<html xmlns:v="urn:schemas-microsoft-com:vml" xmlns:o="urn:schemas-microsoft-com:office:office" xmlns:w="urn:schemas-microsoft-com:office:word" xmlns:x="urn:schemas-microsoft-com:office:excel" xmlns:m="http://schemas.microsoft.com/office/2004/12/omml" xmlns="http://www.w3.org/TR/REC-html40">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<meta name="Generator" content="Microsoft Word 15 (filtered medium)">
<style><!--
/* Font Definitions */
@font-face
{font-family:Helvetica;
panose-1:2 11 6 4 2 2 2 2 2 4;}
@font-face
{font-family:"Cambria Math";
panose-1:2 4 5 3 5 4 6 3 2 4;}
@font-face
{font-family:Calibri;
panose-1:2 15 5 2 2 2 4 3 2 4;}
@font-face
{font-family:Menlo;
panose-1:0 0 0 0 0 0 0 0 0 0;}
@font-face
{font-family:Monaco;}
/* Style Definitions */
p.MsoNormal, li.MsoNormal, div.MsoNormal
{margin:0in;
margin-bottom:.0001pt;
font-size:12.0pt;
font-family:"Times New Roman",serif;}
a:link, span.MsoHyperlink
{mso-style-priority:99;
color:blue;
text-decoration:underline;}
a:visited, span.MsoHyperlinkFollowed
{mso-style-priority:99;
color:purple;
text-decoration:underline;}
span.gmail-
{mso-style-name:gmail-;}
span.EmailStyle18
{mso-style-type:personal;
font-family:"Calibri",sans-serif;
color:#1F497D;}
span.EmailStyle19
{mso-style-type:personal-compose;
font-family:"Calibri",sans-serif;
color:windowtext;}
.MsoChpDefault
{mso-style-type:export-only;
font-family:"Calibri",sans-serif;}
@page WordSection1
{size:8.5in 11.0in;
margin:1.0in 1.0in 1.0in 1.0in;}
div.WordSection1
{page:WordSection1;}
--></style><!--[if gte mso 9]><xml>
<o:shapedefaults v:ext="edit" spidmax="1026" />
</xml><![endif]--><!--[if gte mso 9]><xml>
<o:shapelayout v:ext="edit">
<o:idmap v:ext="edit" data="1" />
</o:shapelayout></xml><![endif]-->
</head>
<body lang="EN-US" link="blue" vlink="purple">
<div class="WordSection1">
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">I understand that changing the starting element to “</span><span style="font-family:"Menlo",serif">InsertedPHIs.being() + StartingPHISize</span><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">”
it will be finite but given that InsrtedPHIs is finite. <o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">I have a case where one element(same element is appened to InsertedPHIs) is added to InsertedPHIs every time fixupDefs is invoked. I traced the issue why this
was happening.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">template <class RangeType><o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">MemoryAccess *MemorySSAUpdater::tryRemoveTrivialPhi(MemoryPhi *Phi,<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"> RangeType &Operands) {<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"> // Detect equal or self arguments<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"> MemoryAccess *Same = nullptr;<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"> for (auto &Op : Operands) {<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"> // If the same or self, good so far<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"> if (Op == Phi || Op == Same)<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"> continue;<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"> // not the same, return the phi since it's not eliminatable by us<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"> if (Same)<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"> return Phi;<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"> Same = cast<MemoryAccess>(Op);<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"> }<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"> // Never found a non-self reference, the phi is undef<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"> if (Same == nullptr)<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"> return MSSA->getLiveOnEntryDef();<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"> if (Phi) {<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"> Phi->replaceAllUsesWith(Same);<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"> removeMemoryAccess(Phi);<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"> }<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"> // We should only end up recursing in case we replaced something, in which<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"> // case, we may have made other Phis trivial.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"> return recursePhi(Same);<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">}<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">In above function what happens when Same is not null and Phi is null. I have one particular case where it retruns non null MemoryPhi element.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"> // See if we can avoid the phi by simplifying it.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"> auto *Result = tryRemoveTrivialPhi(Phi, PhiOps);<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"> // If we couldn't simplify, we may have to create a phi<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"> if (Result == Phi) {<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"> if (!Phi)<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"> Phi = MSSA->createMemoryPhi(BB);<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"> // These will have been filled in by the recursive read we did above.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"> if (PHIExistsButNeedsUpdate) {<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"> std::copy(PhiOps.begin(), PhiOps.end(), Phi->op_begin());<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"> std::copy(pred_begin(BB), pred_end(BB), Phi->block_begin());<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"> } else {<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"> unsigned i = 0;<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"> for (auto *Pred : predecessors(BB))<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"> Phi->addIncoming(PhiOps[i++], Pred);<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"> }<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"> Result = Phi;<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"> }<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"> if (MemoryPhi *MP = dyn_cast<MemoryPhi>(Result))<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"> InsertedPHIs.push_back(MP);<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"> // Set ourselves up for the next variable by resetting visited state.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"> VisitedBlocks.erase(BB);<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"> return Result;<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">In above code which is part of “MemoryAccess *MemorySSAUpdater::getPreviousDefRecursive(BasicBlock *BB)” function. If Result is non null and Phi is null then
