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<p class="MsoNormal"><span lang="EN-US" style="font-size:10.5pt;font-family:"Calibri",sans-serif;color:#1F497D">As far as I know, dead register is not only means none would read it , it may means register is clobbered.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:10.5pt;font-family:"Calibri",sans-serif;color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:10.5pt;font-family:"Calibri",sans-serif;color:#1F497D">http://lists.llvm.org/pipermail/llvm-dev/2018-February/120994.html<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:10.5pt;font-family:"Calibri",sans-serif;color:#1F497D"><o:p> </o:p></span></p>
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<p class="MsoNormal"><b><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri",sans-serif">From:</span></b><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri",sans-serif"> Matthias Braun [mailto:matze@braunis.de]
<br>
<b>Sent:</b> Tuesday, October 23, 2018 2:04 AM<br>
<b>To:</b> Yuchao (Michael) <michael.yuchao@huawei.com><br>
<b>Cc:</b> llvm-dev@lists.llvm.org; sunqiang (I) <sunqiang13@huawei.com><br>
<b>Subject:</b> Re: [llvm-dev] Question about MachineCSE optimization<o:p></o:p></span></p>
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<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">Yes<o:p></o:p></span></p>
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<p class="MsoNormal"><span lang="EN-US">OP_X ..., implicit-def dead %X<o:p></o:p></span></p>
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<p class="MsoNormal"><span lang="EN-US">... potentially more defs/uses of %X<o:p></o:p></span></p>
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<p class="MsoNormal"><span lang="EN-US">OP_X ..., implicit-def dead %X<o:p></o:p></span></p>
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<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
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<p class="MsoNormal"><span lang="EN-US">is good to merge. Also note that:<o:p></o:p></span></p>
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<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
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<p class="MsoNormal"><span lang="EN-US">%X = DEF<o:p></o:p></span></p>
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<p class="MsoNormal"><span lang="EN-US">OP_X ..., implicit-def dead %X<o:p></o:p></span></p>
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<p class="MsoNormal"><span lang="EN-US">use %X<o:p></o:p></span></p>
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<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
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<p class="MsoNormal"><span lang="EN-US">is not valid! (The def overrides the register value but the "dead" flag promises noone will read it, even though in this case another use comes below).<o:p></o:p></span></p>
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<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
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<p class="MsoNormal"><span lang="EN-US">- Matthias<o:p></o:p></span></p>
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<br>
<o:p></o:p></span></p>
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<p class="MsoNormal"><span lang="EN-US">On Oct 22, 2018, at 4:59 AM, Yuchao (Michael) <<a href="mailto:michael.yuchao@huawei.com">michael.yuchao@huawei.com</a>> wrote:<o:p></o:p></span></p>
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<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
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<p class="MsoNormal"><span lang="EN-US" style="font-size:10.5pt;font-family:"Calibri",sans-serif;color:#1F497D">Hi<span class="apple-converted-space"> </span></span><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri",sans-serif">mbraun</span><span lang="EN-US"><o:p></o:p></span></p>
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<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri",sans-serif"> </span><span lang="EN-US"><o:p></o:p></span></p>
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<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri",sans-serif">Thanks for your information.</span><span lang="EN-US"><o:p></o:p></span></p>
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<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri",sans-serif"> </span><span lang="EN-US"><o:p></o:p></span></p>
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<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri",sans-serif">What about following situation:</span><span lang="EN-US"><o:p></o:p></span></p>
