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I think you can also avoid the RDF computation using a more directed form of control dependence testing such as described in </div>
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<div style="color: rgb(0, 0, 0); font-family: Calibri; font-size: 14px;"><font face="Calibri,sans-serif">Keshav Pingali and Gianfranco Bilardi. 1997. Optimal control dependence computation and the Roman chariots problem. ACM Trans. Program. Lang. Syst. 19,
3 (May 1997), 462-491. DOI=<a href="http://dx.doi.org/10.1145/256167.256217">http://dx.doi.org/10.1145/256167.256217</a> </font></div>
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<div><font face="Calibri,sans-serif"><font face="Calibri">However one challenge seems to be fixing the SSA graph after deleting essentially arbitrary connected regions of the control flow graph. It may be that </font>the<font face="Calibri"> common case where
deleted control flow has a single entry and a common post-dominator which seems straight forward but in the general case it seems much harder. Any prior experience on that problem?</font></font></div>
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<div><font face="Calibri,sans-serif"><font face="Calibri">david</font></font></div>
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<span style="font-weight:bold">From: </span>Hal Finkel <<a href="mailto:hfinkel@anl.gov">hfinkel@anl.gov</a>><br>
<span style="font-weight:bold">Date: </span>Friday, January 29, 2016 at 4:50 PM<br>
<span style="font-weight:bold">To: </span>Daniel Berlin <<a href="mailto:dberlin@dberlin.org">dberlin@dberlin.org</a>><br>
<span style="font-weight:bold">Cc: </span>LLVM Dev Mailing list <<a href="mailto:llvm-dev@lists.llvm.org">llvm-dev@lists.llvm.org</a>>, David Callahan <<a href="mailto:dcallahan@fb.com">dcallahan@fb.com</a>><br>
<span style="font-weight:bold">Subject: </span>Re: [llvm-dev] DCE in the presence of control flow.<br>
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<b>From: </b>"Daniel Berlin" <<a href="mailto:dberlin@dberlin.org">dberlin@dberlin.org</a>><br>
<b>To: </b>"Hal Finkel" <<a href="mailto:hfinkel@anl.gov">hfinkel@anl.gov</a>><br>
<b>Cc: </b>"LLVM Dev Mailing list" <<a href="mailto:llvm-dev@lists.llvm.org">llvm-dev@lists.llvm.org</a>>, "David Callahan" <<a href="mailto:dcallahan@fb.com">dcallahan@fb.com</a>><br>
<b>Sent: </b>Friday, January 29, 2016 12:48:37 AM<br>
<b>Subject: </b>Re: [llvm-dev] DCE in the presence of control flow.<br>
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<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>
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<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>
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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" 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>
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A 1-2% reduction in code size seems like it might well be worth a post-dom calculation.</div>
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<div id="DWT2476">1-2% more cases != 1-2% reduction in code size. In particular, it assumes nothing else will catch those cases :)</div>
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Fair enough.<br>
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<div>The cases are mostly caught by SimplifyCFG/etc anyway<br>
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<div>In any case, here are the numbers from when it was turned on by default:</div>
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<div><a href="https://gcc.gnu.org/ml/gcc-patches/2004-01/msg00675.html" target="_blank">https://gcc.gnu.org/ml/gcc-patches/2004-01/msg00675.html</a><br>
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<div id="DWT2475">Note: GCC is at least 3x faster at computing post-dom than LLVM</div>
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Why?<br>
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Also, what exactly is the algorithm using post-dom info?<br>
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<div>So, to review for a second, right now, the algorithm answers the question when is a branch necessary with "always" :)<br>
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<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>
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<div>(IE </div>
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<div>for each block in RDF(useful block):<br>
mark terminator of block as useful</div>
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FWIW, Keith Cooper's slide deck on this has a nice explanation: <a href="https://www.cs.rice.edu/~keith/512/2011/Lectures/L04Dead-1up.pdf">
https://www.cs.rice.edu/~keith/512/2011/Lectures/L04Dead-1up.pdf</a><br>
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We might be able to do this without precomputing an RDF, however. For example, you could solve a data-flow problem on the reverse CFG, where for each block you solve for the "next" live instruction. Then a branch is alive only if the next live instruction for
each successor is different. You'd need to have "next-live-instruction phi nodes" for cases where there's not a unique answer, etc.<br>
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Thanks again,<br>
Hal<br>
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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>
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Yes. Loops with all dead instructions includes "loops with no side-effects outside of the loop" </div>
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Thanks again,<br>
Hal<span class=""><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>
<p class="MsoNormal"><span style="font-size: 10pt; font-family: Consolas;"> </span></p>
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</span><span class="">-- <br>
<div><span></span>Hal Finkel<br>
Assistant Computational Scientist<br>
Leadership Computing Facility<br>
Argonne National Laboratory<span></span><br>
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-- <br>
<div><span name="x"></span>Hal Finkel<br>
Assistant Computational Scientist<br>
Leadership Computing Facility<br>
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