[llvm-dev] Adding support for self-modifying branches to LLVM?

Philip Reames via llvm-dev llvm-dev at lists.llvm.org
Tue Feb 9 08:07:39 PST 2016

On 02/09/2016 06:57 AM, Jonas Wagner wrote:
> Hi,
> 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.
> 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.
> Some results:
> - On SPEC, patchpoints introduce an overhead of ~25% compared to a 
> version where ASan checks are removed.
> - This is almost half of the cost of the checks themselves.
> - The results are similar for NOPs of size 1 and 5 bytes.
> - 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.
> 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.
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.

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.)

> Best,
> Jonas
> On Thu, Jan 21, 2016 at 11:52 PM Jonas Wagner <jonas.wagner at epfl.ch 
> <mailto:jonas.wagner at epfl.ch>> wrote:
>     Hello,
>             There is some data on this, e.g, in “High System-Code
>             Security with Low Overhead”
>             <http://dslab.epfl.ch/proj/asap/#publications>. 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.
>         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.
>     Interesting. Indeed this was measured by removing some checks and
>     then re-optimizing the program.
>     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.
>     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.
>     I'll try to measure this, and will come back when I have data.
>     Best,
>     Jonas

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