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<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri",sans-serif">3)</span><span lang="EN-US"><o:p></o:p></span></p>
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<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri",sans-serif">OP_X …, implicit-def dead %X </span><span lang="EN-US"><o:p></o:p></span></p>
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<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri",sans-serif">with</span><span lang="EN-US"><o:p></o:p></span></p>
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<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri",sans-serif">OP_X …, implicit-def dead %X</span><span lang="EN-US"><o:p></o:p></span></p>
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<p class="MsoNormal"><span lang="EN-US" style="font-size:10.5pt;font-family:"Calibri",sans-serif;color:#1F497D"> </span><span lang="EN-US"><o:p></o:p></span></p>
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<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri",sans-serif">Between two instructions, there may be another instruction(not OP_X) which also def %X,</span><span lang="EN-US"><o:p></o:p></span></p>
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<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri",sans-serif">Is it correct to do CSE in this situation?</span><span lang="EN-US"><o:p></o:p></span></p>
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<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri",sans-serif"> </span><span lang="EN-US"><o:p></o:p></span></p>
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<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri",sans-serif">Best wishes</span><span lang="EN-US"><o:p></o:p></span></p>
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<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri",sans-serif">Michael</span><span lang="EN-US"><o:p></o:p></span></p>
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<p class="MsoNormal"><span lang="EN-US" style="font-size:10.5pt;font-family:"Calibri",sans-serif;color:#1F497D"> </span><span lang="EN-US"><o:p></o:p></span></p>
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<p class="MsoNormal"><b><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri",sans-serif">From:</span></b><span class="apple-converted-space"><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri",sans-serif"> </span></span><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri",sans-serif"><a href="mailto:mbraun@apple.com">mbraun@apple.com</a>
[<a href="mailto:mbraun@apple.com">mailto:mbraun@apple.com</a>]<span class="apple-converted-space"> </span><br>
<b>Sent:</b><span class="apple-converted-space"> </span>Monday, October 22, 2018 5:51 AM<br>
<b>To:</b><span class="apple-converted-space"> </span>Matthias Braun <<a href="mailto:mbraun@apple.com">mbraun@apple.com</a>><br>
<b>Cc:</b><span class="apple-converted-space"> </span>Yuchao (Michael) <<a href="mailto:michael.yuchao@huawei.com">michael.yuchao@huawei.com</a>>;
<a href="mailto:llvm-dev@lists.llvm.org">llvm-dev@lists.llvm.org</a>; sunqiang (I) <<a href="mailto:sunqiang13@huawei.com">sunqiang13@huawei.com</a>><br>
<b>Subject:</b><span class="apple-converted-space"> </span>Re: [llvm-dev] Question about MachineCSE optimization</span><span lang="EN-US"><o:p></o:p></span></p>
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<p class="MsoNormal"><span lang="EN-US"> <o:p></o:p></span></p>
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<p class="MsoNormal"><span lang="EN-US"> <o:p></o:p></span></p>
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<p class="MsoNormal"><span lang="EN-US"><br>
<br>
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<o:p></o:p></span></p>
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<p class="MsoNormal"><span lang="EN-US">On Oct 21, 2018, at 2:48 PM, mbraun via llvm-dev <<a href="mailto:llvm-dev@lists.llvm.org"><span style="color:purple">llvm-dev@lists.llvm.org</span></a>> wrote:<o:p></o:p></span></p>
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<p class="MsoNormal"><span lang="EN-US"> <o:p></o:p></span></p>
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<p class="MsoNormal"><span lang="EN-US">As far as I understand it (though I didn</span>’<span lang="EN-US">t write the code so I may be missing something) this is about making sure liveness flags (kill, dead) are updated correctly when merging instruction.
