<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>"Mehdi Amini via llvm-dev" <llvm-dev@lists.llvm.org><br><b>To: </b>"Daniel Berlin" <dberlin@dberlin.org><br><b>Cc: </b>"David Callahan via llvm-dev" <llvm-dev@lists.llvm.org><br><b>Sent: </b>Friday, March 25, 2016 5:43:12 PM<br><b>Subject: </b>Re: [llvm-dev] RFC: New aggressive dead code elimination pass<br><br>
<br class=""><div><blockquote class=""><div class="">On Mar 25, 2016, at 3:30 PM, Daniel Berlin via llvm-dev <<a href="mailto:llvm-dev@lists.llvm.org" class="" target="_blank">llvm-dev@lists.llvm.org</a>> wrote:</div><br class="Apple-interchange-newline"><div class=""><div dir="ltr" class=""><div class="">Make most things update post-dominance info and preserve it.</div><div class=""><br class=""></div><div class="">Our other alternative to not take up too much time seems to be invasive surgery on how BB successors/predecessors work so they are a constant time array. I say this because GCC recomputes post-dominators roughly 15-19 times per compilation, and can do it about 3-5x faster than we can. All profiling i've done basically says all our time is spent trying to get at successors and predecessors in dominance/post-dominance respectively, which takes basically no time in gcc, because the edge lists are an array.</div><div class=""><br class=""></div><div class="">(Note that if you look at generic dominance code, like <a href="http://www.cs.princeton.edu/%7Erwerneck/dominators/" class="" target="_blank">http://www.cs.princeton.edu/~rwerneck/dominators/</a>, it's much faster than what we do on the same graphs. This is true even though we use the same algorithms .....)</div><div class=""><br class=""></div><div class="">Given it seems unlikely we are going to change the internal representation anytime soon (or at least i've not seen a proposal), updating post-dom seems the easier answer.</div></div></div></blockquote><div><br class=""></div><div>Are we talking about the basic-blocks edges only? I'm not sure changing the IR for the BBs would be a lot more work than preserving dominance analysis everywhere, or I am totally underestimating one / overestimating the other?</div><div id="DWT8363"><br class=""></div></div></blockquote><br>I'm also curious about this, especially because I'd naively think that:<br><br> representation change == a little thinking and (potentially) a lot of typing<br> preserving post dom == a lot of thinking and a little typing<br><br>and, thus, while updating the analysis might be the *right* thing to do, it is probably not easier (especially once you factor in the time taken to fix bugs where we subtly get it wrong). Maybe in the long run, we should do both?<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><div></div><div>-- </div><div>Mehdi</div><div><br class=""></div><div><br class=""></div><br class=""><blockquote class=""><div class=""><div dir="ltr" class=""><div class=""><br class=""></div></div><div class="gmail_extra"><br class=""><div class="gmail_quote">On Thu, Mar 24, 2016 at 11:56 PM, Eric Christopher <span dir="ltr" class=""><<a href="mailto:echristo@gmail.com" target="_blank" class="">echristo@gmail.com</a>></span> wrote:<br class=""><blockquote class="gmail_quote" style="margin: 0pt 0pt 0pt 0.8ex; border-left: 1px solid rgb(204, 204, 204); padding-left: 1ex;"><p dir="ltr" class="">What do you have in mind here?</p><div class="HOEnZb"><div class="h5">
<br class=""><div class="gmail_quote"><div dir="ltr" class="">On Thu, Mar 24, 2016, 7:28 PM Daniel Berlin via llvm-dev <<a href="mailto:llvm-dev@lists.llvm.org" target="_blank" class="">llvm-dev@lists.llvm.org</a>> wrote:<br class=""></div><blockquote class="gmail_quote" style="margin: 0pt 0pt 0pt 0.8ex; border-left: 1px solid rgb(204, 204, 204); padding-left: 1ex;"><div dir="ltr" class="">Yeah, that was gonna be my question.<br class="">If so, my view is we should just bite the bullet and start threading post dominance through the compiler.<div class="">(assuming anyone wants to help. I'm tackling the memoryssa updating stuff with george ATM).<br class=""></div><div class=""><br class=""></div><div class=""><br class=""></div></div><div class="gmail_extra"><br class=""><div class="gmail_quote">On Thu, Mar 24, 2016 at 7:04 PM, Hal Finkel <span dir="ltr" class=""><<a href="mailto:hfinkel@anl.gov" target="_blank" class="">hfinkel@anl.gov</a>></span> wrote:<br class=""><blockquote class="gmail_quote" style="margin: 0pt 0pt 0pt 0.8ex; border-left: 1px solid rgb(204, 204, 204); padding-left: 1ex;">[+Danny]<br class="">
