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</o:shapelayout></xml><![endif]--></head><body lang=EN-US link=blue vlink=purple style='word-wrap:break-word'><div class=WordSection1><p class=MsoNormal>The highlighted transformations are all correct. If there<span style='font-family:"Times New Roman",serif'>’</span>s a bug in LLVM it<span style='font-family:"Times New Roman",serif'>’</span>s somewhere else. But it<span style='font-family:"Times New Roman",serif'>’</span>s more likely it<span style='font-family:"Times New Roman",serif'>’</span>s a bug in the source program.<o:p></o:p></p><p class=MsoNormal>You can use Alive2 to check correctness of optimizations,, e.g.: <a href="https://alive2.llvm.org/ce/z/rbZbY7">https://alive2.llvm.org/ce/z/rbZbY7</a><o:p></o:p></p><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>Nuno<o:p></o:p></p><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal><o:p> </o:p></p><div style='border:none;border-top:solid #E1E1E1 1.0pt;padding:3.0pt 0cm 0cm 0cm'><p class=MsoNormal><b>From:</b> llvm-dev <llvm-dev-bounces@lists.llvm.org> <b>On Behalf Of </b>Rohit Aggarwal via llvm-dev<br><b>Sent:</b> 20 August 2021 13:45<br><b>To:</b> Florian Hahn <florian_hahn@apple.com><br><b>Cc:</b> llvm-dev <llvm-dev@lists.llvm.org><br><b>Subject:</b> Re: [llvm-dev] After InstCombine pass, expected output got changed<o:p></o:p></p></div><p class=MsoNormal><o:p> </o:p></p><div><div><p class=MsoNormal>Hi Florian,<o:p></o:p></p><div><p class=MsoNormal><o:p> </o:p></p></div><div><p class=MsoNormal><o:p> </o:p></p></div></div><p class=MsoNormal><o:p> </o:p></p><div><div><p class=MsoNormal>On Thu, Aug 19, 2021 at 3:46 PM Florian Hahn <<a href="mailto:florian_hahn@apple.com">florian_hahn@apple.com</a>> wrote:<o:p></o:p></p></div><blockquote style='border:none;border-left:solid #CCCCCC 1.0pt;padding:0cm 0cm 0cm 6.0pt;margin-left:4.8pt;margin-right:0cm'><div><div><p class=MsoNormal><o:p> </o:p></p></div><div><p class=MsoNormal><br><br><o:p></o:p></p><blockquote style='margin-top:5.0pt;margin-bottom:5.0pt'><p class=MsoNormal style='margin-bottom:12.0pt'>On 19 Aug 2021, at 11:01, Rohit Aggarwal via llvm-dev <<a href="mailto:llvm-dev@lists.llvm.org" target="_blank">llvm-dev@lists.llvm.org</a>> wrote:<o:p></o:p></p></blockquote></div><blockquote style='margin-top:5.0pt;margin-bottom:5.0pt'><div><p class=MsoNormal><o:p></o:p></p><div><p class=MsoNormal>Hi All,<o:p></o:p></p><div><p class=MsoNormal><o:p> </o:p></p></div><div><p class=MsoNormal>Thanks Florian for response.<o:p></o:p></p></div><div><p class=MsoNormal><o:p> </o:p></p></div><div><div><div><p class=MsoNormal> Adding more details to the issue:<o:p></o:p></p></div><div><ul type=disc><li class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;mso-list:l3 level1 lfo1'>I revert the commit <a href="https://github.com/llvm/llvm-project/commit/2fd3037ac615643fe8058292d2b89bb19a49cb2f" target="_blank">[ConstantFold] Allow propagation of poison for and/or i1</a> then it is working fine.<o:p></o:p></li><li class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;mso-list:l3 level1 lfo1'>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.<o:p></o:p></li><li class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;mso-list:l3 level1 lfo1'>Using both release branch and commit reverted branch, a poison value is generated by a shift left instruction due to overflow (UB is present) <o:p></o:p></li></ul></div></div></div></div></div></blockquote><div><p class=MsoNormal><o:p> </o:p></p></div><p class=MsoNormal>Ok so it sounds like this is the reason for the different output and the transformation is correct.<o:p></o:p></p></div></blockquote><div><p class=MsoNormal> >> Yes, this poison which is generated is handled differently in llvm-trunk and reverted commit code down the pipeline. <o:p></o:p></p></div><div><p class=MsoNormal> <o:p></o:p></p></div><blockquote style='border:none;border-left:solid #CCCCCC 1.0pt;padding:0cm 0cm 0cm 6.0pt;margin-left:4.8pt;margin-right:0cm'><div><blockquote style='margin-top:5.0pt;margin-bottom:5.0pt'><div><div><div><ul type=disc><li class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;mso-list:l2 level1 lfo2'>Now in the release branch, this poison value is propagated down to 'or' inst. and is constantfolds %or = or true poison -> poison <o:p></o:p></li></ul></div></div></div></blockquote></div></blockquote><div><p class=MsoNormal><o:p> </o:p></p></div><div><ul type=disc><li class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;mso-list:l1 level1 lfo3'>where as in the reverted code, %or = or true poison -> true<o:p></o:p></li></ul></div><div><p class=MsoNormal> >> Here is the main reason, where the problem started <o:p></o:p></p></div><blockquote style='border:none;border-left:solid #CCCCCC 1.0pt;padding:0cm 0cm 0cm 6.0pt;margin-left:4.8pt;margin-right:0cm'><div><div><blockquote style='margin-top:5.0pt;margin-bottom:5.0pt'><div><div><div><div><div><ul type=disc><li class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;mso-list:l0 level1 lfo4'>In the release branch, the poison value propagates to its uses<o:p></o:p></li><li class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;mso-list:l0 level1 lfo4'>There is a store instruction which now has a poison value<o:p></o:p></li><li class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;mso-list:l0 level1 lfo4'>In visitStoreInst, store instruction got removed due to poison value<o:p></o:p></li><li class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;mso-list:l0 level1 lfo4'>But in revert branch, store is not getting remove as it has defined value<o:p></o:p></li></ul><div><p class=MsoNormal>See the below diff for more clarity.<o:p></o:p></p></div><div><p class=MsoNormal><o:p></o:p></p></div></div></div><div><p class=MsoNormal><o:p> </o:p></p></div></div></div></div></blockquote></div></div></blockquote><div><p class=MsoNormal>Reattaching the image. PFA for more clarity.<o:p></o:p></p></div><div><p class=MsoNormal><o:p> </o:p></p></div><div><p class=MsoNormal>Thanks,<o:p></o:p></p></div><div><p class=MsoNormal>Rohit Aggarwal<o:p></o:p></p></div></div></div><p class=MsoNormal><o:p> </o:p></p><p style='margin:0cm'><span style='font-family:"Arial",sans-serif;color:#222222;background:yellow'>Disclaimer:- </span><span style='font-size:12.0pt;color:#222222;background:white'>This footer text is to convey that this email is sent by one of the users of IITH. So, do not mark it as SPAM.</span><o:p></o:p></p></div></body></html>