[LLVMdev] Indirect call site profiling

[Das, Dibyendu]

povray in spec cpu 2006 gains if you do indirect function call promotion followed by inlining AFAIK. However, this may require PGO to specialize the top few calls.

From: llvmdev-bounces at cs.uiuc.edu [mailto:llvmdev-bounces at cs.uiuc.edu] On Behalf Of Xinliang David Li
Sent: Monday, October 27, 2014 10:02 PM
To: betulb at codeaurora.org
Cc: llvmdev
Subject: Re: [LLVMdev] Indirect call site profiling



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

> On 10/24/14, 8:26 PM, betulb at codeaurora.org<mailto: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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