<div class=""><div class=""><br class="">
<hr id="zwchr"><br class="">
> From: "Justin Bogner via llvm-dev" <<a href="mailto:llvm-dev@lists.llvm.org" target="_blank" class="">llvm-dev@lists.llvm.org</a>><br class="">
> To: "David Callahan via llvm-dev" <<a href="mailto:llvm-dev@lists.llvm.org" target="_blank" class="">llvm-dev@lists.llvm.org</a>><br class="">
> Sent: Wednesday, March 23, 2016 12:36:50 PM<br class="">
> Subject: Re: [llvm-dev] RFC: New aggressive dead code elimination pass<br class="">
><br class="">
> David Callahan via llvm-dev <<a href="mailto:llvm-dev@lists.llvm.org" target="_blank" class="">llvm-dev@lists.llvm.org</a>> writes:<br class="">
> > Hi,<br class="">
> ><br class="">
> > I have a new variant of Aggressive Dead Code Elimination that also<br class="">
> > removes dead branching. It is designed to minimize the cost of<br class="">
> > control-dependence analysis in the common case where almost the<br class="">
> > entire<br class="">
> > program is live. It also can optionally remove dead but<br class="">
> > may-be-infinite loops.<br class="">
> ><br class="">
> > When enabled for –O3 (replacing current ADCE pass) and removing<br class="">
> > loops,<br class="">
> > impact on SPEC2006 is in the noise but it impacts internal<br class="">
> > benchmarks<br class="">
> > suites 1-2% with a comparable increase in compile time. My<br class="">
> > expectation would be to enable –O3 only until we have some<br class="">
> > experience<br class="">
> > with cost but I suspect it should be fine –O2.<br class="">
><br class="">
> Just to clarify, you're saying that both runtime and compile time<br class="">
> impact<br class="">
> were in the noise on SPEC, right?<br class="">
><br class="">
> > What information would the community like to see about such a<br class="">
> > change<br class="">
> > before I put up a diff and (including tweaks to unit tests).<br class="">
><br class="">
> I'm not sure that there's much to discuss in the abstract - it's much<br class="">
> easier to evaluate this kind of thing when there's a patch to refer<br class="">
> to.<br class="">
> Presumably people will want to try the patch out on their own<br class="">
> internal<br class="">
> benchmarks as well.<br class="">
<br class="">
</div></div>+1<br class="">
<br class="">
Does it use post-dominance information?<br class="">
<br class="">
-Hal<br class="">
<div class=""><div class=""><br class="">
><br class="">
> > Thanks<br class="">
> > david<br class="">
> > _______________________________________________<br class="">
> > LLVM Developers mailing list<br class="">
> > <a href="mailto:llvm-dev@lists.llvm.org" target="_blank" class="">llvm-dev@lists.llvm.org</a><br class="">
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><br class="">
<br class="">
</div></div><span class=""><font class="" color="#888888">--<br class="">
Hal Finkel<br class="">
Assistant Computational Scientist<br class="">
Leadership Computing Facility<br class="">
Argonne National Laboratory<br class="">
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_______________________________________________<br class="">LLVM Developers mailing list<br class=""><a href="mailto:llvm-dev@lists.llvm.org" class="" target="_blank">llvm-dev@lists.llvm.org</a><br class="">http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev<br class=""></div></blockquote></div><br class=""><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>