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<div class="moz-cite-prefix">On 02/23/2016 09:48 PM, Xinliang David
Li wrote:<br>
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cite="mid:CALRgJCMd_GddzRc2Pt1JP2pL878RFFRVLotxaTHnXJMghy-zWQ@mail.gmail.com"
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<div>Hi Phillip, </div>
<a moz-do-not-send="true" href="http://reviews.llvm.org/D10825">http://reviews.llvm.org/D10825</a>
tries to fix similar issues. Looks like there are missing cases.
Can you create a small reproducer?<br>
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This doesn't quite look the same. That review is about continuing
the loop chain itself into the most profitable successor. My case
is about continuing the preheader's chain into the (previously
formed) loop chain.<br>
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cite="mid:CALRgJCMd_GddzRc2Pt1JP2pL878RFFRVLotxaTHnXJMghy-zWQ@mail.gmail.com"
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<div><br>
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<div>Cong has also improved loop rotation based on better cost
model -- it is not yet enabled (-mllvm
-precise-rotation-cost=true to turn on). If possible, can you
also give it a try?</div>
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Loop rotation is not the problem here. The layout of the loop
(header->latch->header cycle) is exactly what it should be. <br>
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cite="mid:CALRgJCMd_GddzRc2Pt1JP2pL878RFFRVLotxaTHnXJMghy-zWQ@mail.gmail.com"
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<div>thanks,</div>
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<div>David</div>
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<div class="gmail_extra"><br>
<div class="gmail_quote">On Tue, Feb 23, 2016 at 5:19 PM, Philip
Reames via llvm-dev <span dir="ltr"><<a
moz-do-not-send="true"
href="mailto:llvm-dev@lists.llvm.org" target="_blank"><a class="moz-txt-link-abbreviated" href="mailto:llvm-dev@lists.llvm.org">llvm-dev@lists.llvm.org</a></a>></span>
wrote:<br>
<blockquote class="gmail_quote" style="margin:0 0 0
.8ex;border-left:1px #ccc solid;padding-left:1ex">I'm
getting some odd behavior out of MBP and was hoping someone
knowledge of the code might be able to give some guidance.
Fair warning, I'm trying to describe a problem in code I
don't really understand, so if something doesn't make sense,
assume I misunderstood something.<br>
<br>
The problematic case I'm seeing is that cold blocks are
being placed between the preheader and header of a hot
loop. This has the result of adding a bunch of cold code
spread through out the code rather than grouped all together
at the end of the function.<br>
<br>
From what I can tell tracing through the code, the critical
decision that goes wrong is when we're visiting the
preheader after forming a (correct) chain for the loop
itself. When selecting a successor to merge with, we appear
to not be considering the loop even though the loop hasn't
been rotated and the header would be ideal for fallthrough.<br>
<br>
In particular, we're printing the "(prob) (non-cold CFG
conflict)" debug output message for the successor of the
preheader which is the header. If I'm reading this code
correctly, it's identifying the fact there's a global more
important predecessor for the header (i.e. the latch block),
but it doesn't seem to be account for the fact that the
latch block has already been combined into the header's
chain. Unless I'm misreading something, we *should* be able
to merge the loop chain with the preheader chain in it's
entirety right?<br>
<br>
At least one on example I've looked at, adding an early exit
from BadCFGConflict loop when Pred and Succ are part of the
same chain does appear to give the expected result, but I
don't understand the code well enough to reason about
whether that is generally a correct thing to do or not.<br>
<br>
Philip<br>
<br>
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