<html>
<head>
<meta content="text/html; charset=utf-8" http-equiv="Content-Type">
</head>
<body text="#000000" bgcolor="#FFFFFF">
<div class="moz-cite-prefix">On 12/07/2015 02:16 AM, James Molloy
wrote:<br>
</div>
<blockquote
cite="mid:CALCTSA1YHhVGN8OmLnJuxboh3Zv1kZ93R743qGZMbEmmG3-0MQ@mail.gmail.com"
type="cite">
<div dir="ltr">Hi Philip,
<div><br>
</div>
<div>I've taken a closer look at this now. There are several
problems with LVI:</div>
<div><br>
</div>
<div> 1. getValueAt() is not as powerful as getValueAtEnd(). I
think this is an oversight - it looks like the intention is
that getValueAt() should be more powerful than getValueAtEnd()
because it has the ability to use assume intrinsics and range
metadata. However, if the range metadata and assume intrinsics
give no answer (undefined), we don't fall back on our other
analyses (getBlockValue()).</div>
</div>
</blockquote>
This is a known (to me at least) problem. I've got a series of
patches stuck in review right now which will eventually fix this.<br>
<blockquote
cite="mid:CALCTSA1YHhVGN8OmLnJuxboh3Zv1kZ93R743qGZMbEmmG3-0MQ@mail.gmail.com"
type="cite">
<div dir="ltr">
<div> 2. LVI does not look through select instructions.</div>
</div>
</blockquote>
Again, I have a patch ready once review gets unblocked. (To be
clear, I think the next unblocking step is mine; the cycle time is
just long enough to get it out of my active list.)<br>
<blockquote
cite="mid:CALCTSA1YHhVGN8OmLnJuxboh3Zv1kZ93R743qGZMbEmmG3-0MQ@mail.gmail.com"
type="cite">
<div dir="ltr">
<div> 3. LVI does not handle loop PHIs at all.</div>
</div>
</blockquote>
I'm not sure what you're describing here. LVI can prove limited
recursive conditions. What are you trying to describe here? Do you
have a specific example case?<br>
<blockquote
cite="mid:CALCTSA1YHhVGN8OmLnJuxboh3Zv1kZ93R743qGZMbEmmG3-0MQ@mail.gmail.com"
type="cite">
<div dir="ltr">
<div><br>
</div>
<div>I have patches for (1) and (2) and will send them for
review irrespective of what happens with this review, as I
think they're obviously good things to do. I think (3) is the
real deal breaker - my motivating example has this chain of
selects threaded over a loop, and LVI just cannot deal with
loops at all.</div>
<div><br>
</div>
<div>I think I recall someone (Jiangning Liu?) attempting to add
support for loops to LVI, but having difficulty retaining the
lattice structure and making sure it always converged.</div>
</div>
</blockquote>
Yep, that would probably be challenging. It does become easier once
my "intersect" patch lands - since it can be a separable analysis
which is then intersected - but it's not a small project. <br>
<blockquote
cite="mid:CALCTSA1YHhVGN8OmLnJuxboh3Zv1kZ93R743qGZMbEmmG3-0MQ@mail.gmail.com"
type="cite">
<div dir="ltr">
<div><br>
</div>
<div>So at the moment I'm not sure how else to implement this
other than what I've already uploaded - even pushing the icmp
queries up the use-def graph would eventually end up at the
head of a loop - somewhere we need to do some analysis that
can pass through loop PHIs.</div>
</div>
</blockquote>
Ok, thank you for looking at the options. I'm still a bit unhappy
about the chosen approach, but with your explanation I see why you
settled on the given approach. I withdraw my objections. <br>
<blockquote
cite="mid:CALCTSA1YHhVGN8OmLnJuxboh3Zv1kZ93R743qGZMbEmmG3-0MQ@mail.gmail.com"
type="cite">
<div dir="ltr">
<div><br>
</div>
<div>Cheers,</div>
<div><br>
</div>
<div>James</div>
</div>
<br>
<div class="gmail_quote">
<div dir="ltr">On Sat, 5 Dec 2015 at 19:34 James Molloy <<a
moz-do-not-send="true" href="mailto:james@jamesmolloy.co.uk"><a class="moz-txt-link-abbreviated" href="mailto:james@jamesmolloy.co.uk">james@jamesmolloy.co.uk</a></a>>
wrote:<br>
</div>
<blockquote class="gmail_quote" style="margin:0 0 0
.8ex;border-left:1px #ccc solid;padding-left:1ex">Hi Philip,<br>
<br>
Thanks for taking a look!<br>
<br>
I agree that searching an arbitrary size value graph would be
horrendously expensive. What I have implemented is to search
through unlimited selects and phis (should be cheap, these
aren't commonly chained deeply, see GetUnderlyingObjects), and
will look through only a certain number of binary operators.
The default depth is 1, which will go through one add but no
more (configurable by the user). I think overall this should
restrict the potential for bloat. <br>
<br>
I have a motivating example which is a chain of selects and
phis in a loop. The loop part is important - it's what seems
to confuse instcombine currently. What I can do is take a look
to see what's causing CVP / LVI to not support this case and
report back. Thanks for the hint!<br>
<br>
I'm not sure the last approach you suggested would work for
the loop use case, although I haven't thought majorly hard
about it just yet. <br>
<br>
Thanks again,<br>
<br>
James<br>
<div class="gmail_quote">
<div dir="ltr">On Fri, 4 Dec 2015 at 20:36, Philip Reames
via llvm-commits <<a moz-do-not-send="true"
href="mailto:llvm-commits@lists.llvm.org"
target="_blank">llvm-commits@lists.llvm.org</a>>
wrote:<br>
</div>
<blockquote class="gmail_quote" style="margin:0 0 0
.8ex;border-left:1px #ccc solid;padding-left:1ex">reames
added a subscriber: reames.<br>
reames added a comment.<br>
<br>
At a high level, I'm a bit unsure about the formulation
chosen here. I'm worried that seeking to find a set of
constant values through a potentially large value graph
might be expensive.<br>
<br>
A couple of specific questions:<br>
<br>
- The examples you've listed should be entirely handled by
LazyValueInfo and either JumpThreading or CVP. Do you
have a strong reason InstCombine needs to perform the same
optimization?<br>
- Have you considered phrasing this as pushing the query
(i.e. icmp) back along the inputs? Doing this might a)
let you terminate earlier or b) unswitch the comparison if
the select is only used by the compare.<br>
<br>
<br>
<a moz-do-not-send="true"
href="http://reviews.llvm.org/D15232" rel="noreferrer"
target="_blank">http://reviews.llvm.org/D15232</a><br>
<br>
<br>
<br>
_______________________________________________<br>
llvm-commits mailing list<br>
<a moz-do-not-send="true"
href="mailto:llvm-commits@lists.llvm.org"
target="_blank">llvm-commits@lists.llvm.org</a><br>
<a moz-do-not-send="true"
href="http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-commits"
rel="noreferrer" target="_blank">http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-commits</a><br>
</blockquote>
</div>
</blockquote>
</div>
</blockquote>
<br>
</body>
</html>