[PATCH] D76132: [LoopUnrollAndJam] Changed safety checks to consider more than 2-levels loop nest.

[Whitney Tsang via Phabricator via llvm-commits]

Whitney added a comment.

In D76132#1943966 <https://reviews.llvm.org/D76132#1943966>, @Meinersbur wrote:

> In D76132#1943872 <https://reviews.llvm.org/D76132#1943872>, @Whitney wrote:
>
> > Thanks @Meinersbur ! I mostly used your code directly, except
> >
> >   for (unsigned d = 1; d < UnrollLevel; ++d) {
> >         // Check if dependence is carried by an outer loop.
> >         // That is, changing
> >         //   (0,>,>,*,*)
> >         // to
> >         //   (0,>,>=,*,*)
> >         // will still not violate the dependency.
> >         if (D->getDirection(d) == Dependence::DVEntry::GT)
> >           return true;
> >       }
> >
> >
> > which I think should be safe as long as the one dependence is not EQ then should be safe.
> >
> >   for i
> >     for j        <= unroll loop
> >       for k
> >          A[i][j][k]
> >          A[i-1][j+1][k]
> >
> >
> > Loop-j should be safe to unroll and jam. Am I right?
>
>
> Yes, that's what the cod above would be testing.


My above example was not good. 
Consider this example:

  for i
    for j        <= unroll loop
      for k
         A[i][j][k]
         A[i-1][j+1][k-1]

The dependence direction vector would be [LT, GT, LT].

After unroll and jam loop-j:

  for i
     for j  += 2
       for k
          A[i][j][k]
          A[i-1][j+1][k-1]
          A[i][j+1][k]
          A[i-1][j+2][k-1]

I think loop-j is safe to unroll and jam.
However if we only allow GT for loop-i, then this would be consider not safe.


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https://reviews.llvm.org/D76132