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<p class="MsoNormal"><span style="font-size:11.0pt;color:#1F497D">What kind of perf. uplifts are expected and in what benchmarks/apps ?<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;color:#1F497D"><o:p> </o:p></span></p>
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<p class="MsoNormal"><b><span style="font-size:11.0pt">From:</span></b><span style="font-size:11.0pt"> llvm-dev [mailto:llvm-dev-bounces@lists.llvm.org]
<b>On Behalf Of </b>David Callahan via llvm-dev<br>
<b>Sent:</b> Monday, August 01, 2016 11:15 PM<br>
<b>To:</b> LLVM Dev Mailing list <llvm-dev@lists.llvm.org><br>
<b>Cc:</b> Nadav Rotem <nrotem@fb.com><br>
<b>Subject:</b> [llvm-dev] [LLVM] New Dead Code Elimination<o:p></o:p></span></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal"><span style="color:black">I have a rewrite of the aggressive dead code elimination pass which handles control flow and allows may-be-infinite loops to be removed under optional flag. Chandler suggested rather than a large change a series
of small changes and, while that may not get us to small changes easily, there has been no further commentary on the diff:
<a href="https://reviews.llvm.org/D18762">https://reviews.llvm.org/D18762</a>. Given that, I will plan on proposing a series of diffs and would like to recruit some people who are interested in the change to help review those changes.
<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:black"> <o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:black">I expect to stage as follows:<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:black"> <o:p></o:p></span></p>
<p class="MsoListParagraphCxSpFirst" style="text-indent:-.25in;mso-list:l0 level1 lfo2">
<![if !supportLists]><span style="color:black"><span style="mso-list:Ignore">1.<span style="font:7.0pt "Times New Roman"">
</span></span></span><![endif]><span style="color:black">Rewrite base algorithm to introduce data structures needed to hold extra information (no functional change)<o:p></o:p></span></p>
<p class="MsoListParagraphCxSpMiddle" style="text-indent:-.25in;mso-list:l0 level1 lfo2">
<![if !supportLists]><span style="color:black"><span style="mso-list:Ignore">2.<span style="font:7.0pt "Times New Roman"">
</span></span></span><![endif]><span style="color:black">Rewrite base algorithm to allow removing of control flow (under option, off by default). This variant will not be correct in the general case.<o:p></o:p></span></p>
<p class="MsoListParagraphCxSpMiddle" style="text-indent:-.25in;mso-list:l0 level1 lfo2">
<![if !supportLists]><span style="color:black"><span style="mso-list:Ignore">3.<span style="font:7.0pt "Times New Roman"">
</span></span></span><![endif]><span style="color:black">Code to rewrite the control flow graph and remove dead basic blocks<o:p></o:p></span></p>
<p class="MsoListParagraphCxSpMiddle" style="text-indent:-.25in;mso-list:l0 level1 lfo2">
<![if !supportLists]><span style="color:black"><span style="mso-list:Ignore">4.<span style="font:7.0pt "Times New Roman"">
</span></span></span><![endif]><span style="color:black">Identify Phi nodes with non-instruction reaching definitions and keep the associated source block live. Identify Phi nodes with multiple-live values from dead blocks<o:p></o:p></span></p>
<p class="MsoListParagraphCxSpLast" style="text-indent:-.25in;mso-list:l0 level1 lfo2">
<![if !supportLists]><span style="color:black"><span style="mso-list:Ignore">5.<span style="font:7.0pt "Times New Roman"">
</span></span></span><![endif]><span style="color:black">Remove unreachable code discovered by post-order traversal to avoid.<o:p></o:p></span></p>
<p class="MsoNormal" style="margin-left:.25in;text-indent:.25in"><span style="color:black"> (code is functionally correct at this point)<o:p></o:p></span></p>
<p class="MsoListParagraphCxSpFirst" style="text-indent:-.25in;mso-list:l0 level1 lfo2">
<![if !supportLists]><span style="color:black"><span style="mso-list:Ignore">6.<span style="font:7.0pt "Times New Roman"">
</span></span></span><![endif]><span style="color:black">Use post-order traversal to identify loop bottoms. By default this will be kept live but include switch to allow loops to be removed.
<o:p></o:p></span></p>
<p class="MsoListParagraphCxSpLast" style="text-indent:-.25in;mso-list:l0 level1 lfo2">
<![if !supportLists]><span style="color:black"><span style="mso-list:Ignore">7.<span style="font:7.0pt "Times New Roman"">
</span></span></span><![endif]><span style="color:black">Improve handling of live values out of dead regions<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:black"> <o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:black">Please respond if you are willing to help or have suggestions on staging or approach.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:black"> <o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:black">Thanks<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:black">David<o:p></o:p></span></p>
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