The situations to consider are probably:<o:p></o:p></span></p>
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<p class="MsoNormal"><span lang="EN-US"> <o:p></o:p></span></p>
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<p class="MsoNormal"><span lang="EN-US">1)<o:p></o:p></span></p>
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<p class="MsoNormal"><span lang="EN-US">OP_X </span>…<span lang="EN-US">, implicit-def %X<o:p></o:p></span></p>
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<p class="MsoNormal"><span lang="EN-US">with<o:p></o:p></span></p>
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<p class="MsoNormal"><span lang="EN-US">OP_X </span>…<span lang="EN-US">, implicit-def dead %X<o:p></o:p></span></p>
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<p class="MsoNormal"><span lang="EN-US"> <o:p></o:p></span></p>
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<p class="MsoNormal"><span lang="EN-US">The merged instruction probably should not have a dead flag set. The 2nd if appears to be about this situation:<o:p></o:p></span></p>
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<p class="MsoNormal"><span lang="EN-US"> <o:p></o:p></span></p>
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<p class="MsoNormal"><span lang="EN-US"> <o:p></o:p></span></p>
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<p class="MsoNormal"><span lang="EN-US">2)<o:p></o:p></span></p>
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<p class="MsoNormal"><span lang="EN-US">OP_X </span>…<span lang="EN-US">, implicit-def dead %X<o:p></o:p></span></p>
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<p class="MsoNormal"><span lang="EN-US">Use killed %X<o:p></o:p></span></p>
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<p class="MsoNormal">…<span lang="EN-US"><o:p></o:p></span></p>
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<p class="MsoNormal"><span lang="EN-US">OP_X </span>…<span lang="EN-US">, implicit-def %X ; this gets merged with the previous OP_X<o:p></o:p></span></p>
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<p class="MsoNormal"><span lang="EN-US">Use %X<o:p></o:p></span></p>
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<p class="MsoNormal"><span lang="EN-US">This example should of course read:<o:p></o:p></span></p>
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<p class="MsoNormal"><span lang="EN-US"> <o:p></o:p></span></p>
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<p class="MsoNormal"><span lang="EN-US">OP_X </span>…<span lang="EN-US">, implicit-def %X<o:p></o:p></span></p>
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<p class="MsoNormal"><span lang="EN-US">Use killed %X<o:p></o:p></span></p>
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<p class="MsoNormal">…<span lang="EN-US"><o:p></o:p></span></p>
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<p class="MsoNormal"><span lang="EN-US">OP_X </span>…<span lang="EN-US">, implicit-def %X ; this gets merged with the previous OP_X<o:p></o:p></span></p>
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<p class="MsoNormal"><span lang="EN-US">Use %X<o:p></o:p></span></p>
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<p class="MsoNormal"><span lang="EN-US"> <o:p></o:p></span></p>
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<p class="MsoNormal"><span lang="EN-US">Once we merge the 2nd OP_X with the first one, we have to drop the `killed` flags on all users of %X between the first OP_X and the position where we had the 2nd OP_X before it was merged.<o:p></o:p></span></p>
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<p class="MsoNormal"><span lang="EN-US"><br>
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<o:p></o:p></span></p>
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<p class="MsoNormal"><span lang="EN-US"> <o:p></o:p></span></p>
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<p class="MsoNormal"><span lang="EN-US">Once we merge the 2nd OP_X with the first one, we have to drop the `killed` flags on the %X users.<o:p></o:p></span></p>
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<p class="MsoNormal"><span lang="EN-US"> <o:p></o:p></span></p>
