[llvm-dev] After InstCombine pass, expected output got changed

[Rohit Aggarwal via llvm-dev]

Hi All,

Thanks Florian for response.

 Adding more details to the issue:

   - I revert the commit  [ConstantFold] Allow propagation of poison for
   and/or i1
   <https://github.com/llvm/llvm-project/commit/2fd3037ac615643fe8058292d2b89bb19a49cb2f#>
then
   it is working fine.
   - In this commit, a check is removed to stop the constantfold of
   instruction to poison value incase of integer ADD or OR instruction if
   either of operands are poison.
   - Using both release branch and commit reverted branch, a poison value
   is generated by a shift left instruction due to overflow (UB is present)
   - Now in the release branch, this poison value is propagated down to
   'or' inst. and  is constantfolds %or = or  true poison  -> poison
   - The poison value propagates to its uses
   - There is a store instruction which now has a poison value
   - In visitStoreInst, store instruction got removed due to poison value
   - But in revert branch, store is not getting remove as it has defined
   value

See the below diff for more clarity.
[image: image.png]

Thanks,
Rohit Aggarwal

On Thu, Aug 19, 2021 at 2:19 PM Florian Hahn <florian_hahn at apple.com> wrote:

> 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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