llvm dev - Jan 2019 - Issues with using scalar evolution with newer versions of LLVM IR

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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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.
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev

Thank You..
I used following command to generate .bc or .ll

/Documents/clang+llvm-4.0.0-x86_64-linux-gnu-ubuntu-16.04/bin/clang -O0
-emit-llvm -S -o vec4.ll vecsum.c
/Documents/clang+llvm-7.0.0-x86_64-linux-gnu-ubuntu-16.04/bin/clang -O0
-emit-llvm -S -o vec7.ll vecsum.c


On Wed, Jan 16, 2019 at 6:49 AM Sanjoy Das <sanjoy at
playingwithpointers.com>
wrote:
> 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.
> > _______________________________________________
> > LLVM Developers mailing list
> > llvm-dev at lists.llvm.org
> > http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
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