Can you please attach WIP patch you have?

Created attachment 45703 [details]
patch for tls counters (incomplete - no runtime bits)

Also I think google's code to reduce cacheline conflicts is https://gcc.gnu.org/ml/gcc-patches/2012-05/msg00959.html

(In reply to Jan Hubicka from comment #2)
> Created attachment 45703 [details]
> patch for tls counters (incomplete - no runtime bits)

Isn't the patch only a refactoring that is eliminating tls_model from tree_decl_with_vis and moving that into cgraph_node?

I'm just looking at the google/gcc-4.9 branch:
https://android.googlesource.com/toolchain/gcc/+/master/gcc-4.9/

and they have a sampling approach:

/* Transform:

   ORIGINAL CODE

   Into:

   __gcov_sample_counter++;
   if (__gcov_sample_counter >= __gcov_sampling_period)
     {
       __gcov_sample_counter = 0;
       ORIGINAL CODE
     }

which effectively updates edge counters just for a limited time. I would expect
size increase:

Removing basic block 9
Removing basic block 10
main (int argc)
{
  unsigned int PROF_sample.2;
  unsigned int PROF_sample.1;
  long int PROF_edge_counter_6;
  long int PROF_edge_counter_7;
  long int PROF_edge_counter_8;
  long int PROF_edge_counter_9;

  <bb 2>:
  __gcov_indirect_call_profiler_v2 (1005944783, main);
  __gcov_indirect_call_callee = 0B;
  if (argc_2(D) != 0)
    goto <bb 3>;
  else
    goto <bb 6>;

  <bb 3>:
  a = 123;
  PROF_sample.2_13 = __gcov_sample_counter;
  PROF_sample.2_14 = PROF_sample.2_13 + 1;
  __gcov_sample_counter = PROF_sample.2_14;
  PROF_sample.2_15 = __gcov_sampling_period;
  if (PROF_sample.2_14 >= PROF_sample.2_15)
    goto <bb 5>;
  else
    goto <bb 4>;

  <bb 4>:
  goto <bb 8>;

  <bb 5>:
  __gcov_sample_counter = 0;
  PROF_edge_counter_6 = __gcov0.main[0];
  PROF_edge_counter_7 = PROF_edge_counter_6 + 1;
  __gcov0.main[0] = PROF_edge_counter_7;
  goto <bb 8>;

  <bb 6>:
  a = 0;
  PROF_sample.1_10 = __gcov_sample_counter;
  PROF_sample.1_11 = PROF_sample.1_10 + 1;
  __gcov_sample_counter = PROF_sample.1_11;
  PROF_sample.1_12 = __gcov_sampling_period;
  if (PROF_sample.1_11 >= PROF_sample.1_12)
    goto <bb 7>;
  else
    goto <bb 4>;

  <bb 7>:
  __gcov_sample_counter = 0;
  PROF_edge_counter_8 = __gcov0.main[1];
  PROF_edge_counter_9 = PROF_edge_counter_8 + 1;
  __gcov0.main[1] = PROF_edge_counter_9;

  <bb 8>:
  return 0;
}

> 
> which effectively updates edge counters just for a limited time. I would
> expect

Ah now, it's really doing sampling. I guess it can lead to quite some profile inconsistencies..

> Ah now, it's really doing sampling. I guess it can lead to quite some profile
> inconsistencies..
Yep, it is not coolest solution. I would not worry too much about
precision loss unless you get some weird interference between the
sampling counter and actual program behaviour.  Adding conditionals
everywhere is not very good and I am not sure how well CPU will predict
such branches.

Honza

Btw, use of TLS has

 * size of counters overhead (one could use char sized TLS counters and
   update the global ones with locking on overflow)
 * tear-down/build-up cost at thread termination/creation

the advantage is of course it's simple implementation-wise.

GCC 10.1 has been released.

GCC 10.2 is released, adjusting target milestone.

GCC 10.3 is being released, retargeting bugs to GCC 10.4.

GCC 10.4 is being released, retargeting bugs to GCC 10.5.