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<div class="moz-cite-prefix">On 6/19/16 12:36 PM, Mehdi Amini wrote:<br>
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<div class="">On Jun 19, 2016, at 8:05 AM, John Criswell <<a
moz-do-not-send="true" href="mailto:jtcriswel@gmail.com"
class=""><a class="moz-txt-link-abbreviated" href="mailto:jtcriswel@gmail.com">jtcriswel@gmail.com</a></a>> wrote:</div>
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<div class="moz-cite-prefix">On 6/19/16 4:28 AM, Mehdi
Amini via llvm-dev wrote:<br class="">
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<div class="">On Jun 18, 2016, at 10:44 PM, Yuxi
Chen via llvm-dev <<a moz-do-not-send="true"
href="mailto:llvm-dev@lists.llvm.org" class="">llvm-dev@lists.llvm.org</a>>
wrote:</div>
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<div class="">
<div class="">Hi All,
<div class=""><br class="">
</div>
<div class="">When I use llvm, I encounter a
problem like "unable to schedule pass A
required by C"</div>
<div class="">I investigated deeper. It's like:</div>
<div class="">I have three passes, say A, B,
C(all are on function level)</div>
<div class="">A would modify IR code. (change
instruction order)</div>
<div class=""><br class="">
</div>
<div class="">For pass B, </div>
<div class="">I would use the result of pass A,
I use addRequired<B>(), and
&getAnalysis<B>(), it works. </div>
<div class=""><br class="">
</div>
<div class="">void getAnalysisUsage(AU){</div>
<div class=""><span class="Apple-tab-span"></span>AU.addRequired<A>();</div>
<div class="">}</div>
<div class=""><br class="">
</div>
<div class=""><br class="">
</div>
<div class="">For pass C, it will use the
results of pass A and B. </div>
<div class="">I use the way as used for pass B,
but it failed, even for LoopInfo analysis
pass(which is the built-in analysis pass).</div>
<div class="">void getAnalysisUsage(AU){</div>
<div class=""><span class=""><span
class="Apple-tab-span"></span>AU.addRequired<A>();</span></div>
<div class=""><span class=""><span
class="Apple-tab-span"></span>AU.addRequired<B>();</span></div>
<div class=""><span class="">}</span></div>
<div class=""><br class="">
</div>
<div class=""><br class="">
</div>
<div class="">It seems because A would modify IR
code, so for pass C, I need first load pass A
then pass B, otherwise it will be
invalidated. </div>
<div class="">However, when I change the using
order, I still got error "unable to schedule
pass A required by C".</div>
<div class=""><br class="">
</div>
<div class="">Does anyone encounter the same
problem before and have a solution?</div>
</div>
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<div class="">
<div class="">
<div class="">Any help is appreciated. </div>
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<div class=""><br class="">
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<div class="">Depending on other transformations isn’t
recommended, and isn’t supported by the
soon-new-passmanager I believe.</div>
<div class="">The expectation is that the passes are
added in order to the pass manager by the client.</div>
</div>
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<br class="">
Depending on transformation passes isn't supported by the
legacy PassManager, either. </div>
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<div><br class="">
</div>
<div>Really? What about LCSSA for example? My impression was
that the legacy PM does not make any distinction between
transformations and analysis.</div>
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<br>
The legacy PassManager doesn't make distinctions between analysis
and transform passes per se. A pass is a pass; analysis passes are
just passes that never modify the IR whereas transform passes are
passes that may modify the IR.<br>
<br>
Since LLVM 2.0, the PassManager did not support the feature of using
getAnalysisUsage<>() to dictate which transform passes were
required to be run before a given pass. While one could write a
pass that could act as both a transform pass and analysis pass, the
PassManager didn't guarantee that it could schedule such passes (I
believe primarily due to impossible-to-schedule pass schedules). I
was told that the PassManager was really designed so that
getAnalysisUsage<>() would specify analysis passes; whoever
set up the pass pipeline was responsible for ensuring that
prerequisite transforms were run when they needed to be run.<br>
<br>
Now, while the "do not schedule transform passes using
getAnalysisUsage<>()" rule has been presented on the mailing
list multiple times over the years, there are passes in LLVM that
break it. I believe some passes require UnifyExitNodes. If you say
LCSSA does, I'd believe it. However, in most cases, you don't need
this feature, and breaking the rule can generate difficult-to-debug
messages from the PassManager. Therefore, while the rule isn't
followed all the time, I tell people who run into the problem not to
require transform passes. Historically, debugging bad pass
pipelines has been no fun, and it's terribly difficult for new LLVM
users.<br>
<br>
Regards,<br>
<br>
John Criswell<br>
<br>
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<div><br class="">
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<div>-- </div>
<div>Mehdi</div>
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<blockquote type="cite" class="">
<div class="">
<div bgcolor="#FFFFFF" text="#000000" class="">Occasionally
some passes can get away with it, but it often results in
unschedule-able pass pipelines as above.<br class="">
<br class="">
If your transform pass does something to the code, other
passes should either infer what it did by examining the
IR. the IR contains the definitive information about the
program (because it is the program).<br class="">
<br class="">
Alternatively, you could create an analysis pass upon
which both your transform and analysis passes depend. The
transform pass would update this new analysis pass with
information on what it transformed; your later analysis
passes could then query this information. This approach
is fragile, but it could work.<br class="">
<br class="">
Regards,<br class="">
<br class="">
John Criswell<br class="">
<br class="">
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<div class="">In you case, I expect that it would
“work” by removing the dependency from C to A. If C
requires B and B requires A, by scheduling C you’ll
get A, B, C in sequence.</div>
<div class=""><br class="">
</div>
<div class="">— </div>
<div class="">Mehdi</div>
<div class=""><br class="">
</div>
<div class=""><br class="">
</div>
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<div class="">
<div class="">
<div class=""><br class="">
</div>
<div class="">Best,</div>
<div class="">Yuxi</div>
</div>
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<pre class="moz-signature" cols="72">--
John Criswell
Assistant Professor
Department of Computer Science, University of Rochester
<a moz-do-not-send="true" class="moz-txt-link-freetext" href="http://www.cs.rochester.edu/u/criswell">http://www.cs.rochester.edu/u/criswell</a></pre>
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<pre class="moz-signature" cols="72">--
John Criswell
Assistant Professor
Department of Computer Science, University of Rochester
<a class="moz-txt-link-freetext" href="http://www.cs.rochester.edu/u/criswell">http://www.cs.rochester.edu/u/criswell</a></pre>
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