[llvm-dev] LoopVectorize fails to vectorize more complex loops

[Caballero, Diego via llvm-dev]

Agreed on the aliasing issue. You could also rewrite your loop like this:

   for (j = 0; j < m; j++)
     {
       TYPE red = 0.0;
       for (i = 0; i < n; i++)
           red += data[j][i];
       mean[j] = red;
     }

Diego

> -----Original Message-----
> From: llvm-dev [mailto:llvm-dev-bounces at lists.llvm.org] On Behalf Of
> Nema, Ashutosh via llvm-dev
> Sent: Friday, July 6, 2018 7:48 PM
> To: Alex Susu <alex.susu at gmail.com>
> Cc: llvm-dev at lists.llvm.org
> Subject: Re: [llvm-dev] LoopVectorize fails to vectorize more complex loops
> 
> Hi Alex,
> 
> This seems like LICM problem, where it's not able to promote the memory
> reference to scalar because of unknown aliasing.
> This result recurrence store remain in the loop body and loop vectorization
> fails to vectorize.
> Loop Versioning LICM pass can help in this case.
> 
> - Ashutosh
> 
> -----Original Message-----
> From: llvm-dev [mailto:llvm-dev-bounces at lists.llvm.org] On Behalf Of Alex
> Susu via llvm-dev
> Sent: Saturday, July 7, 2018 6:22 AM
> To: llvm-dev <llvm-dev at lists.llvm.org>
> Subject: [llvm-dev] LoopVectorize fails to vectorize more complex loops
> 
>    Hello.
>      Could you please tell me why the first loop of the following program (also
> maybe the commented loop) doesn't get vectorized with LoopVectorize
> (from a recent LLVM build from the SVN repository from Jun 2018)?
> 
> typedef short TYPE;
> TYPE data[1400][1200];
> 
> void kernel_covariance(int m, int n, TYPE mean[1200]) {
>    int i, j, k;
> 
>    for (j = 0; j < m; j++)
>      {
>        mean[j] = 0.0;
>        for (i = 0; i < n; i++)
>          mean[j] += data[j][i];
>        //mean[j] /= float_n;
>      }
> 
>    // This loop gets vectorized
>    for (i = 0; i < n; i++)
>      for (j = 0; j < m; j++)
>        data[i][j] -= mean[j];
> 
>    /*
>    // This loop doesn't get vectorized either:
>    for (i = 0; i < m; i++)
>      for (j = i; j < m; j++)
>        {
>          cov[i][j] = 0.0;
>          for (k = 0; k < n; k++)
>            cov[i][j] += data[i][k] * data[j][k];
>            sum += data[i][k] * data[j][k];
>          cov[i][j] /= (float_n - 1.0);
>          cov[j][i] = cov[i][j];
>        }
>    */
> }
> 
>    For the first loop I get the following debug info from clang and opt:
>      LV: Checking a loop in "kernel_covariance" from test.c:10:7
>      LV: Loop hints: force=? width=0 unroll=0
>      LV: Found a loop: for.body3.us
>      LV: PHI is not a poly recurrence.
>      LV: PHI is not a poly recurrence.
>      LV: Found an unidentified PHI.  %2 = phi i16 [ 0, %for.body.us ], [ %add.us,
> %for.body3.us ], !dbg !48
>      LV: Can't vectorize the instructions or CFG
>      LV: Not vectorizing: Cannot prove legality.
> 
>    Thank you,
>      Alex
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