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<p class="MsoNormal"><span lang="EN-US">In short:<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">Shall analysis of a SCEV expression be allowed even though part of the IR that was analyzed to build the SCEV expression has been removed?<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">In more detail with background:<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">About a month ago I landed <a href="https://reviews.llvm.org/D87494">
https://reviews.llvm.org/D87494</a> where SCEV is used to do additional debug-info salvaging in LSR. The idea was to compute (and store) a SCEV of what llvm.dbg.value intrinsics inside the loop refers to and then after LSR transformation compare these SCEVs
with the current SCEVs of PHI-nodes of the loop header. If any of these match (with a possible offset) then the llvm.dbg.value intrinsic can be updated to refer to the PHI-node (with a possible offset in the DIExpression).<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">This use-case obviously depends on being able to analyze a SCEV where parts of the corresponding IR has been removed.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">Now last week <a href="https://bugs.llvm.org/show_bug.cgi?id=48166">
https://bugs.llvm.org/show_bug.cgi?id=48166</a> was reported which raises the question if this is allowed use-case or not.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">Clearly if the Value that a SCEVUnknown refers to has been deleted then that renders the entire SCEV useless. But if only IR that SCEV has successfully “analyzed through” is removed then I would argue that the SCEV expression
should still be analyzable.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">Thoughts, comments and clarifications are welcome.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">-Markus<o:p></o:p></span></p>
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