<div dir="ltr"><br><div class="gmail_extra"><br><div class="gmail_quote">On Thu, Jan 28, 2016 at 10:09 PM, Hal Finkel <span dir="ltr"><<a href="mailto:hfinkel@anl.gov" target="_blank">hfinkel@anl.gov</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left-width:1px;border-left-color:rgb(204,204,204);border-left-style:solid;padding-left:1ex"><div><div style="font-family:arial,helvetica,sans-serif;font-size:10pt;color:rgb(0,0,0)"><br><hr><blockquote style="border-left-width:2px;border-left-style:solid;border-left-color: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" <<a href="mailto:llvm-dev@lists.llvm.org" target="_blank">llvm-dev@lists.llvm.org</a>><br><b>To: </b>"Daniel Berlin" <<a href="mailto:dberlin@dberlin.org" target="_blank">dberlin@dberlin.org</a>>, "LLVM Dev Mailing list" <<a href="mailto:llvm-dev@lists.llvm.org" target="_blank">llvm-dev@lists.llvm.org</a>><br><b>Sent: </b>Thursday, January 28, 2016 11:35:49 PM<span class=""><br><b>Subject: </b>Re: [llvm-dev] DCE in the presence of control flow.<br><br>
<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" target="_blank">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" target="_blank">dcallahan@fb.com</a>>, LLVM Dev Mailing list <<a href="mailto:llvm-dev@lists.llvm.org" target="_blank">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
</div></div></span></span></blockquote><br>A 1-2% reduction in code size seems like it might well be worth a post-dom calculation.</div></div></blockquote><div><br></div><div>1-2% more cases != 1-2% reduction in code size. In particular, it assumes nothing else will catch those cases :)</div><div> </div><div>The cases are mostly caught by SimplifyCFG/etc anyway<br></div><div><br></div><div>In any case, here are the numbers from when it was turned on by default:</div><div><br></div><div><a href="https://gcc.gnu.org/ml/gcc-patches/2004-01/msg00675.html">https://gcc.gnu.org/ml/gcc-patches/2004-01/msg00675.html</a><br></div><div><br></div><div>Note: GCC is at least 3x faster at computing post-dom than LLVM</div><div><br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left-width:1px;border-left-color:rgb(204,204,204);border-left-style:solid;padding-left:1ex"><div><div style="font-family:arial,helvetica,sans-serif;font-size:10pt;color:rgb(0,0,0)"> Also, what exactly is the algorithm using post-dom info?<br><br></div></div></blockquote><div><br></div><div>So, to review for a second, right now, the algorithm answers the question when is a branch necessary with "always" :)<br></div><div><br></div><div>The real answer is "when an already-necessary operation depends on its existence". This is of course, requires control-dependence to answer.</div><div>So if you take our current DCE algorithm, and instead of marking terminators always-live, it simply marks control dependent edges of those operands as necessary, and branches that generate those edges.</div><div><br></div><div>(IE </div><div><br></div><div>for each block in RDF(useful block):<br> mark terminator of block as useful</div><div><br></div><div><br></div><div><br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left-width:1px;border-left-color:rgb(204,204,204);border-left-style:solid;padding-left:1ex"><div><div style="font-family:arial,helvetica,sans-serif;font-size:10pt;color:rgb(0,0,0)">I suppose that we're looking for cases where we have a CFG diamond with one having only dead instructions, and loops with all dead instructions, etc.</div></div></blockquote><div><br>Yes. Loops with all dead instructions includes "loops with no side-effects outside of the loop" </div><div> </div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left-width:1px;border-left-color:rgb(204,204,204);border-left-style:solid;padding-left:1ex"><div><div style="font-family:arial,helvetica,sans-serif;font-size:10pt;color:rgb(0,0,0)"><span style="font-family:arial,sans-serif;font-size:small;color:rgb(34,34,34)"> </span><br></div></div></blockquote><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left-width:1px;border-left-color:rgb(204,204,204);border-left-style:solid;padding-left:1ex"><div><div style="font-family:arial,helvetica,sans-serif;font-size:10pt;color:rgb(0,0,0)">Thanks again,<br>Hal<span class=""><br><blockquote style="border-left-width:2px;border-left-style:solid;border-left-color: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"><span><div><div><div><br>
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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" target="_blank">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:10pt;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>
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<br>_______________________________________________<br>LLVM Developers mailing list<br><a href="mailto:llvm-dev@lists.llvm.org" target="_blank">llvm-dev@lists.llvm.org</a><br><a href="http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev" target="_blank">http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev</a><br></blockquote><br><br><br></span><span class="">-- <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></span></div></div></blockquote></div><br></div></div>