InsertedPHIs is updated but Result is not added to the Basic Block BB.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">My question is what is the intended behavior when Result is non null and Phi is null??<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">Below is the dump for the sample case I’m observing. It’s the state of InsertedPHIs after every invocation of fixupDefs.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:Monaco">For reference I included, some dumps that I introduced, below:</span><o:p></o:p></p>
<p class="MsoNormal"> <o:p></o:p></p>
<p class="MsoNormal"><span style="font-family:Monaco">Inserted PHIs</span><o:p></o:p></p>
<p class="MsoNormal"><span style="font-family:Monaco">at 0: 470 = MemoryPhi({if.then753,274},{Perl_hv_placeholders_get.exit,275})</span><o:p></o:p></p>
<p class="MsoNormal"> <o:p></o:p></p>
<p class="MsoNormal"><span style="font-family:Monaco">Inserted PHIs</span><o:p></o:p></p>
<p class="MsoNormal"><span style="font-family:Monaco">at 0: 470 = MemoryPhi({if.then753,274},{Perl_hv_placeholders_get.exit,275})</span><o:p></o:p></p>
<p class="MsoNormal"> <o:p></o:p></p>
<p class="MsoNormal"><span style="font-family:Monaco">at 1: 470 = MemoryPhi({if.then753,274},{Perl_hv_placeholders_get.exit,275})</span><o:p></o:p></p>
<p class="MsoNormal"> <o:p></o:p></p>
<p class="MsoNormal"><span style="font-family:Monaco">Inserted PHIs</span><o:p></o:p></p>
<p class="MsoNormal"><span style="font-family:Monaco">at 0: 470 = MemoryPhi({if.then753,274},{Perl_hv_placeholders_get.exit,275})</span><o:p></o:p></p>
<p class="MsoNormal"> <o:p></o:p></p>
<p class="MsoNormal"><span style="font-family:Monaco">at 1: 470 = MemoryPhi({if.then753,274},{Perl_hv_placeholders_get.exit,275})</span><o:p></o:p></p>
<p class="MsoNormal"> <o:p></o:p></p>
<p class="MsoNormal"><span style="font-family:Monaco">at 2: 470 = MemoryPhi({if.then753,274},{Perl_hv_placeholders_get.exit,275})</span><o:p></o:p></p>
<p class="MsoNormal"> <o:p></o:p></p>
<p class="MsoNormal"><span style="font-family:Monaco">Inserted PHIs</span><o:p></o:p></p>
<p class="MsoNormal"><span style="font-family:Monaco">at 0: 470 = MemoryPhi({if.then753,274},{Perl_hv_placeholders_get.exit,275})</span><o:p></o:p></p>
<p class="MsoNormal"> <o:p></o:p></p>
<p class="MsoNormal"><span style="font-family:Monaco">at 1: 470 = MemoryPhi({if.then753,274},{Perl_hv_placeholders_get.exit,275})</span><o:p></o:p></p>
<p class="MsoNormal"> <o:p></o:p></p>
<p class="MsoNormal"><span style="font-family:Monaco">at 2: 470 = MemoryPhi({if.then753,274},{Perl_hv_placeholders_get.exit,275})</span><o:p></o:p></p>
<p class="MsoNormal"> <o:p></o:p></p>
<p class="MsoNormal"><span style="font-family:Monaco">at 3: 470 = MemoryPhi({if.then753,274},{Perl_hv_placeholders_get.exit,275})</span><span style="font-size:9.0pt;font-family:"Helvetica",sans-serif"><br>
<br>
</span><o:p></o:p></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><b><span style="font-size:11.0pt;font-family:"Calibri",sans-serif">From:</span></b><span style="font-size:11.0pt;font-family:"Calibri",sans-serif"> Daniel Berlin [mailto:dberlin@dberlin.org]
<br>
<b>Sent:</b> Saturday, September 23, 2017 10:42 PM<br>
<b>To:</b> Godala, Bhargav-reddy <Bhargav-reddy.Godala@amd.com><br>
<b>Cc:</b> llvm-dev@lists.llvm.org<br>
<b>Subject:</b> Re: [llvm-dev] Potential infinite loop in MemorySSAUpdater<o:p></o:p></span></p>
<p class="MsoNormal"><o:p> </o:p></p>
<div>
<p class="MsoNormal"><o:p> </o:p></p>
<div>
<p class="MsoNormal"><o:p> </o:p></p>
<div>
<p class="MsoNormal">On Sat, Sep 23, 2017 at 9:55 AM, Godala, Bhargav-reddy <<a href="mailto:Bhargav-reddy.Godala@amd.com" target="_blank">Bhargav-reddy.Godala@amd.com</a>> wrote:<o:p></o:p></p>
