<html><head><meta http-equiv="Content-Type" content="text/html charset=us-ascii"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class=""><br class=""><div><blockquote type="cite" class=""><div class="">On Sep 12, 2017, at 6:44 AM, Jonas Paulsson via llvm-dev <<a href="mailto:llvm-dev@lists.llvm.org" class="">llvm-dev@lists.llvm.org</a>> wrote:</div><br class="Apple-interchange-newline"><div class="">
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<div bgcolor="#FFFFFF" text="#000000" class=""><p class="">Hi Ghassan,<br class="">
</p>
<blockquote type="cite" cite="mid:MWHPR04MB09455C555208ACFB159BE0918C690@MWHPR04MB0945.namprd04.prod.outlook.com" class="">
<div id="divtagdefaultwrapper" style="font-size: 12pt; font-family: Calibri, Helvetica, sans-serif;" dir="ltr" class=""><p class="">As for live-through information, we found that the machine
scheduler does call initLiveThru() and here is a pointer to
the code:</p><p class=""><br class="">
</p><div class=""><font style="font-family:
Calibri,Helvetica,sans-serif,serif,"EmojiFont";" face="Calibri,Helvetica,sans-serif" size="3" class=""><span style="font-size:12pt;" id="divtagdefaultwrapper" class=""></span></font><br class="webkit-block-placeholder"></div>
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<div style="margin-top:0;margin-bottom:0;" class=""><a href="https://gitlab.com/CSUS_LLVM/LLVM_DRAGONEGG/blob/master/Generic/llvmTip/llvm-master/lib/CodeGen/MachineScheduler.cpp#L921" target="_blank" rel="noopener noreferrer" id="LPlnk344791" previewremoved="true" moz-do-not-send="true" class=""><span id="LPlnk344791" class="">https://gitlab.com/CSUS_LLVM/LLVM_DRAGONEGG/blob/master/Generic/llvmTip/llvm-master/lib/CodeGen/MachineScheduler.cpp#L921</span></a></div>
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<font size="3" class=""><font face="Calibri,Helvetica,sans-serif" class=""><font face="Calibri,Helvetica,sans-serif" class=""><font face="Calibri,Helvetica,sans-serif" class=""><font face="Calibri,Helvetica,sans-serif" class=""><font face="Calibri,Helvetica,sans-serif" class=""><font face="Calibri,Helvetica,sans-serif" class=""><font face="Calibri,Helvetica,sans-serif" class=""><font face="Calibri,Helvetica,sans-serif" class="">The fir<font face="Calibri,Helvetica,sans-serif" class="">st part of
the comment <font face="Calibri,Helvetica,sans-serif" class="">above
initLiveThru() says "The register tracker is
unaware of global liveness so ignores normal<font face="Calibri,Helvetica,sans-serif" class=""> </font>live-thru
ranges.</font></font></font></font></font></font></font></font></font></font></font>.."<font face="Calibri,Helvetica,sans-serif" class="">. It is then
of course confusing to see the<font face="Calibri,Helvetica,sans-serif" class="">se methods
like initLiveThru()...<br class="">
<br class="">
</font><font face="Calibri,Helvetica,sans-serif" class="">My
understanding is that (please correct me if I'm
wrong)<br class="">
<font face="Calibri,Helvetica,sans-serif" class="">1. All
instructions are traversed bottom<font face="Calibri,Helvetica,sans-serif" class="">-up
during DAG building. <font face="Calibri,Helvetica,sans-serif" class="">While
doing this <font face="Calibri,Helvetica,sans-serif" class="">re<font face="Calibri,Helvetica,sans-serif" class="">g
pressure is <font face="Calibri,Helvetica,sans-serif" class="">tracked</font>
based on <font face="Calibri,Helvetica,sans-serif" class="">just
looking at </font>those
instructions. So if a def has no use
in an mbb it is a "live-out" reg, and
if there is a use with no def, it
would become "live-in"<font face="Calibri,Helvetica,sans-serif" class="">.
This </font>is then a kind of local
live-through concept, in co<font face="Calibri,Helvetica,sans-serif" class="">ntrast
to a true live-through analysis<font face="Calibri,Helvetica,sans-serif" class=""> which would be aware o<font face="Calibri,Helvetica,sans-serif" class="">f
registers not used/defed in the
region as well.</font></font></font><br class=""></font></font></font></font></font></font></font></div></div></blockquote><div>Yes, the first pass during DAG construction determines the maximum register pressure and the list of live-out values. I think the code consults liveintervals to differentiate dead-defs from true live-outs or detect killing uses that aren't marked as such.</div><br class=""><blockquote type="cite" class=""><div class=""><div bgcolor="#FFFFFF" text="#000000" class=""><font face="Calibri,Helvetica,sans-serif" class=""><font face="Calibri,Helvetica,sans-serif" class=""><font face="Calibri,Helvetica,sans-serif" class=""><font face="Calibri,Helvetica,sans-serif" class=""><font face="Calibri,Helvetica,sans-serif" class=""><font face="Calibri,Helvetica,sans-serif" class=""><font face="Calibri,Helvetica,sans-serif" class="">
</font></font></font></font></font></font></font>2.
We should ideally have an analysis <font face="Calibri,Helvetica,sans-serif" class=""><font face="Calibri,Helvetica,sans-serif" class="">of global li<font face="Calibri,Helvetica,sans-serif" class="">veness </font></font>so
that t<font face="Calibri,Helvetica,sans-serif" class="">h<font face="Calibri,Helvetica,sans-serif" class="">e
regpressure trackers could be properly
initialized, but this is currently missing. Of
course, one might hope that<font face="Calibri,Helvetica,sans-serif" class=""> it
wouldn't be too hard to extend LiveInter<font face="Calibri,Helvetica,sans-serif" class="">vals
to also provide this information...</font></font>
It <font face="Calibri,Helvetica,sans-serif" class="">would
be interesting to merely try this and see
how valuable it <font face="Calibri,Helvetica,sans-serif" class="">wou<font face="Calibri,Helvetica,sans-serif" class="">ld</font></font>
<font face="Calibri,Helvetica,sans-serif" class="">be...<br class=""></font></font></font></font></font></div></div></blockquote><div>Again a global minimum amount of spills/reloads is not necessarily better than a good schedule inside a loop with extra spills/reloads outside the loop. But would certainly be worth exploring.</div><div>Writing a naive function to determine all live values at a certain point is easy: Just iterate over all vregs and check for each whether the point in question is covered by a live interval. However to do this efficiently so it can be used in production would probably require a concept such as keeping live-in/live-out lists on basic blocks up-to-date. To put that into production we should first demonstrate that it is worth it.</div><div><br class=""></div><div>- Matthias</div><div><br class=""></div></div></body></html>