[LLVMdev] Indirect call site profiling (original) (raw)

betulb at codeaurora.org betulb at codeaurora.org
Sun Oct 26 11:49:05 PDT 2014

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)?

I can't give numbers, but we do collect data from 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.

Spec does have programs such as gcc, vortex and others which use indirect calls. I'm planning to have an RFC soon on the feature. I'll follow it w/ the patch for the profiler changes for clang, llvm and compiler-rt.

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

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