llvm dev - Aug 2021 - After InstCombine pass, expected output got changed

Hi All,

I am running the instcombine pass on the given lR and executing the binary.

*./build_release13x/bin/opt -enable-new-pm=false  -instcombine sample.ll -S
-o sample_instcombined_llvm.ll && ./build_release13x/bin/clang
sample_instcombined_llvm.ll && ./a.out*

But before passing to instcombine, the output is *CE6DB4AC* and after
passing to instcombine value changes to *3B83FBC*.

I tried to debug the issue and found that due to the occurence of poison
value, some instructions(or, store,..) are getting deleted/erased.

I also observed that there are active changes happening related to poison
values in the community. Is this behaviour expected one? Or Could it be a
side effect of those changes?

Thanks and Regards,
Rohit Aggarwal
Compiler Engineer - AMD

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Hi,
> On Aug 18, 2021, at 21:21, Rohit Aggarwal via llvm-dev <llvm-dev at
lists.llvm.org> wrote:
> 
> 
> Hi All,
> 
> I am running the instcombine pass on the given lR and executing the binary.
> 
> ./build_release13x/bin/opt -enable-new-pm=false  -instcombine sample.ll -S
-o sample_instcombined_llvm.ll && ./build_release13x/bin/clang
sample_instcombined_llvm.ll && ./a.out
> 
> But before passing to instcombine, the output is CE6DB4AC and after passing
to instcombine value changes to 3B83FBC.
> 
> I tried to debug the issue and found that due to the occurence of poison
value, some instructions(or, store,..) are getting deleted/erased.
> 
> I also observed that there are active changes happening related to poison
values in the community. Is this behaviour expected one? Or Could it be a side
effect of those changes
There are 2 potential causes for the difference: 1) your input has undefined
behavior and/or the result depends on a poison value or 2) a bug in instcombine.

Without narrowing things down to the problematic pattern, it is very hard to say
which one it is. I’d suggest you try to isolate the change in behavior to the
problematic instcombine pattern and minimize the input IR to trigger the
problematic pattern.

Cheers,
Florian
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