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

Xinliang David Li xinliangli at gmail.com
Mon Oct 27 09:32:08 PDT 2014

On Sun, Oct 26, 2014 at 11:49 AM, <betulb at codeaurora.org> wrote:

> 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.

IIRC, gap is another C program benefit from this.

David

> 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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