<blockquote style="border:none;border-left:solid #CCCCCC 1.0pt;padding:0in 0in 0in 6.0pt;margin-left:4.8pt;margin-right:0in">
<div>
<p class="MsoNormal"><span class="gmail-"><o:p> </o:p></span></p>
<div>
<div>
<p class="MsoNormal"><span style="font-size:9.0pt;font-family:"Helvetica",sans-serif;color:black">With regards<br>
Bhargav Reddy Godala<br>
Software Engineer 2<br>
Bangalore, India<br>
E-mail: <a href="mailto:Bhargav-reddy.Godala@amd.com" target="_blank">Bhargav-reddy.Godala@amd.com</a> Ext 30678<o:p></o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-size:9.0pt;font-family:"Helvetica",sans-serif;color:black"><o:p> </o:p></span></p>
</div>
<p class="MsoNormal"><o:p> </o:p></p>
</div>
<p class="MsoNormal"><o:p> </o:p></p>
<div>
<blockquote style="margin-top:5.0pt;margin-bottom:5.0pt">
<div>
<p class="MsoNormal">On 23-Sep-2017, at 9:27 PM, Daniel Berlin <<a href="mailto:dberlin@dberlin.org" target="_blank">dberlin@dberlin.org</a>> wrote:<o:p></o:p></p>
</div>
<p class="MsoNormal"><o:p> </o:p></p>
<div>
<div>
<p class="MsoNormal"><o:p> </o:p></p>
<div>
<p class="MsoNormal"><o:p> </o:p></p>
<div>
<p class="MsoNormal">On Sat, Sep 23, 2017 at 8:38 AM, Godala, Bhargav-reddy via llvm-dev <<a href="mailto:llvm-dev@lists.llvm.org" target="_blank">llvm-dev@lists.llvm.org</a>> wrote:<o:p></o:p></p>
<blockquote style="border:none;border-left:solid #CCCCCC 1.0pt;padding:0in 0in 0in 6.0pt;margin-left:4.8pt;margin-right:0in">
<div>
<p class="MsoNormal">Hi, <o:p></o:p></p>
<div>
<p class="MsoNormal"><o:p> </o:p></p>
</div>
<div>
<p class="MsoNormal">Can some one explain the intended behaviour of following loop in void MemorySSAUpdater::insertDef(MemoryDef *MD, bool RenameUses) function.<o:p></o:p></p>
</div>
<div>
<p class="MsoNormal"><o:p> </o:p></p>
</div>
<div>
<div>
<p class="MsoNormal"><span style="font-family:"Menlo",serif"> while (!FixupList.empty()) {</span><o:p></o:p></p>
</div>
<div>
<p class="MsoNormal"><span style="font-family:"Menlo",serif"> unsigned StartingPHISize = InsertedPHIs.size();</span><o:p></o:p></p>
</div>
<div>
<p class="MsoNormal"><span style="font-family:"Menlo",serif"> fixupDefs(FixupList);</span><o:p></o:p></p>
</div>
<div>
<p class="MsoNormal"><span style="font-family:"Menlo",serif"> FixupList.clear();</span><o:p></o:p></p>
</div>
<div>
<p class="MsoNormal"><span style="font-family:"Menlo",serif"> // Put any new phis on the fixup list, and process them</span><o:p></o:p></p>
</div>
<div>
<p class="MsoNormal"><span style="font-family:"Menlo",serif"> FixupList.append(InsertedPHIs.end() - StartingPHISize, InsertedPHIs.end());</span><o:p></o:p></p>
</div>
<div>
<p class="MsoNormal"><span style="font-family:"Menlo",serif"> }</span><o:p></o:p></p>
</div>
<div>
<p class="MsoNormal"><o:p> </o:p></p>
</div>
<div>
<p class="MsoNormal">With the latest code on trunk compilation of perlbench SPEC CPU 2017 INT benchmark with “-O3 -inline-threshold=1000 and -enable-gvn-hoist” options is looping infinitely on the above loop.<o:p></o:p></p>
</div>
</div>
</div>
</blockquote>
<div>
<p class="MsoNormal"><o:p> </o:p></p>
</div>
<div>
<p class="MsoNormal"> <o:p></o:p></p>
</div>
<blockquote style="border:none;border-left:solid #CCCCCC 1.0pt;padding:0in 0in 0in 6.0pt;margin-left:4.8pt;margin-right:0in">
<div>
<div>
<div>
<p class="MsoNormal"><o:p> </o:p></p>
</div>
<div>
<p class="MsoNormal">Above loop never terminates unless elements from InsertedPHIs are removed as and when they are processed.<o:p></o:p></p>
</div>
</div>
