Re: atomic update of profile counters (issue7000044)

[Jan Hubicka <hubicka@ucw.cz>]

> 2013-11-19  Rong Xu  <xur@google.com>
> 
> 	* gcc/gcov-io.h: Add atomic function macros for compiler use.
> 	* gcc/common.opt (fprofile-generate-atomic): New option.
> 	* gcc/tree-profile.c (gimple_init_edge_profiler): Support for
>         atomic counter update.
> 	(gimple_gen_edge_profiler): Ditto.
> 	* libgcc/libgcov-profiler.c 
> 	(__gcov_interval_profiler_atomic): Ditto.
> 	(__gcov_pow2_profiler_atomic): Ditto.
> 	(__gcov_one_value_profiler_body_atomic): Ditto.
> 	(__gcov_one_value_profiler_atomic): Ditto.
> 	(__gcov_indirect_call_profiler_atomic): Ditto.
> 	(__gcov_indirect_call_profiler_v2_atomic): Ditto.
> 	(__gcov_time_profiler_atomic): Ditto.
> 	(__gcov_average_profiler_atomic): Ditto.
> 	* gcc/gcc.c: Link libatomic when -fprofile-generate-atomic used.
> 	* libgcc/Makefile.in: Add atomic objects.
> 
> Index: gcc/common.opt
> ===================================================================
> --- gcc/common.opt	(revision 205053)
> +++ gcc/common.opt	(working copy)
> @@ -1684,6 +1684,15 @@ fprofile-correction
>  Common Report Var(flag_profile_correction)
>  Enable correction of flow inconsistent profile data input
>  
> +; fprofile-generate-atomic=0: default and disable atomical update.
> +; fprofile-generate-atomic=1: atomically update edge profile counters.
> +; fprofile-generate-atomic=2: atomically update value profile counters.
> +; fprofile-generate-atomic=3: atomically update edge and value profile counters.
> +; other values will be ignored (fall back to the default of 0).
> +fprofile-generate-atomic=
> +Common Joined UInteger Report Var(flag_profile_generate_atomic) Init(3) Optimization
> +fprofile-generate-atomic=[0..3] Atomical increments of profile counters.

Instead magic numbers I would preffer flags, like "edge" and "value"
and permiting combinations like -ffprofile-generate-atomic=edge,value

I wonder about the option name, I suppose this option also combine with -ftest-coverage
(and no longer too useful -fprofile-arcs), so the profile-generate prefix may be bit
misleading here (giving an impression that it is not useful in this case).
But I can't think of something really better.

Other thing I wonder about is that we may want to implement alternative solution with
TLS or smaller per-thread buffers and locked updates.  It would be bit difficult to 
extend -fprofile-generate-atomic to this...

> @@ -155,6 +155,9 @@ gimple_init_edge_profiler (void)
>    tree ic_profiler_fn_type;
>    tree average_profiler_fn_type;
>    tree time_profiler_fn_type;
> +  const char *fn_name;
> +  bool profile_gen_value_atomic = (flag_profile_generate_atomic == 2 ||
> +                                   flag_profile_generate_atomic == 3);
>  
>    if (!gcov_type_node)
>      {

Will this work with optimization attributes?

The rest of patch looks OK, lets settle option name and get it in.
Sorry for dropping the ball for 4.8.