atomic update of profile counters (issue7000044)

Andrew Pinski pinskia@gmail.com
Thu Jan 3 01:25:00 GMT 2013


On Wed, Jan 2, 2013 at 5:15 PM, Rong Xu <xur@google.com> wrote:
> Hi,
>
> Here is a new patch. The only difference is to declare
> __atomic_fetch_add as weak. This is
> needed for targets without sync/atomic builtin support. The patch
> contains a call to the builtin regardless of the new options
> -fprofile-gen-atomic. This results in a unsat in these targets even
> for regular profile-gen built.
>
> With this new patch, if the user uses -fprofile-gen-atomic in these
> target, the generated code will seg fault.
>
> We think a better solution is to emit the builtin call only in these
> targets with the support, and give warning for non-supported target.
> But I did not find any target hook for this. Does anyone know how to
> do this?

Why not use libatomic for those targets?

Thanks,
Andrew Pinski



>
> Thanks,
>
> -Rong
>
>
> On Fri, Dec 28, 2012 at 11:35 AM, Xinliang David Li <davidxl@google.com> wrote:
>> It would be great if this can make into gcc4.8. The patch has close to
>> 0 impact on code stability.
>>
>> David
>>
>> On Fri, Dec 28, 2012 at 11:32 AM, Rong Xu <xur@google.com> wrote:
>>> 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



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