</div>
</blockquote>
<div>
<p class="MsoNormal"><o:p> </o:p></p>
</div>
<div>
<p class="MsoNormal">Yes, the loop is slightly off.<o:p></o:p></p>
</div>
</div>
</div>
</div>
</div>
</blockquote>
<blockquote style="margin-top:5.0pt;margin-bottom:5.0pt">
<div>
<div>
<div>
<div>
<div>
<p class="MsoNormal"><o:p> </o:p></p>
</div>
<div>
<p class="MsoNormal">The intention is to process any new phis added by fixupdefs.<o:p></o:p></p>
</div>
<div>
<p class="MsoNormal">However, it really should be InsertedPHIs.start() + StartingPHISize.<o:p></o:p></p>
</div>
</div>
</div>
</div>
</div>
</blockquote>
<div>
<p class="MsoNormal"><o:p> </o:p></p>
</div>
<p class="MsoNormal">Even in that case it still is infinite, as there is no call to clear already processed elements(MemoryPhi) in InsertedPHIs. fixupDefs function only inserts new elements but not remove any that are processed.<o:p></o:p></p>
</div>
</div>
</blockquote>
<div>
<p class="MsoNormal"><o:p> </o:p></p>
</div>
<div>
<p class="MsoNormal">It is not iterating over inserted phis, it is iterating over the phis added by fixupdefs. <o:p></o:p></p>
</div>
<div>
<p class="MsoNormal"><o:p> </o:p></p>
</div>
<div>
<p class="MsoNormal">Thus, it does not matter whether things are removed from insertedphis, and it definitely is not infinite in that case.<o:p></o:p></p>
</div>
<div>
<p class="MsoNormal">1. insertedphis represents the set of phis inserted during updating for a given def/use. It should never be cleared until a new insertuse/insertdef call happens.<o:p></o:p></p>
</div>
<div>
<p class="MsoNormal">You can see it is cleared at the beginning of each insertuse/insertdef.<o:p></o:p></p>
</div>
<div>
<p class="MsoNormal"><o:p> </o:p></p>
</div>
<div>
<p class="MsoNormal">2. Prior to this loop, insertedphis will contain the initial set of phis required by a new def insertion. This in turn may require new phis. That is what this loop does. It fixes up the defs that inserting a new phi requires, and iteratively
processes any new phis that process creates.<o:p></o:p></p>
</div>
<div>
<p class="MsoNormal"><o:p> </o:p></p>
</div>
<div>
<p class="MsoNormal">The number of phis you must have to inserted is bounded by the number of basic blocks. At worst, you will insert a single phi at every merge point. This is a finite number. So the number of phis the loop must go through is finite.<o:p></o:p></p>
</div>
<div>
<p class="MsoNormal"><o:p> </o:p></p>
</div>
<div>
<p class="MsoNormal">So let's go through the loop itself.<o:p></o:p></p>
</div>
<div>
<p class="MsoNormal"><o:p> </o:p></p>
</div>
<div>
<p class="MsoNormal">At the beginning, the fixup list will contain a single def.<o:p></o:p></p>
</div>
<div>
<p class="MsoNormal">Let's look at a fixed version of the loop:<o:p></o:p></p>
</div>
<div>
<div>
<p class="MsoNormal"><o:p> </o:p></p>
</div>
<div>
<p class="MsoNormal"> while (!FixupList.empty()) {<o:p></o:p></p>
</div>
<div>
<p class="MsoNormal"> unsigned StartingPHISize = InsertedPHIs.size();<o:p></o:p></p>
</div>
<div>
<p class="MsoNormal"> fixupDefs(FixupList);<o:p></o:p></p>
</div>
<div>
<p class="MsoNormal"> FixupList.clear();<o:p></o:p></p>
</div>
<div>
<p class="MsoNormal"> // Put any new phis on the fixup list, and process them<o:p></o:p></p>
</div>
<div>
<p class="MsoNormal"> FixupList.append(InsertedPHIs.begin() + StartingPHISize, InsertedPHIs.end());<o:p></o:p></p>
</div>
<div>
<p class="MsoNormal"> }<o:p></o:p></p>
</div>
</div>
<div>
<p class="MsoNormal"><o:p> </o:p></p>
</div>
<div>
<p class="MsoNormal"><o:p> </o:p></p>
