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Re: atomic update of profile counters (issue7000044)
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