Re: atomic update of profile counters (issue7000044)

[Rong Xu <xur at google dot com>]

Hi Honza,

In the other thread of discussion (similar patch in google-4_7
branch), you said you were thinking if to let this patch into trunk in
stage 3. Can you give some update?

Thanks,

-Rong

On Fri, Dec 21, 2012 at 10:37 AM, Rong Xu <xur@google.com> wrote:
> On Fri, Dec 21, 2012 at 1:25 AM, Jan Hubicka <hubicka@ucw.cz> wrote:
>>> Hi,
>>>
>>> This patch adds support of atomic update of profiles counters. The goal is to improve
>>> the poor counter values for highly thread programs.
>>>
>>> The atomic update is under a new option -fprofile-gen-atomic=<N>
>>> N=0: default, no atomic update
>>> N=1: atomic update edge counters.
>>> N=2: atomic update some of value profile counters (currently indirect-call and one value profile).
>>> N=3: both edge counter and the above value profile counters.
>>> Other value: fall back to the default.
>>>
>>> This patch is a simple porting of the version in google-4_7 branch. It uses __atomic_fetch_add
>>> based on Andrew Pinski's suggestion. Note I did not apply to all the value profiles as
>>> the indirect-call profile is the most relevant one here.
>>>
>>> Test with bootstrap.
>>>
>>> Comments and suggestions are welcomed.
>>>
>>> Thanks,
>>>
>>> -Rong
>>>
>>>
>>> 2012-12-20  Rong Xu  <xur@google.com>
>>>
>>>       * libgcc/libgcov.c (__gcov_one_value_profiler_body_atomic): New
>>>         function. Atomic update profile counters.
>>>       (__gcov_one_value_profiler_atomic): Ditto.
>>>       (__gcov_indirect_call_profiler_atomic): Ditto.
>>>       * gcc/gcov-io.h: Macros for atomic update.
>>>       * gcc/common.opt: New option.
>>>       * gcc/tree-profile.c (gimple_init_edge_profiler): Atomic
>>>         update profile counters.
>>>       (gimple_gen_edge_profiler): Ditto.
>>
>> The patch looks resonable.  Eventually we probably should provide rest of the value counters
>> in thread safe manner.  What happens on targets not having atomic operations?
>
> From http://gcc.gnu.org/onlinedocs/gcc/_005f_005fsync-Builtins.html#_005f_005fsync-Builtins,
> it says:
>       "If a particular operation cannot be implemented on the target
> processor, a warning is generated and a call an external function is
> generated. "
>
> So I think there will be a warning and eventually a link error of unsat.
>
> Thanks,
>
> -Rong
>
>
>>
>> Honza