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<p class="MsoNormal"><span lang="EN-US" style="color:#1F497D">I don’t have a test case for that special case right now. The problem is that the code wants to determine condition’s type before see it, which is impossible IIUC, and calling getConstantFwdRef()
earlier doesn’t help there because it also requires for a type. I’ll commit a patch for the bug I posted, but more efforts will be needed to solve it thoroughly .<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="color:#1F497D">-Mindong<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="color:#1F497D"><o:p> </o:p></span></p>
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<p class="MsoNormal" align="left" style="text-align:left"><b><span lang="EN-US" style="font-size:11.0pt">From:</span></b><span lang="EN-US" style="font-size:11.0pt"> Eli Friedman [mailto:efriedma@quicinc.com]
<br>
<b>Sent:</b> Tuesday, July 21, 2020 3:57 AM<br>
<b>To:</b> chenmindong <chenmindong1@huawei.com>; llvm-dev@lists.llvm.org<br>
<b>Subject:</b> RE: [llvm-dev] BitcodeReader.cpp bug under LTO<o:p></o:p></span></p>
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<p class="MsoNormal" align="left" style="text-align:left"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal" align="left" style="text-align:left"><span lang="EN-US" style="font-size:11.0pt">The issue is specifically the case where the condition of a select constant expression is itself a select or shuffle constant expression? The simplest solution
is probably to just call getConstantFwdRef() early, to “set” the type of each expression. We know the result type of the constant expression when we first see it; the ambiguity is only the type of the condition operand.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt">-Eli<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt"><o:p> </o:p></span></p>
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<p class="MsoNormal" align="left" style="margin-left:36.0pt;text-align:left"><b><span lang="EN-US" style="font-size:11.0pt">From:</span></b><span lang="EN-US" style="font-size:11.0pt"> chenmindong <<a href="mailto:chenmindong1@huawei.com">chenmindong1@huawei.com</a>>
<br>
<b>Sent:</b> Sunday, July 19, 2020 10:27 PM<br>
<b>To:</b> Eli Friedman <<a href="mailto:efriedma@quicinc.com">efriedma@quicinc.com</a>>;
<a href="mailto:llvm-dev@lists.llvm.org">llvm-dev@lists.llvm.org</a><br>
<b>Subject:</b> [EXT] RE: [llvm-dev] BitcodeReader.cpp bug under LTO<o:p></o:p></span></p>
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<p class="MsoNormal" align="left" style="margin-left:36.0pt;text-align:left"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal" style="margin-left:36.0pt"><span lang="EN-US" style="color:#1F497D">Hi Eli,<o:p></o:p></span></p>
<p class="MsoNormal" style="margin-left:36.0pt"><span lang="EN-US" style="color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal" style="margin-left:36.0pt"><span lang="EN-US" style="color:#1F497D">Thanks for the advice! By delaying processing the “select” until we have resolved other records(like “aggregate ” in this case) as you did for “shuffle”, the test case
passes now. But I wonder if it’s an ultimate solution: what if the selector of a “select” is the output of another forward-reference “select” that hasn’t been resolved yet? We still cannot determine its type then. Is it possible?<o:p></o:p></span></p>
<p class="MsoNormal" style="margin-left:36.0pt"><span lang="EN-US" style="color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal" style="margin-left:36.0pt"><span lang="EN-US" style="color:#1F497D">Regards,<o:p></o:p></span></p>
<p class="MsoNormal" style="margin-left:36.0pt"><span lang="EN-US" style="color:#1F497D">Mindong<o:p></o:p></span></p>
<p class="MsoNormal" style="margin-left:36.0pt"><span lang="EN-US" style="color:#1F497D"><o:p> </o:p></span></p>
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<p class="MsoNormal" align="left" style="margin-left:36.0pt;text-align:left"><b><span lang="EN-US" style="font-size:11.0pt">From:</span></b><span lang="EN-US" style="font-size:11.0pt"> Eli Friedman [<a href="mailto:efriedma@quicinc.com">mailto:efriedma@quicinc.com</a>]
<br>
<b>Sent:</b> Friday, July 17, 2020 3:06 AM<br>
<b>To:</b> chenmindong <<a href="mailto:chenmindong1@huawei.com">chenmindong1@huawei.com</a>>; llvm-dev <<a href="mailto:llvm-dev@lists.llvm.org">llvm-dev@lists.llvm.org</a>><br>
<b>Subject:</b> RE: [llvm-dev] BitcodeReader.cpp bug under LTO<o:p></o:p></span></p>
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<p class="MsoNormal" align="left" style="margin-left:36.0pt;text-align:left"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal" style="margin-left:36.0pt"><span lang="EN-US" style="font-size:11.0pt">The DelayedShuffles code in BitcodeReader::parseConstants is solving a sort of similar issue; you might be able to borrow the same approach.<o:p></o:p></span></p>
