AutoFDO profile toolchain is open-sourced

[Ilya Palachev]

On 21.04.2015 14:57, Diego Novillo wrote:
> >From the autofdo page: https://github.com/google/autofdo
>
> [ ... ]
> Inputs:
>
> --profile: PERF_PROFILE collected using linux perf (with last branch record).
> In order to collect this profile, you will need to have an Intel CPU that
> have last branch record (LBR) support. You also need to have your linux
> kernel configured with LBR support. To profile:
> # perf record -c PERIOD -e EVENT -b -o perf.data -- ./command
> EVENT is refering to BR_INST_RETIRED:TAKEN if available. For some
> architectures, BR_INST_EXEC:TAKEN also works.
> [ ... ]
>
> The important one for autofdo is -b. It asks perf to use LBR registers
> for branch tracking (assuming your architecture supports it).

Thanks! It worked. Now big programs produce big gcov files. Sorry for 
this confusing message.

But why create_gcov does not inform about that (no branch events were 
found)? It creates empty gcov file and says nothing :(

Moreover, in the mentioned README it is said that perf should also be 
executed with option -e BR_INST_RETIRED:TAKEN.
I tried to add it but perf said that

    invalid or unsupported event: 'BR_INST_RETIRED:TAKEN'
    Run 'perf list' for a list of valid events

For my architecture x86_64 the perf list contains

    $ sudo perf list | grep -i br
       branch-instructions OR branches                    [Hardware event]
       branch-misses                                      [Hardware event]
       branch-loads                                       [Hardware
    cache event]
       branch-load-misses                                 [Hardware
    cache event]
       branch-instructions OR cpu/branch-instructions/    [Kernel PMU event]
       branch-misses OR cpu/branch-misses/                [Kernel PMU event]
       mem:<addr>[:access] [Hardware breakpoint]
       syscalls:sys_enter_brk                             [Tracepoint event]
       syscalls:sys_exit_brk                              [Tracepoint event]

There is no BR_INST_RETIRED:TAKEN there. Do you use some specific 
configuration of perf for that?

However, I tried to use option "-e branch-instructions". Before that the 
following error was obtained:

    E0421 15:57:39.308374 11551 perf_parser.cc:210] Mapped 50% of
    samples, expected at least 95%

and now it disappeared (because of option "-e branch-instructions").

Though, the performance decreases after adding option 
"-fauto-profile=file.gcov" or "-fprofile-use=file.gcov" to the list of 
compiler options.
The program becomes 10% slower than before.
Can you explain that? Maybe I should configure perf so that it will be 
able to collect events BR_INST_RETIRED:TAKEN ? How can it be done?

--
Best regards,
Ilya Palachev