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<div>Thanks</div>
<div>Also I found that some cases are also caught by a specialized routine to remove dead loops which is missing the case I noticed.</div>
<div>odavd</div>
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<span style="font-weight:bold">From: </span>Daniel Berlin <<a href="mailto:dberlin@dberlin.org">dberlin@dberlin.org</a>><br>
<span style="font-weight:bold">Date: </span>Thursday, January 28, 2016 at 8:45 PM<br>
<span style="font-weight:bold">To: </span>David Callahan <<a href="mailto:dcallahan@fb.com">dcallahan@fb.com</a>>, LLVM Dev Mailing list <<a href="mailto:llvm-dev@lists.llvm.org">llvm-dev@lists.llvm.org</a>><br>
<span style="font-weight:bold">Subject: </span>Re: [llvm-dev] DCE in the presence of control flow.<br>
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<div>The post dominators computation costs more on llvm than GCC because of how the respective cfgs work under the covers. Even for GCC, when we implemented cd-dce, it only catches 1-2% more cases. It's not really worth the cost in llvm unless postdom comes
free
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<div dir="ltr">On Wed, Jan 27, 2016, 1:56 PM David Callahan via llvm-dev <<a href="mailto:llvm-dev@lists.llvm.org">llvm-dev@lists.llvm.org</a>> wrote:<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).<u></u><u></u></p>
<p class="MsoNormal"><u></u> <u></u></p>
<p 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:10.0pt;font-family:Consolas">isa<TerminatorInst>(I)).
</span>Doing more requires some approximation to control dependence.<u></u><u></u></p>
<p class="MsoNormal"><u></u> <u></u></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.<u></u><u></u></p>
<p class="MsoNormal"><u></u> <u></u></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?<u></u><u></u></p>
<p class="MsoNormal"><u></u> <u></u></p>
<p class="MsoNormal">Thanks<u></u><u></u></p>
<p class="MsoNormal">David<u></u><u></u></p>
<p class="MsoNormal"><span style="font-size:10.0pt;font-family:Consolas"> </span></p>
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