<html><head><style type='text/css'>p { margin: 0; }</style></head><body><div style='font-family: arial,helvetica,sans-serif; font-size: 10pt; color: #000000'><br><hr id="zwchr"><blockquote style="border-left: 2px solid rgb(16, 16, 255); margin-left: 5px; padding-left: 5px; color: rgb(0, 0, 0); font-weight: normal; font-style: normal; text-decoration: none; font-family: Helvetica,Arial,sans-serif; font-size: 12pt;"><b>From: </b>"David Callahan via llvm-dev" <llvm-dev@lists.llvm.org><br><b>To: </b>"LLVM Dev Mailing list" <llvm-dev@lists.llvm.org><br><b>Sent: </b>Wednesday, January 27, 2016 3:56:33 PM<br><b>Subject: </b>[llvm-dev] DCE in the presence of control flow.<br><br>
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<p class="MsoNormal">I have been looking at some internal codes looking for differences between Clang (specifically 3.7.1) and gcc (typically 4.8.1 but sometimes later).</p>
<p class="MsoNormal"> </p>
<p id="DWT1743" class="MsoNormal">One area where I bumped into was dead code elimination in the presence of complex control flow. I note that the “aggressive dead code elimination” (ADCE.cpp) treats all branch operations as live (<span style="font-size: 10pt; font-family: Consolas;">isa<TerminatorInst>(I)).
</span>Doing more requires some approximation to control dependence.</p></div></blockquote><br>Can you provide a small example where this matters?<br><br> -Hal<br><blockquote style="border-left: 2px solid rgb(16, 16, 255); margin-left: 5px; padding-left: 5px; color: rgb(0, 0, 0); font-weight: normal; font-style: normal; text-decoration: none; font-family: Helvetica,Arial,sans-serif; font-size: 12pt;"><div><p class="MsoNormal"></p>
<p class="MsoNormal"> </p>
<p class="MsoNormal">I note SimplifyCFG indirectly handles some simple cases. It will speculate the contents of a basic block into a predecessor but this is insufficient for more complex structures. This probably cherry-picks the most common cases by frequency.</p>
<p class="MsoNormal"> </p>
<p class="MsoNormal">Have their been prior attempts strengthen dead code elimination w.r.t. control flow? If so, any guidance on what went wrong?</p>
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<p class="MsoNormal">Thanks</p>
<p class="MsoNormal">David</p>
<p class="MsoNormal"><span style="font-size: 10pt; font-family: Consolas;"> </span></p>
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<br>_______________________________________________<br>LLVM Developers mailing list<br>llvm-dev@lists.llvm.org<br>http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev<br></blockquote><br><br><br>-- <br><div><span name="x"></span>Hal Finkel<br>Assistant Computational Scientist<br>Leadership Computing Facility<br>Argonne National Laboratory<span name="x"></span><br></div></div></body></html>