llvm dev - Feb 2021 - Obtaining bounds of loops

Hi,

I am trying to test out the getBounds() method from LoopInfo. However, the 
method doesn't seem to obtain the bounds of simple loops like -

        for (int j = 0; j < 10; j ++)
            B[j] = 6;

        int i = 10, j = 2,

        while ( i != j)
              j++;

  My code snippet in the function pass is as follows (which gives the output 
as "No value for loop bounds" for all the loops):

       LoopInfo &LI =
getAnalysis<LoopInfoWrapperPass>().getLoopInfo();

       for (auto loop = LI.begin(), e = LI.end(); loop != e; ++loop )
       {
           ScalarEvolution *SE = 
&getAnalysis<ScalarEvolutionWrapperPass>().getSE();
           auto b = (*loop)->getBounds(*SE);

           if (!b.hasValue())
                {
                   errs() << "No value for loop bounds\n";
                }
       }

  Is there something that I am missing here?


  Thanks,
  Bodhi

Hi Bodhi,

Well, probably you can't get the bounds because your loops are not rotated
[1]
You can get that with -loop-rotate pass.

FWIW, this is required by `getInductionVariable()` [2]
as if the loop is not rotated, you won't have the comparison on the latch.
In general, I think that `getInductionVariable()` is weird, which
results in e.g., some loop optimizations writing _their own_
getInductionVariable() because the "canonical" one can't do the
job. For
example, loop interchange [3]

Best,
Stefanos

[1]  https://llvm.org/docs/LoopTerminology.html#rotated-loops
[2]
https://github.com/llvm/llvm-project/blob/95d13c01ecba5c9dba8ea1bd875c4179cbaea9e2/llvm/lib/Analysis/LoopInfo.cpp#L299
[3]
https://github.com/llvm/llvm-project/blob/95d13c01ecba5c9dba8ea1bd875c4179cbaea9e2/llvm/lib/Transforms/Scalar/LoopInterchange.cpp#L298

Στις Δευ, 22 Φεβ 2021 στις 6:48 μ.μ., ο/η bodhisatwa via llvm-dev <
llvm-dev at lists.llvm.org> έγραψε:
> Hi,
>
> I am trying to test out the getBounds() method from LoopInfo. However, the
> method doesn't seem to obtain the bounds of simple loops like -
>
>         for (int j = 0; j < 10; j ++)
>             B[j] = 6;
>
>         int i = 10, j = 2,
>
>         while ( i != j)
>               j++;
>
>   My code snippet in the function pass is as follows (which gives the
> output
> as "No value for loop bounds" for all the loops):
>
>        LoopInfo &LI =
getAnalysis<LoopInfoWrapperPass>().getLoopInfo();
>
>        for (auto loop = LI.begin(), e = LI.end(); loop != e; ++loop )
>        {
>            ScalarEvolution *SE >
&getAnalysis<ScalarEvolutionWrapperPass>().getSE();
>            auto b = (*loop)->getBounds(*SE);
>
>            if (!b.hasValue())
>                 {
>                    errs() << "No value for loop bounds\n";
>                 }
>        }
>
>   Is there something that I am missing here?
>
>
>   Thanks,
>   Bodhi
>
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL:
<http://lists.llvm.org/pipermail/llvm-dev/attachments/20210222/8e426d0e/attachment.html>

JFYI, the infrastructure on Loop itself is very very limited. Honestly, 
it probably shouldn't exist at all.

ScalarEvolution exists to analyze loops.  In particular, it can compute 
symbolic execute counts for a much much broader family of loops.

Without knowing your use case, I can't say for sure, but I suspect you 
want to be using ScalarEvolution.

Philip

On 2/22/21 8:48 AM, bodhisatwa via llvm-dev wrote:
> Hi,
>
> I am trying to test out the getBounds() method from LoopInfo. However, 
> the method doesn't seem to obtain the bounds of simple loops like -
>
>        for (int j = 0; j < 10; j ++)
>            B[j] = 6;
>
>        int i = 10, j = 2,
>
>        while ( i != j)
>              j++;
>
>  My code snippet in the function pass is as follows (which gives the 
> output as "No value for loop bounds" for all the loops):
>
>       LoopInfo &LI =
getAnalysis<LoopInfoWrapperPass>().getLoopInfo();
>
>       for (auto loop = LI.begin(), e = LI.end(); loop != e; ++loop )
>       {
>           ScalarEvolution *SE = 
> &getAnalysis<ScalarEvolutionWrapperPass>().getSE();
>           auto b = (*loop)->getBounds(*SE);
>
>           if (!b.hasValue())
>                {
>                   errs() << "No value for loop bounds\n";
>                }
>       }
>
>  Is there something that I am missing here?
>
>
>  Thanks,
>  Bodhi
>
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev