[LLVMdev] Trip count and Loop Vectorizer

[Arnold Schwaighofer]

Sriram,

The problem is that you want to unroll/vectorize many loops with non-constant loop count - it is a trade-off of which case you estimate as more likely.


int foo(int *ptr, int n) {
  for ( .. i <n)
    ptr[i] = ...
}

The question is: is it more likely to have “n” such that unrolling is beneficial or not.

Now, you could probably write an analysis that bounds the loop count (for the purpose of this heuristic) based on the only possible legal access in the loop. In your example you have an access to an alloca of which the size is known, so you could infer that n must be smaller than 8 (because you know the range of the other dimension). The question is how often does such an example occur, where this is possible, to make such an effort justifiable?


Best,
Arnold

On Sep 27, 2013, at 12:17 PM, Murali, Sriram <sriram.murali at intel.com> wrote:

> Hi Nadav,
> Thanks for the response. I forgot to mention that there is an upper limit of 16 for the Trip Count check,
> TinyTripCountVectorThreshold = 16;
> if (TC > 0u && TC < TinyTripCountVectorThreshold). So right now, any loop with Trip Count as 0, or with  value >=16, LV with unroll. With the change to the lower bound, it will also include the loop with 0 trip count.
> SCEV returns 0 trip count for this case, because it identifies that there is no backedge taken.
>  
> ScalarEvolution::ComputeExitLimitFromCond () {
> …  
>  if (L->contains(FBB) == !CI->getZExtValue())
>    { }
> else
>    // The backedge is never taken.
>    return getConstant(CI->getType(), 0);
> }
> From: Nadav Rotem [mailto:nrotem at apple.com] 
> Sent: Friday, September 27, 2013 1:03 PM
> To: Murali, Sriram
> Cc: llvmdev at cs.uiuc.edu
> Subject: Re: [LLVMdev] Trip count and Loop Vectorizer
>  
> Hi Sriram, 
>  
> Thanks for performing this analysis. The problem here, both for memcpy and the vectorizer, is that we can’t predict the size of “n”, even though the only use of ’n’ is for the loop bound for the alloca [4 x [8 x i32]]. If you change the unroll condition to TC >= 0 then you will disable loop unrolling for all loops because getSmallConstantTripCount returns an unsigned number. You can control the unroll factor using metadata or using the command line tools. 
>  
> Thanks,
> Nadav
>  
>  
> On Sep 27, 2013, at 9:41 AM, Murali, Sriram <sriram.murali at intel.com> wrote:
> 
> 
> Hi,
> I am trying to get a small loop to *not vectorize* for cases where it doesn’t make sense. For instance, this loop:
> void foo(int a[4][8], int n)
> {   
>     int b[4][8];
>     for(int i = 0; i < 4; i++) {
>         for(int j = 0; j < n; j++) {
>             a[i][j] = b[i][j];           
>         }
>     }
> }
> * Has maximum of 8ints copy. LLVM tries to use Memcpy for the inner loop. It is not helpful to perform memcpy for such small moves, especially when the outer loop is unrolled since the trip count is constant (4). The 4 calls to memcpy is not efficient.
> * Therefore, I disabled the memcpy optimization for such cases, and found that LLVM  LoopVectorizer successfully vectorizes and unrolls the inner loop. However, in order to take the fast path (vmovups) it must copy at least 32 ints, where as in this case we only do an 8int copy.
> ** Upon closer look, LoopVectorizer obtains the TripCount for the innerloop using getSmallConstantTripCount(Loop,…). This value is 0 for the loop with unknown trip count. Loop unrolling is disabled when TC > 0. Should this be changed to TC >= 0 (which does the job for this testcase)? Or is there a better way to disable loop unrolling for such trivial loops, at least the ones with known array size?
>  
> Thanks for your feedback
>  
> Sriram
>  
> --
> Sriram Murali
> SSG/DPD/ECDL/DMP
> +1 (519) 772 – 2579
>  
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