[LLVMdev] Indirect call site profiling

[John Criswell]

On 10/24/14, 8:26 PM, betulb at codeaurora.org wrote:
> Hi All,
>
> We've been working on enhancing LLVM's instrumentation based profiling by
> adding indirect call target profiling support. Our goal is to add
> instrumentation around indirect call sites, so that we may track the
> frequently taken target addresses and their call frequencies.
>
> The acquired data has uses in optimization of indirect function call
> heavy applications. Our initial findings show that using the profile data
> in optimizations would help improve the performance of some of the spec
> benchmarks notably.

Can you quantify "notably?" Also, do you profile on one set of inputs 
and then test the optimization on another set of inputs (e.g., the test 
and train runs)?

>   We have a proof of concept implementation,  which we
> plan to put it up for review. However, I’d like to inquire prior if there
> are any plans or ongoing work done in the community to enable indirect
> call target profiling support or not. Please inform if cfe-dev is a better
> candidate for posting PGO related emails.

Interesting. I did not think SPEC had many programs with a lot of 
indirect function calls.

It would be interesting to see what your optimization would do on an 
operating system kernel like FreeBSD or Linux. The VFS (file system) 
layer uses function pointers a lot, but I'm not sure if it's the 
dominant overhead.

Have you tried on C++ programs? They should be making heavy use of 
indirect function calls as well.

If you make your software public, please let me know. Adapting your work 
for kernel execution and trying it out on a kernel might be a nice 
project for one of our students.

Regards,

John Criswell

-- 
John Criswell
Assistant Professor
Department of Computer Science, University of Rochester
http://www.cs.rochester.edu/u/criswell