? 0 : -1));
>>
>> MPM.add(createLCSSAPass());
>>
>> MPM.add(createIndVarSimplifyPass());        // Canonicalize indvars
>>
>> MPM.add(createLoopUnrollPass());
>>
>>
>> Digging deeper I found, that it fails in UnrollRuntimeLoopRemainder
>> function, where it is unable to calculate the BackEdge taken amount.
>>
>> Can anybody explain what is need to get the outer loop unrolled by 2? It
>> would be a great help.
>>
>>
>> Well, I can at least explain what is happening... runtime unrolling needs
>> to be able to symbolically compute the trip count to avoid inserting a
>> branch after every iteration.  SCEV isn't able to prove that your loop
>> isn't an infinite loop (consider the case of vectorizable_elements==SIZE_MAX),
>> therefore it can't compute the trip count.  Therefore, we don't unroll.
>>
>> There's a few different angles you could use to attack this: you could
>> teach the unroller to unroll loops with an uncomputable trip count, or you
>> can make the trip count of your loop computable somehow.  Changing the
>> unroller is probably straightforward (see the recently committed r284044).
>> Making the trip count computable is more complicated... it's probably
>> possible to teach SCEV to reason about the overflow in the pointer
>> computation, or maybe you could version the loop.
>>
>> -Eli
>>
>> --
>> Employee of Qualcomm Innovation Center, Inc.
>> Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a Linux Foundation Collaborative Project
>>
>>
>
>
> --
> Kind regards,
> Charith Mendis
>
> Graduate Student,
> CSAIL,
> Massachusetts Institute of Technology
>



-- 
Kind regards,
Charith Mendis

Graduate Student,
CSAIL,
Massachusetts Institute of Technology
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