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<p class="MsoNormal"><span lang="EN-US"> <o:p></o:p></span></p>
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<p class="MsoNormal"><span lang="EN-US"> <o:p></o:p></span></p>
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<p class="MsoNormal"><span lang="EN-US">Though admittedly I am at a loss right now why this is only performed for implicit operands and not for all operands</span>…<span lang="EN-US"><o:p></o:p></span></p>
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<p class="MsoNormal"><span lang="EN-US"> <o:p></o:p></span></p>
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<p class="MsoNormal"><span lang="EN-US">- Matthias<o:p></o:p></span></p>
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<p class="MsoNormal"><span lang="EN-US">On Oct 20, 2018, at 1:38 AM, Yuchao (Michael) via llvm-dev <<a href="mailto:llvm-dev@lists.llvm.org"><span style="color:purple">llvm-dev@lists.llvm.org</span></a>> wrote:<o:p></o:p></span></p>
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<p class="MsoNormal"><span lang="EN-US"> <o:p></o:p></span></p>
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<p class="MsoNormal" style="text-align:justify;text-justify:inter-ideograph"><span lang="EN-US" style="font-size:10.5pt;font-family:"Calibri",sans-serif">Hi<span class="apple-converted-space"> </span></span><span lang="EN-US"><o:p></o:p></span></p>
<p class="MsoNormal" style="text-align:justify;text-justify:inter-ideograph"><span lang="EN-US" style="font-size:10.5pt;font-family:"Calibri",sans-serif"> </span><span lang="EN-US"><o:p></o:p></span></p>
<p class="MsoNormal" style="text-align:justify;text-justify:inter-ideograph"><span lang="EN-US" style="font-size:10.5pt;font-family:"Calibri",sans-serif">I am trying to fix a bug in MachineCSE and have one question about following code:</span><span lang="EN-US"><o:p></o:p></span></p>
<p class="MsoNormal" style="text-align:justify;text-justify:inter-ideograph"><span lang="EN-US" style="font-size:10.5pt;font-family:"Calibri",sans-serif"> </span><span lang="EN-US"><o:p></o:p></span></p>
<p class="MsoNormal" style="text-align:justify;text-justify:inter-ideograph"><span lang="EN-US" style="font-size:10.5pt;font-family:"Calibri",sans-serif">*********************************************</span><span lang="EN-US"><o:p></o:p></span></p>
<p class="MsoNormal" style="text-align:justify;text-justify:inter-ideograph"><span lang="EN-US" style="font-size:10.5pt;font-family:"Calibri",sans-serif">// Go through implicit defs of CSMI and MI, if a def is not dead at MI,</span><span lang="EN-US"><o:p></o:p></span></p>
<p class="MsoNormal" style="text-align:justify;text-justify:inter-ideograph"><span lang="EN-US" style="font-size:10.5pt;font-family:"Calibri",sans-serif">// we should make sure it is not dead at CSMI.</span><span lang="EN-US"><o:p></o:p></span></p>
<p class="MsoNormal" style="text-align:justify;text-justify:inter-ideograph"><span lang="EN-US" style="font-size:10.5pt;font-family:"Calibri",sans-serif">if (MO.isImplicit() && !MO.isDead() && CSMI->getOperand(i).isDead())</span><span lang="EN-US"><o:p></o:p></span></p>
<p class="MsoNormal" style="text-align:justify;text-justify:inter-ideograph"><span lang="EN-US" style="font-size:10.5pt;font-family:"Calibri",sans-serif"> ImplicitDefsToUpdate.push_back(i);</span><span lang="EN-US"><o:p></o:p></span></p>
<p class="MsoNormal" style="text-align:justify;text-justify:inter-ideograph"><span lang="EN-US" style="font-size:10.5pt;font-family:"Calibri",sans-serif"> </span><span lang="EN-US"><o:p></o:p></span></p>
<p class="MsoNormal" style="text-align:justify;text-justify:inter-ideograph"><span lang="EN-US" style="font-size:10.5pt;font-family:"Calibri",sans-serif">// Keep track of implicit defs of CSMI and MI, to clear possibly</span><span lang="EN-US"><o:p></o:p></span></p>
<p class="MsoNormal" style="text-align:justify;text-justify:inter-ideograph"><span lang="EN-US" style="font-size:10.5pt;font-family:"Calibri",sans-serif">// made-redundant kill flags.</span><span lang="EN-US"><o:p></o:p></span></p>