<p class="MsoNormal" style="margin-left:36.0pt"><span lang="EN-US" style="font-size:11.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal" style="margin-left:36.0pt"><span lang="EN-US" style="font-size:11.0pt">-Eli<o:p></o:p></span></p>
<p class="MsoNormal" style="margin-left:36.0pt"><span lang="EN-US" style="font-size:11.0pt"><o:p> </o:p></span></p>
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<p class="MsoNormal" align="left" style="margin-left:72.0pt;text-align:left"><b><span lang="EN-US" style="font-size:11.0pt">From:</span></b><span lang="EN-US" style="font-size:11.0pt"> llvm-dev <<a href="mailto:llvm-dev-bounces@lists.llvm.org">llvm-dev-bounces@lists.llvm.org</a>>
<b>On Behalf Of </b>chenmindong via llvm-dev<br>
<b>Sent:</b> Thursday, July 16, 2020 7:54 AM<br>
<b>To:</b> <a href="mailto:llvm-dev@lists.llvm.org">llvm-dev@lists.llvm.org</a><br>
<b>Subject:</b> [EXT] [llvm-dev] BitcodeReader.cpp bug under LTO<o:p></o:p></span></p>
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<p class="MsoNormal" align="left" style="margin-left:72.0pt;text-align:left"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal" style="margin-left:72.0pt"><span lang="EN-US">Hi guys,<o:p></o:p></span></p>
<p class="MsoNormal" style="margin-left:72.0pt"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal" style="margin-left:72.0pt"><span lang="EN-US">We have found a bug of BitcodeReader.cpp in processing an LTO bitcode file. As LLVM doesn’t emit use-list for LTO bitcode files, many forward references will happen when BitcodeReader processes
the bitcode file, and LLVM uses placeholders for those forward references and resolve them later.
<o:p></o:p></span></p>
<p class="MsoNormal" style="margin-left:72.0pt"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal" style="margin-left:72.0pt"><span lang="EN-US">When parseConstants() reads in a CST_CODE_CE_SELECT record, e.g.<o:p></o:p></span></p>
<p class="MsoNormal" style="margin-left:72.0pt"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal" style="margin-left:72.0pt"><span lang="EN-US">select <selty><cond>, <ty><val1>, <ty><val2><o:p></o:p></span></p>
<p class="MsoNormal" style="margin-left:72.0pt"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal" style="margin-left:72.0pt"><span lang="EN-US">If “ty” here is a vector type and “cond” is a forward reference, LLVM uses i1 as the placeholder type of “cond” if it cannot find “cond” in ValueList, as the code follows:<o:p></o:p></span></p>
<p class="MsoNormal" style="margin-left:72.0pt"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal" style="margin-left:72.0pt"><span lang="EN-US"> Type *SelectorTy = Type::getInt1Ty(Context);<o:p></o:p></span></p>
<p class="MsoNormal" style="margin-left:72.0pt"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal" style="margin-left:72.0pt"><span lang="EN-US"> // The selector might be an i1 or an <n x i1><o:p></o:p></span></p>
<p class="MsoNormal" style="margin-left:72.0pt"><span lang="EN-US"> // Get the type from the ValueList before getting a forward ref.<o:p></o:p></span></p>
<p class="MsoNormal" style="margin-left:72.0pt"><span lang="EN-US"> if (VectorType *VTy = dyn_cast<VectorType>(CurTy))<o:p></o:p></span></p>
<p class="MsoNormal" style="margin-left:72.0pt"><span lang="EN-US"> if (Value *V = ValueList[Record[0]])<o:p></o:p></span></p>
<p class="MsoNormal" style="margin-left:72.0pt"><span lang="EN-US"> if (SelectorTy != V->getType())<o:p></o:p></span></p>
<p class="MsoNormal" style="margin-left:72.0pt"><span lang="EN-US"> SelectorTy = VectorType::get(SelectorTy, VTy->getNumElements());
<o:p></o:p></span></p>
<p class="MsoNormal" style="margin-left:72.0pt"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal" style="margin-left:72.0pt"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal" style="margin-left:72.0pt"><span lang="EN-US">However, the program aborts in RAUW() if we find “selty” is a vector type later, because LLVM are trying to replace an i1 placeholder with an <n x i1> value.<o:p></o:p></span></p>
<p class="MsoNormal" style="margin-left:72.0pt"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal" style="margin-left:72.0pt"><span lang="EN-US">A rough idea is to create a BitcodeReader-specific RAUW which doesn’t check type legitimacy and any other suggestion is welcome.<o:p></o:p></span></p>
<p class="MsoNormal" style="margin-left:72.0pt"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal" style="margin-left:72.0pt"><span lang="EN-US">Bugzilla link:
<a href="https://bugs.llvm.org/show_bug.cgi?id=46750">https://bugs.llvm.org/show_bug.cgi?id=46750</a><o:p></o:p></span></p>
<p class="MsoNormal" style="margin-left:72.0pt"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal" style="margin-left:72.0pt"><span lang="EN-US">Regards,<o:p></o:p></span></p>
<p class="MsoNormal" style="margin-left:72.0pt"><span lang="EN-US">Mindong<o:p></o:p></span></p>
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