Re: atomic update of profile counters (issue7000044)

[Rong Xu <xur@google.com>]

Does libatomic support all targets?
I think it's a good idea to change the driver to link in this library
if the option is specified.
But still, we need to make the builtin weak.

Thanks,

-Rong

On Wed, Jan 2, 2013 at 5:25 PM, Andrew Pinski <pinskia@gmail.com> wrote:
> 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