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<p class="MsoNormal">Hi Sanjoy,<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">SCEV is behaving inconsistently when forming SCEV for this zext instruction in the attached test case-<o:p></o:p></p>
<p class="MsoNormal">%conv5 = zext i32 %dec to i64<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">If we request a SCEV for the instruction, it returns-<o:p></o:p></p>
<p class="MsoNormal">(zext i32 {{-1,+,1}<nw><%for.body>,+,-1}<nw><%for.body7> to i64)<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">This can be seen by invoking-<o:p></o:p></p>
<p class="MsoNormal">$ opt -analyze -scalar-evolution inconsistent-scev-zext.ll<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">But when computing the backedge taken count of the containing loop for.body7, it is able to push zext inside the AddRec and forms the following for the same instruction-<o:p></o:p></p>
<p class="MsoNormal">{(zext i32 {-1,+,1}<%for.body> to i64),+,-1}<nw><%for.body7><o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">This allows ScalarEvolution to compute a valid backedge taken count for the loop.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">The ‘simplification’ is done by this piece of code inside getZeroExtendExpr()-<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal"> // Cache knowledge of AR NW, which is propagated to this<o:p></o:p></p>
<p class="MsoNormal"> // AddRec. Negative step causes unsigned wrap, but it<o:p></o:p></p>
<p class="MsoNormal"> // still can't self-wrap.<o:p></o:p></p>
<p class="MsoNormal"> const_cast<SCEVAddRecExpr *>(AR)->setNoWrapFlags(SCEV::FlagNW);<o:p></o:p></p>
<p class="MsoNormal"> // Return the expression with the addrec on the outside.<o:p></o:p></p>
<p class="MsoNormal"> return getAddRecExpr(<o:p></o:p></p>
<p class="MsoNormal"> getExtendAddRecStart<SCEVZeroExtendExpr>(AR, Ty, this,<o:p></o:p></p>
<p class="MsoNormal"> Depth + 1),<o:p></o:p></p>
<p class="MsoNormal"> getSignExtendExpr(Step, Ty, Depth + 1), L,<o:p></o:p></p>
<p class="MsoNormal"> AR->getNoWrapFlags());<o:p></o:p></p>
<p class="MsoNormal"> }<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">I believe it is wrong for ScalarEvolution to use a simplified SCEV for internal analysis and return a non-simplified SCEV to the client.
<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Thanks,<o:p></o:p></p>
<p class="MsoNormal">Pankaj<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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