[LLVMdev] Trip count and Loop Vectorizer

Nadav Rotem nrotem at apple.com
Fri Sep 27 10:02:36 PDT 2013

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. 


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
> +1 (519) 772 – 2579
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