<p class="MsoNormal" style="text-align:justify;text-justify:inter-ideograph"><span lang="EN-US" style="font-size:10.5pt;font-family:"Calibri",sans-serif">if (MO.isImplicit() && !MO.isDead() && OldReg == NewReg)</span><span lang="EN-US"><o:p></o:p></span></p>
<p class="MsoNormal" style="text-align:justify;text-justify:inter-ideograph"><span lang="EN-US" style="font-size:10.5pt;font-family:"Calibri",sans-serif"> ImplicitDefs.push_back(OldReg);</span><span lang="EN-US"><o:p></o:p></span></p>
<p class="MsoNormal" style="text-align:justify;text-justify:inter-ideograph"><span lang="EN-US" style="font-size:10.5pt;font-family:"Calibri",sans-serif"> </span><span lang="EN-US"><o:p></o:p></span></p>
<p class="MsoNormal" style="text-align:justify;text-justify:inter-ideograph"><span lang="EN-US" style="font-size:10.5pt;font-family:"Calibri",sans-serif">if (OldReg == NewReg) {</span><span lang="EN-US"><o:p></o:p></span></p>
<p class="MsoNormal" style="text-align:justify;text-justify:inter-ideograph"><span lang="EN-US" style="font-size:10.5pt;font-family:"Calibri",sans-serif"> --NumDefs;</span><span lang="EN-US"><o:p></o:p></span></p>
<p class="MsoNormal" style="text-align:justify;text-justify:inter-ideograph"><span lang="EN-US" style="font-size:10.5pt;font-family:"Calibri",sans-serif"> continue;</span><span lang="EN-US"><o:p></o:p></span></p>
<p class="MsoNormal" style="text-align:justify;text-justify:inter-ideograph"><span lang="EN-US" style="font-size:10.5pt;font-family:"Calibri",sans-serif">}</span><span lang="EN-US"><o:p></o:p></span></p>
<p class="MsoNormal" style="text-align:justify;text-justify:inter-ideograph"><span lang="EN-US" style="font-size:10.5pt;font-family:"Calibri",sans-serif">**********************************************</span><span lang="EN-US"><o:p></o:p></span></p>
<p class="MsoNormal" style="text-align:justify;text-justify:inter-ideograph"><span lang="EN-US" style="font-size:10.5pt;font-family:"Calibri",sans-serif"> </span><span lang="EN-US"><o:p></o:p></span></p>
<p class="MsoNormal" style="text-align:justify;text-justify:inter-ideograph"><span lang="EN-US" style="font-size:10.5pt;font-family:"Calibri",sans-serif">From Above source code, it seems that CSE does not handle instruction which</span><span lang="EN-US"><o:p></o:p></span></p>
<p class="MsoNormal" style="text-align:justify;text-justify:inter-ideograph"><span lang="EN-US" style="font-size:10.5pt;font-family:"Calibri",sans-serif">have one operand that is ‘implicit+isDead+isDef’.</span><span lang="EN-US"><o:p></o:p></span></p>
<p class="MsoNormal" style="text-align:justify;text-justify:inter-ideograph"><span lang="EN-US" style="font-size:10.5pt;font-family:"Calibri",sans-serif"> </span><span lang="EN-US"><o:p></o:p></span></p>
<p class="MsoNormal" style="text-align:justify;text-justify:inter-ideograph"><span lang="EN-US" style="font-size:10.5pt;font-family:"Calibri",sans-serif">Base on my understanding, implicit dead register is mean clobbered, so it is not safe to</span><span lang="EN-US"><o:p></o:p></span></p>
<p class="MsoNormal" style="text-align:justify;text-justify:inter-ideograph"><span lang="EN-US" style="font-size:10.5pt;font-family:"Calibri",sans-serif">do CSE. Is this right?</span><span lang="EN-US"><o:p></o:p></span></p>
<p class="MsoNormal" style="text-align:justify;text-justify:inter-ideograph"><span lang="EN-US" style="font-size:10.5pt;font-family:"Calibri",sans-serif"> </span><span lang="EN-US"><o:p></o:p></span></p>
<p class="MsoNormal" style="text-align:justify;text-justify:inter-ideograph"><span lang="EN-US" style="font-size:10.5pt;font-family:"Calibri",sans-serif">Best wishes</span><span lang="EN-US"><o:p></o:p></span></p>
<p class="MsoNormal" style="text-align:justify;text-justify:inter-ideograph"><span lang="EN-US" style="font-size:10.5pt;font-family:"Calibri",sans-serif">Michael</span><span lang="EN-US"><o:p></o:p></span></p>
<p class="MsoNormal" style="text-align:justify;text-justify:inter-ideograph"><span lang="EN-US" style="font-size:10.5pt;font-family:"Calibri",sans-serif"> </span><span lang="EN-US"><o:p></o:p></span></p>
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<p class="MsoNormal"><span lang="EN-US"> <o:p></o:p></span></p>
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