Re: [PATCH] decide edge's hotness when there is profile info

[Jan Hubicka <hubicka at ucw dot cz>]

> 2013-11-08  Teresa Johnson  <tejohnson@google.com>
>             Jan Hubicka  <jh@suse.cz>
> 
>         * predict.c (drop_profile): New function.
>         (handle_missing_profiles): Ditto.
>         (counts_to_freqs): Don't overwrite estimated frequencies
>         when function has no profile counts.
>         * predict.h (handle_missing_profiles): Declare.
>         * tree-inline.c (freqs_to_counts): New function.
>         (copy_cfg_body): Invoke freqs_to_counts as needed.
>         * tree-profile.c (tree_profiling): Invoke handle_missing_profiles.
> 
> Index: predict.c
> ===================================================================
> --- predict.c   (revision 204516)
> +++ predict.c   (working copy)
> @@ -2765,6 +2765,107 @@ estimate_loops (void)
>    BITMAP_FREE (tovisit);
>  }
> 
> +/* Drop the profile for NODE to guessed, and update its frequency based on
> +   whether it is expected to be HOT.  */
> +
> +static void
> +drop_profile (struct cgraph_node *node, bool hot)
> +{
> +  struct function *fn = DECL_STRUCT_FUNCTION (node->decl);
> +
> +  if (dump_file)
> +    fprintf (dump_file,
> +             "Dropping 0 profile for %s/%i. %s based on calls.\n",
> +             cgraph_node_name (node), node->order,
> +             hot ? "Function is hot" : "Function is normal");
> +  /* We only expect to miss profiles for functions that are reached
> +     via non-zero call edges in cases where the function may have
> +     been linked from another module or library (COMDATs and extern
> +     templates). See the comments below for handle_missing_profiles.  */
> +  if (!DECL_COMDAT (node->decl) && !DECL_EXTERNAL (node->decl))
> +    warning (0,
> +             "Missing counts for called function %s/%i",
> +             cgraph_node_name (node), node->order);
> +
> +  profile_status_for_function (fn)
> +      = (flag_guess_branch_prob ? PROFILE_GUESSED : PROFILE_ABSENT);
> +  node->frequency
> +      = hot ? NODE_FREQUENCY_HOT : NODE_FREQUENCY_NORMAL;
> +}
> +
> +/* In the case of COMDAT routines, multiple object files will contain the same
> +   function and the linker will select one for the binary. In that case
> +   all the other copies from the profile instrument binary will be missing
> +   profile counts. Look for cases where this happened, due to non-zero
> +   call counts going to 0-count functions, and drop the profile to guessed
> +   so that we can use the estimated probabilities and avoid optimizing only
> +   for size.
> +
> +   The other case where the profile may be missing is when the routine
> +   is not going to be emitted to the object file, e.g. for "extern template"
> +   class methods. Those will be marked DECL_EXTERNAL. Emit a warning in
> +   all other cases of non-zero calls to 0-count functions.  */
> +
> +void
> +handle_missing_profiles (void)
> +{
> +  struct cgraph_node *node;
> +  int unlikely_count_fraction = PARAM_VALUE (UNLIKELY_BB_COUNT_FRACTION);
> +  vec<struct cgraph_node *> worklist;
> +  worklist.create (64);
> +
> +  /* See if 0 count function has non-0 count callers.  In this case we
> +     lost some profile.  Drop its function profile to PROFILE_GUESSED.  */
> +  FOR_EACH_DEFINED_FUNCTION (node)
> +    {
> +      struct cgraph_edge *e;
> +      gcov_type call_count = 0;
> +      struct function *fn = DECL_STRUCT_FUNCTION (node->decl);
> +
> +      if (node->count)
> +        continue;
> +      for (e = node->callers; e; e = e->next_caller)
> +        call_count += e->count;
> +      if (call_count
> +          && fn && fn->cfg
> +          && (call_count * unlikely_count_fraction >= profile_info->runs))
> +        {
> +          bool maybe_hot = maybe_hot_count_p (NULL, call_count);
> +
> +          drop_profile (node, maybe_hot);
> +          worklist.safe_push (node);
> +        }

Perhaps we should add an note/error on mishandled profile when function is not COMDAT?
That way we may notice further bugs in this area.
> +    }
> +
> +  /* Propagate the profile dropping to other 0-count COMDATs that are
> +     potentially called by COMDATs we already dropped the profile on.  */
> +  while (worklist.length () > 0)
> +    {
> +      struct cgraph_edge *e;
> +
> +      node = worklist.pop ();
> +      for (e = node->callees; e; e = e->next_caller)
> +        {
> +          struct cgraph_node *callee = e->callee;
> +          struct function *fn = DECL_STRUCT_FUNCTION (callee->decl);
> +
> +          if (callee->count > 0)
> +            continue;
> +          if (DECL_COMDAT (callee->decl) && fn && fn->cfg
> +              && profile_status_for_function (fn) == PROFILE_READ)

Perhaps we can check here maybe_hot_bb_p for the caller and rely on profile estimate
to give us false only for really known to be unlikely paths? (i.e. EH handling, noreturns
etc.)
> +            {
> +              /* Since there are no non-0 call counts to this function,
> +                 we don't know for sure whether it is hot. Indicate to
> +                 the drop_profile routine that function should be marked
> +                 normal, rather than hot.  */
> +              drop_profile (node, false);
> +              worklist.safe_push (callee);
> +            }
> +        }
> +    }
> +  worklist.release ();

I would add a pointer set so we avoid duplicates in worklist.  This is potentially quadratic
for large programs.

OK, with these changes.

Honza