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<p class="MsoNormal">Thanks for all the feedback !<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">The issue I encountered is likely not happening on the main llvm, but I believe the issue is interesting enough to<o:p></o:p></p>
<p class="MsoNormal">document:<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">- the problematic code construct consists of a cycle of 'dead' basic blocks that conditionally jumps into live blocks.<o:p></o:p></p>
<p class="MsoNormal"> That jump introduce a connection of a dead self-referring 'getelementptr' to live code through a PHI node.<o:p></o:p></p>
<p class="MsoNormal">- during SROA, the scoped noalias analysis (full restrict version) does an unbounded (MaxLookup == 0) 'getUnderlyingObject' call on a pointer.<o:p></o:p></p>
<p class="MsoNormal">- 'getUnderlyingObject' also follows the path to the dead blocks and ends up in the self-referring 'getelementpr'.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Does this mean that 'getUnderlyingObject' should also be prepared to be handling 'invalid instructions' in 'dead code' ?<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Greetings,<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Jeroen Dobbelaere<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal"><b>From:</b> Mehdi AMINI <joker.eph@gmail.com> <br>
<b>Sent:</b> Sunday, March 14, 2021 05:05<br>
<b>To:</b> Jeroen Dobbelaere <dobbel@synopsys.com><br>
<b>Cc:</b> llvm-dev@lists.llvm.org<br>
<b>Subject:</b> Re: [llvm-dev] Invalid llvm-ir and unreachable code<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 Fri, Mar 12, 2021 at 12:47 AM Jeroen Dobbelaere via llvm-dev <<a href="mailto:llvm-dev@lists.llvm.org">llvm-dev@lists.llvm.org</a>> wrote:<o:p></o:p></p>
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<p class="MsoNormal">Hi all,<br>
<br>
I have found a case where an optimization pass is barfing on invalid code in an unreachable<br>
basic block. A self referencing GEP '%x = getelementptr %x, 0, 1' is found inside a cycle of<br>
two unreachable basic blocks)<br>
<br>
The invalid code and the unreachable basic blocks were introduced by the function inliner.<br>
<br>
I am wondering what is valid for these cases ?<br>
(I did not find anything in LangRef, but I might as well have missed it)<br>
<br>
- clearly 'dead code' (aka unreachable basic blocks) is valid in an IR ?<br>
- I assume that the self reference in dead code is not valid ?<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">Yes, self-references are valid in unreachable code, we have test cases for this, it'll pass the verifier. Passes should be resilient against this (even though a pass could always do a quick reachability check and delete all dead blocks
as a pre-step).<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">Mehdi<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">- should inlining do an extra effort of cleaning up such dead code blocks ?<br>
<br>
Thanks,<br>
<br>
Jeroen Dobbelaere<br>
<br>
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