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<p class="MsoNormal">Hi,<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">We have recently encountered incorrect code generation issues and I think it is a SCEVExpander bug exposed by a recent patch to add nuw/nsw flags when generating code for SCEVMulExpr.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">The test case IR looks like this-<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">%1 = load i16, i16* %arrayidx<o:p></o:p></p>
<p class="MsoNormal">%2 = and i16 %1, -32768<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">The SCEV form of %2 is this-<o:p></o:p></p>
<p class="MsoNormal">(-32768 * (%1 /u -32768))<nuw><nsw><o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">It has both nsw/nuw flags. The signed range of the second operand of SCEVMulExpr (%1 /u -32768) is [0, 2). So it can be either 0 or 1.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">But SCEVExpander uses shl to generate the multiply. The generated IR looks something like this-<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">%7 = lshr i16 %gepload, 15<o:p></o:p></p>
<p class="MsoNormal">%8 = shl nuw nsw i16 %7, 15<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">The issue is that %8 is later simplified to zero because according to langref it produces poison otherwise -<o:p></o:p></p>
<p class="MsoNormal"><i>If the nuw keyword is present, then the shift produces a poison value if it shifts out any non-zero bits. If the nsw keyword is present, then the shift produces a poison value if it shifts out any bits that disagree with the resultant
sign bit.<o:p></o:p></i></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Is this a real bug or am I missing something?<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>
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