[llvm-dev] Issues with using scalar evolution with newer versions of LLVM IR

[Sanjoy Das via llvm-dev]

It is hard to tell what's going on from the information you have
provided.  How are you generating vec.bc?  This could either be a
regression in scalar evolution or a change in some optimization pass
that is resulting in a different vec.bc.

-- Sanjoy

On Tue, Jan 15, 2019 at 1:42 PM hameeza ahmed via llvm-dev
<llvm-dev at lists.llvm.org> wrote:
>
> Hello,
>
> I am trying to use scalar evolution pass using following command;
>
> opt -analyze -mem2reg -indvars -loop-simplify -scalar-evolution < vec.bc
> when vec.bc is generated using newer version of LLVM i.e LLVM 6 and 7 i get following message in the end;
> Determining loop execution counts for: @main
> Loop %8: Unpredictable backedge-taken count.
> Loop %8: Unpredictable max backedge-taken count.
> Loop %8: Unpredictable predicated backedge-taken count.
> which means it is unable to compute iteration count.
>
> However, when .bc file is generated using LLVM 4.0 i am getting following;
>
> Determining loop execution counts for: @main
> Loop %4: backedge-taken count is 512
> Loop %4: max backedge-taken count is 512
> Loop %4: Predicated backedge-taken count is 512
>  Predicates:
>
> Loop %4: Trip multiple is 513
>
> means correct computation of loop count...
> Why is that happening with newer version of LLVM?
>
> Please help.
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