</div>
<div>
<p class="MsoNormal">Fixup list contains 1 def. <o:p></o:p></p>
</div>
<div>
<p class="MsoNormal">Let's say Insertedphis looks like this { A, B }<o:p></o:p></p>
</div>
<div>
<p class="MsoNormal">StartingPHISize = 2<o:p></o:p></p>
</div>
<div>
<p class="MsoNormal">we fix up the def, which may create new phis<o:p></o:p></p>
</div>
<div>
<p class="MsoNormal">Let's say insertedphis now looks like this {A, B, C, D}<o:p></o:p></p>
</div>
<div>
<p class="MsoNormal">We clear the fixuplist<o:p></o:p></p>
</div>
<div>
<p class="MsoNormal">We now append only the new phis to the fixup list. That is, those that come *after* StartingPHISize in the list.<o:p></o:p></p>
</div>
<div>
<p class="MsoNormal">Fixuplist will now contain {C, D}<o:p></o:p></p>
</div>
<div>
<p class="MsoNormal">Iteration 1 ends<o:p></o:p></p>
</div>
<div>
<p class="MsoNormal"><o:p> </o:p></p>
</div>
<div>
<p class="MsoNormal">Iteration 2:<o:p></o:p></p>
</div>
<div>
<p class="MsoNormal">Fixup list is {C, D}<br>
Insertedphis looks like {A, B, C, D }<o:p></o:p></p>
</div>
<div>
<p class="MsoNormal">StartingPHISize = 4<o:p></o:p></p>
</div>
<div>
<p class="MsoNormal">we fixup these defs. Assume it creates a new phi<o:p></o:p></p>
</div>
<div>
<div>
<p class="MsoNormal">Insertedphis looks like {A, B, C, D, E }<o:p></o:p></p>
</div>
</div>
<div>
<p class="MsoNormal">We clear the fixup list.<o:p></o:p></p>
</div>
<div>
<p class="MsoNormal">We now append only the new phis to the fixup list.<o:p></o:p></p>
</div>
<div>
<p class="MsoNormal">FIxuplist will now contain {E}<o:p></o:p></p>
</div>
<div>
<p class="MsoNormal">Iteration 2 ends<o:p></o:p></p>
</div>
<div>
<p class="MsoNormal"><o:p> </o:p></p>
</div>
<div>
<p class="MsoNormal">Fixup list is {E}<o:p></o:p></p>
</div>
<div>
<div>
<p class="MsoNormal">Insertedphis looks like {A, B, C, D, E }<o:p></o:p></p>
</div>
</div>
<div>
<p class="MsoNormal">StartingPHISize = 5<o:p></o:p></p>
</div>
<div>
<p class="MsoNormal">we fixup these defs. Assume it creates no new phis<o:p></o:p></p>
</div>
<div>
<div>
<p class="MsoNormal">Insertedphis looks like {A, B, C, D, E }<o:p></o:p></p>
</div>
</div>
<div>
<div>
<p class="MsoNormal">We clear the fixup list.<o:p></o:p></p>
</div>
</div>
<div>
<p class="MsoNormal">InsertedPHIs.begin() + StartingPHISize == InsertedPHIs.end(), so we add nothing<o:p></o:p></p>
</div>
<div>
<p class="MsoNormal">The fixuplist is empty, so the loop ends. <o:p></o:p></p>
</div>
<div>
<p class="MsoNormal"><o:p> </o:p></p>
</div>
<div>
<p class="MsoNormal">We are guaranteed insertedphis can only grow to the number of basic blocks.<o:p></o:p></p>
</div>
<div>
<p class="MsoNormal">Once it contains a phi for every basic block, the loop must terminate, because no new phis can be added.<o:p></o:p></p>
</div>
<div>
<p class="MsoNormal"><o:p> </o:p></p>
</div>
<div>
<p class="MsoNormal">The only way the loop can be finite is if we add phis where phis already exist, as MemorySSA is a single phi per block form,.<o:p></o:p></p>
</div>
<div>
<p class="MsoNormal">That would be a bug if that occurs.<o:p></o:p></p>
</div>
<div>
<p class="MsoNormal"><o:p> </o:p></p>
</div>
<div>
<p class="MsoNormal">Otherwise, the loop must terminate when a phi is in every block other than start.<o:p></o:p></p>
</div>
<div>
<p class="MsoNormal"><o:p> </o:p></p>
</div>
<div>
<p class="MsoNormal"><o:p> </o:p></p>
</div>
<div>
<p class="MsoNormal"><o:p> </o:p></p>
</div>
</div>
<p class="MsoNormal"><o:p> </o:p></p>
</div>
</div>
</div>
</body>
</html>