<div dir="ltr"><br><div class="gmail_extra"><br><div class="gmail_quote">On Mon, Mar 21, 2016 at 6:28 PM, Philip Reames via llvm-dev <span dir="ltr"><<a href="mailto:llvm-dev@lists.llvm.org" target="_blank">llvm-dev@lists.llvm.org</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
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<div>On 03/21/2016 08:56 AM, Jia Chen via
llvm-dev wrote:<br>
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Hi Christian,<br>
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Thank you so much for the reply! Please see my comments inline.<br>
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<div>On 03/21/2016 09:32 AM, Christian
Convey wrote:<br>
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<div dir="ltr">Hi Jia,
<div><br>
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<blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left-width:1px;border-left-color:rgb(204,204,204);border-left-style:solid;padding-left:1ex"><span style="font-size:12.8px">If one looks at existing research
literatures, there are even more algorithm to consider for
doing pointer analysis.</span></blockquote>
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<div>For at least some published AA algorithms, there may be
some uncertainty about software patents and/or copyright. </div>
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<div>At one point I was interested in the status of <a href="https://dl.acm.org/citation.cfm?id=2466483" target="_blank">this AA
implementation</a> by Lian Li et al. IIRC, when I
contacted Lian to ask if there was any chance of getting it
into LLVM, IIRC she said that her employer wouldn't promise
to relinquish all possible claims it had on that algorithm's
IP. So unfortunately, at least in the U.S., an algorithm
being published in an academic journal doesn't remove all
legal risk associated with using it.</div>
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This is news to me. Thanks for sharing it.<br>
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<div>Also, speaking from my own experience, even when an AA
algorithm seems to be described in great detail in some
piece of literature (e.g., a phd thesis), there can still be
a lot of details which are glossed over, or which seem clear
when reading the document but which get a lot more confusing
when one tries to actually implement it. </div>
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<div>That can make implementing such an algorithm take far
longer than one would expect based on just reading the
document. (It's also an argument in favor of requiring
academic papers which describe the behavior of a software
implementation to actually include a working copy of the
source code, IMHO.)</div>
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My personal experience also coincides. And even if the paper does
come with an artifact or source codes, they are usually
proof-of-concept implementations with lots of important real-world
corner cases ignored. <br>
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<div><br>
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<blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left-width:1px;border-left-color:rgb(204,204,204);border-left-style:solid;padding-left:1ex"><span style="font-size:12.8px">So my question here is: what
kind(s) of precision really justify the cost and what
kinds do not? Has anybody done any study in the past to
evaluate what kinds of features in pointer analyses will
benefit what kinds of optimization passes?</span><br>
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<div>At one point I discussed this with Daniel Berlin, but I'm
having trouble find a record of the conversation. IIRC, he
says that he once threw a huge amount of computing power at
doing a <i>full</i> context-sensitive AA on some software,
and the speedup he observed in the resulting program as
underwhelming (10-15%?). <br>
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I kind of expect that. As you mentioned later, most optimization
passes work in a context-insensitive manner (i.e. they won't clone
a function and optimize differently on different clones). Context
sensitivity on the pointer analysis side is probably not going to
help a lot if the client cannot fully capitalize on it. In the
settings of compiler optimization, my guess is that flow
sensitivity, field sensitivity, heap model and external library
annotations are the four aspects that are likely to matter. <br>
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I did some preliminary experiments with licm on c programs over
the last weekend. I chose licm because intuitively that's the
optimization that could have the biggest performance impact. The
result suggested that tbaa, cfl-aa, scev-aa and globals-aa yields
very little additional benefits over basic-aa. Again, both the
methodology and benchmark selection are very immature and the
results need to be double-checked, but my hope is that by looking
at how aa algorithms and their clients interact I may be able to
get some hints on what kind of aa a compiler really wants. <br>
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Just to chime in here, your results match my experience and
expectations with LICM as well. Between basic-aa, and TBAA
(specifically LLVM's implementation thereof), I haven't seen a lot
of cases where an imprecision in the alias analysis prevents
hoisting.<br></div></blockquote><div><br>Yeah, at best, for LICM, it's just going to tell you the best place to insert runtime checks. LICM has a specific goal, and it's usually not AA that prevents proving something loop invariant. Most loads/stores are also either trivially loop invariant, or impossible to prove loop invariant.</div><div><br></div><div>More general PRE and GVN-like opts (basically, load elimination, load hoisting, dead store elimination, store sinking) are where i expect better AA to help the most off the top of my head. But to figure out the gains, you'd really need to instrument at runtime to figure out what the theoretical maximum gain is (IE when things are equivalent that it is not proving equivalent).</div><div><br></div><div><br></div><div> <br></div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div text="#000000" bgcolor="#FFFFFF">
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*However*, if you're interested in LICM specifically, I have
*definitely* seen cases where the precision of AliasSetTracker (our
grouping of AA results to prevent O(n^2) queries) prevents hoisting
in spurious cases. AST could use some serious attention, both from
an engineering standpoint and from (possibly) a theoretically one.
</div></blockquote><div><br></div><div><br></div><div>You already know my view on this one: It's going to be remarkably hard to make AST work the way folks want it and have it be incremental and completely agnostic of anything but the AA API.</div><div><br></div><div>It's just really hard if not provably impossible to do this incrementally and avoid duplicate work, and get precise results and ...</div><div><br></div><div><br></div><div>On the other hand, it's pretty easy if you basically say "i provide list of all pointers and statements i care about, you make me some sets", and you let it figure out the answers upfront.</div><div><br></div><div>(it's also not clear to me why AST is the right abstraction for LICM to work on top of, but that's neither here nor there :P)</div><div><br></div><div><br></div><div><br></div></div></div></div>