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<div class="moz-cite-prefix">On 02/09/2016 06:57 AM, Jonas Wagner
wrote:<br>
</div>
<blockquote
cite="mid:CAMRJFfK4LmbL8xFSL+mSpKhh97UdEUuQT1E_b8t=5xa5Sveimw@mail.gmail.com"
type="cite">
<div dir="ltr">Hi,
<div><br>
</div>
<div>I'm coming back to this old thread with data about the
performance of NOPs. Recalling that I was considering
transforming NOP instructions into branches and back, in order
to dynamically enable code. One use case for this was
enabling/disabling individual sanitizer checks (ASan, UBSan)
on demand.</div>
<div><br>
</div>
<div>I wrote a pass which takes an ASan-instrumented program,
and replaces each ASan check with an
llvm.experimental.patchpoint intrinsic. This intrinsic inserts
a NOP of configurable size. It has otherwise no effect on the
program semantics. It does prevent some optimizations,
presumably because instructions cannot be moved across the
patchpoint.</div>
<div><br>
</div>
<div>Some results:</div>
<div>- On SPEC, patchpoints introduce an overhead of ~25%
compared to a version where ASan checks are removed.</div>
<div>- This is almost half of the cost of the checks themselves.</div>
<div>- The results are similar for NOPs of size 1 and 5 bytes.</div>
<div>- Interestingly, the results are similar for NOPs of 0
bytes, too. These are patchpoints that don't insert any code
and only inhibit optimizations. I've only tested this on one
benchmark, though.</div>
<div><br>
</div>
<div>To summarize, only part of the cost of NOPs is due to
executing them. Their effect on optimizations is significant,
too. I guess this would hold for branches and sanitizer checks
as well.</div>
</div>
</blockquote>
I don't think you can really draw strong conclusions from the
experiments you described. What you've ended up measuring is nearly
the impact of not optimizing over patchpoints at the check
locations. This doesn't really tell you much about what a check
(which is likely to inhibit optimization much less) costs over a nop
at the same position. <br>
<br>
One bit of data you could extract from the experiment as constructed
would be the relative cost of extra nops. You do mention that the
results are similar for sizes 1-5 bytes, but similar is very vague
in this context. Are the results statistically indistinguishable?
Or is there a noticeable but small slowdown that results? (Numbers
would be great here.)<br>
<br>
<blockquote
cite="mid:CAMRJFfK4LmbL8xFSL+mSpKhh97UdEUuQT1E_b8t=5xa5Sveimw@mail.gmail.com"
type="cite">
<div dir="ltr">
<div><br>
</div>
<div>Best,</div>
<div>Jonas</div>
<div><br>
</div>
<div><br>
</div>
<div>
<div class="gmail_quote">
<div dir="ltr">On Thu, Jan 21, 2016 at 11:52 PM Jonas Wagner
<<a moz-do-not-send="true"
href="mailto:jonas.wagner@epfl.ch">jonas.wagner@epfl.ch</a>>
wrote:<br>
</div>
<blockquote class="gmail_quote" style="margin:0 0 0
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<div dir="ltr">
<div>
<p style="margin:1.2em 0px!important">Hello,</p>
</div>
</div>
<div dir="ltr">
<div>
<blockquote style="margin:1.2em
0px;border-left-width:4px;border-left-style:solid;border-left-color:rgb(221,221,221);padding:0px
1em;color:rgb(119,119,119);quotes:none">
<blockquote style="margin:1.2em
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<p style="margin:1.2em 0px!important">There is
some data on this, e.g, in <a
moz-do-not-send="true"
href="http://dslab.epfl.ch/proj/asap/#publications"
target="_blank">“High System-Code Security
with Low Overhead”</a>. In this work we found
that, for ASan as well as other instrumentation
tools, most overhead comes from the checks.
Especially for CPU-intensive applications, the
cost of maintaining shadow memory is small.</p>
</blockquote>
<p style="margin:1.2em 0px!important">How did you
measure this? If it was measured by removing the
checks before optimization happens, then what you
may have been measuring is not the execution
overhead of the branches (which is what would be
eliminated by nop’ing them out) but the effect on
the optimizer.</p>
</blockquote>
</div>
</div>
<div dir="ltr">
<div>
<p style="margin:1.2em 0px!important">Interesting.
Indeed this was measured by removing some checks and
then re-optimizing the program.</p>
<p style="margin:1.2em 0px!important">I’m aware of
some impact checks may have on optimization. For
example, I’ve seen cases where much less inlining
happens because functions with checks are larger. Do
you know other concrete examples? This is definitely
something I’ll have to be careful about. Philip
Reames confirms this, too.</p>
<p style="margin:1.2em 0px!important">On the other
hand, we’ve also found that the benefit from
removing a check is roughly proportional to the
number of cycles spent executing that check’s
instructions. Our model of this is not very precise,
but it shows that the cost of executing the check’s
instructions matters.</p>
<p style="margin:1.2em 0px!important">I'll try to
measure this, and will come back when I have data.</p>
<p style="margin:1.2em 0px!important">Best,<br>
Jonas</p>
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