[llvm-dev] Error with perf2bolt in LLVM BOLT

Thu Apr 9 21:59:29 PDT 2020

Hi Rafael,

Thanks a lot for the detailed description. I think this would help me a lot.

Regards,
Venu.

On Fri, Apr 10, 2020 at 12:35 AM Rafael Auler <rafaelauler at fb.com> wrote:

> Hi Venugopal,
>
>
>
> Running a project with source code under a debugger is the fastest way I
> know to create a mental model of the most important classes and how a
> project is organized. I definitely recommend doing that. BOLT’s
> documentation would be a high-level view described in the 2019 CGO paper
> and some slides, but the technical details are only available in source
> code.
>
>
>
> However, I can give a quick overview to get you better prepared. Here are
> some key points:
>
> 1. Binary format is mostly abstracted away by LLVM’s libObject (see
> Binary.h and ObjectFile.h). We are currently working in a new bolt-only
> BinaryFormat abstraction to better encapsulate the gory details of
> manipulating object files.
> 2. llvm-bolt.cpp is the main tool entry point. All class hierarchy where
> real work happens is designed as a library, LLVM-style, and llvm-bolt.cpp
> is the main user of BOLT as a library.
> 3. The main control class in BOLT would be RewriteInstance. This
> represents the concept of a single binary rewrite that was requested by the
> user.
> 4. RewriteInstance will coordinate the entire rewrite process by first
> reading the binary, building BOLT’s IR to represent its contents, perform a
> pipeline of modifications (BinaryPasses) and then rewriting it in a
> separate output file.
>
> If your interest in BOLT is to write a pass, I would suggest setting
> breakpoints and paying special attention to BinaryPassManager. You can find
> the simplest passes and easier to understand under Passes/BinaryPasses.cpp.
> Just copy one of them and register your copy in BinaryPassManager.  Your
> pass will be exposed to BOLT’s view of the world and you can write code to
> dump a snapshot of this view for a quick analysis (look for dump() methods
> in the objects exposed to your pass).
>
> BOLT’s IR top-level class is BinaryFunction. A BinaryFunction holds
> BinaryBasicBlock instances, which holds MCInst instances. Since BOLT
> operates with an augmented MCInst in comparison with the regular MCInst
> from LLVM, we have a special class to deal with operations on instructions
> and abstract the target machine. You can play around by using MCPlusBuilder
> and its subclass X86MCPlusBuilder to do all sort of work, such as
> creating/checking calls, branches, etc. If you want to instrument your
> binary, for example, take a look at Passes/Instrumentation.cpp to see how
> it accomplishes branch instrumentation.
>
> I hope this helps,
>
> Rafael
>
>
>
> *From: *Venugopal Raghavan <venur2005 at gmail.com>
> *Date: *Thursday, April 9, 2020 at 6:10 AM
> *To: *Rafael Auler <rafaelauler at fb.com>
> *Subject: *Re: [llvm-dev] Error with perf2bolt in LLVM BOLT
>
>
>
> Hi Rafael,
>
>
>
> Thanks for your reply. I think I understand the issue now. I had run BOLT
> a week ago successfully and I must have attempted perf2bolt specifying the
> binary that was optimized earlier.
>
>
>
> I think I will do a fresh run.
>
>
>
> I would like to understand the code in BOLT, the general flow and the data
> structures it uses. Is there any documentation on the code structure? What
> would suggest the fastest way to get some understanding of the code. My
> idea was to run llvm-bolt under gdb and step through the code, but I am not
> sure that is the best way. I am not familiar with binary formats and so on,
> so that is another obstacle I need to face.
>
>
>
> Thanks.
>
>
>
> Regards,
>
> Venu.
>
>
>
> On Wed, Apr 8, 2020 at 11:16 PM Rafael Auler <rafaelauler at fb.com> wrote:
>
> Hi Venugopal,
>
>
>
> perf2bolt has strict demands on its inputs when generating the profile
> data file. The input binary to perf2bolt must be the same that was running
> when you launched perf record, and it will try to verify that by checking
> the build id, if the binary has one. It also assumes this will be the
> binary you will later optimize. Ordinarily, BOLT doesn’t optimize a binary
> that was already optimized by BOLT itself. That’s the message you are
> getting. You should try collecting data on a binary that you did not
> already optimize with BOLT.
>
>
>
> That said, if you really want it, it is possible to collect data in
> binaries that were already bolted, but you need some non-standard flags for
> that. You need to use -enable-bat when generating that binary. This flag
> will embed a translation table in your binary that perf2bolt uses to build
> the profile data suitable to be consumed in the original binary. This is
> non-standard because it is only really necessary in some large scale
> deployments where collecting the data in a special “no bolt” configuration
> can be inconvenient.
>
>
>
> Best,
>
> Rafael
>
> *From: *llvm-dev <llvm-dev-bounces at lists.llvm.org> on behalf of Venugopal
> Raghavan via llvm-dev <llvm-dev at lists.llvm.org>
> *Reply-To: *Venugopal Raghavan <venur2005 at gmail.com>
> *Date: *Tuesday, April 7, 2020 at 10:53 PM
> *To: *"llvm-dev at lists.llvm.org" <llvm-dev at lists.llvm.org>
> *Subject: *[llvm-dev] Error with perf2bolt in LLVM BOLT
>
>
>
> Hi,
>
>
>
> I was interested in trying out LLVM BOLT and generated profile data using
> Linux perf using the following:
>
>
>
> perf record -e cycles:u -o perf.data <command>
>
>
>
> This is without the use of LBR so I understand the performance
> improvements may not be much but this was more for becoming familiar with
> BOLT's commands.
>
>
>
> I then run:
>
>
>
> perf2bolt -nl -p perf.data -o perf.fdata <binary>
>
>
>
> and I get the following:
>
>
>
> PERF2BOLT: Starting data aggregation job for perf.data
> PERF2BOLT: spawning perf job to read events without LBR
> PERF2BOLT: spawning perf job to read mem events
> PERF2BOLT: spawning perf job to read process events
> PERF2BOLT: spawning perf job to read task events
> BOLT-INFO: Target architecture: x86_64
> *BOLT-ERROR: input file was processed by BOLT. Cannot re-optimize.*
>
>
>
> Not sure why I get the above error. Can someone who has used BOLT help me?
>
>
>
> Thanks.
>
>
>
> Regards,
>
> Venugopal Raghavan.
>
>
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