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<p class="MsoNormal">I think it’s reasonable to expect that IR generated by frontends doesn’t do this.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Not sure about transforms; I can imagine that we might speculate a load without proving all the bits are well-defined.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">-Eli<o:p></o:p></p>
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<p class="MsoNormal"><b>From:</b> llvm-dev <llvm-dev-bounces@lists.llvm.org> <b>On Behalf Of
</b>Juneyoung Lee via llvm-dev<br>
<b>Sent:</b> Sunday, September 20, 2020 3:54 PM<br>
<b>To:</b> llvm-dev <llvm-dev@lists.llvm.org><br>
<b>Subject:</b> [EXT] Re: [llvm-dev] Is it valid to dereference a pointer that have undef bits in its offset?<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">> %p2 = gep %p, (undef & 8)<o:p></o:p></p>
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<p class="MsoNormal">A silly typo: undef & 8 -> undef & 7<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">On Mon, Sep 21, 2020 at 7:47 AM Juneyoung Lee <<a href="mailto:juneyoung.lee@sf.snu.ac.kr">juneyoung.lee@sf.snu.ac.kr</a>> wrote:<o:p></o:p></p>
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<p class="MsoNormal">Hello all,<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">Is it valid to dereference a pointer that has undef bits in its offset?<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">For example,<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">%p = alloca [8 x i8]<o:p></o:p></p>
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<p class="MsoNormal">%p2 = gep %p, (undef & 8)<o:p></o:p></p>
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<p class="MsoNormal">store 0, %p2<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">undef & 8 is always less than 8, so technically it will store zero to one of the array's elements.<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">The reason is that I want to improve no-undef analysis by suggesting that a pointer that is passed to load/store is well-defined, by making it raise UB when a pointer with undef bits is given.<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">A suggested patch is here: <a href="https://reviews.llvm.org/D87994" target="_blank">
https://reviews.llvm.org/D87994</a><br clear="all">
<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">I wonder whether there is a case using this to do something that I'm not aware.<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">Thanks,<o:p></o:p></p>
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<p class="MsoNormal">Juneyoung<o:p></o:p></p>
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<o:p></o:p></p>
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<p class="MsoNormal">-- <o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal"><span style="font-size:7.5pt">Juneyoung Lee</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-size:7.5pt">Software Foundation Lab, Seoul National University</span><o:p></o:p></p>
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