[llvm-dev] Pattern not recognized as reduction

[Himanshu Shukla via llvm-dev]

Reduction Not Captured By LLVM

CODE_1

------------------------------------------------------------
------------------------------------------------------------
--------------------

#include <stdio.h>

int main()

{

       int sum[1000]={1,2,3,4};

       for (int i=1;i<1000;i++)

       {

               sum[0]  +=sum[i-1];

       }

}

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------------------------------------------------------------
--------------------

In the code given above, we tried to run the vectorization pass of LLVM.
The remark emitted by vectorizer says the following :-

REMARKS_1

------------------------------------------------------------
-------------------------------------------------------------------

test6.c:8:11: remark: loop not vectorized: value that could not be
identified as reduction is used outside the loop [-Rpass-analysis=loop-
vectorize]

  sum[0] += sum[i-1];

         ^

test6.c:6:3: remark: loop not vectorized: unsafe dependent memory
operations in loop. Use #pragma loop distribute(enable) to allow loop
distribution to attempt to

     isolate the offending operations into a separate loop
[-Rpass-analysis=loop-vectorize]

 for(int i=100;i<1000;i++)

 ^

------------------------------------------------------------
-------------------------------------------------------------------

The command used for capturing the remarks from above code is

clang -O3 -ffast-math -mavx2 -Rpass=loop-vectorize
-Rpass-analysis=loop-vectorize file.c



On the other hand , if we replace in the same code sum[0] by x and making
very slight changes , it gets vectorized and prints the following remarks :





CODE_2

------------------------------------------------------------
---------------------------------------------------------

#include <stdio.h>

int main()

{

      int sum[1000]={1,2,3,4};

      int x = 0;

      for (int i=1;i<1000;i++)

      {

              x  +=sum[i-1];

      }

      sum[0] = x;

      return sum[0];

}

------------------------------------------------------------
------------------------------------------------------------
--------------------

REMARKS_2

------------------------------------------------------------
-------------------------------------------------------------------

test6.c:6:3: remark: vectorized loop (vectorization width: 8, interleaved
count: 4) [-Rpass=loop-vectorize]

 for(int i=100;i<1000;i++)

 ^

------------------------------------------------------------
-------------------------------------------------------------------




Q : Why sum[0] += sum[i-1] is not identified as reduction by LLVM ?
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