Re: [PATCH] rs6000 branch probability changes

[David Edelsohn <dje dot gcc at gmail dot com>]

On Sat, Jul 1, 2017 at 9:06 AM, Jan Hubicka <hubicka@ucw.cz> wrote:
>> >> > * config/rs6000/rs6000.c (emit_unlikely_jump): Adjust to new branch
>> >> > probability data type.
>> >> >
>> >> > Index: rs6000.c
>> >> > ===================================================================
>> >> > --- rs6000.c (revision 249839)
>> >> > +++ rs6000.c (working copy)
>> >> > @@ -23514,10 +23514,9 @@
>> >> >  static void
>> >> >  emit_unlikely_jump (rtx cond, rtx label)
>> >> >  {
>> >> > -  int very_unlikely = REG_BR_PROB_BASE / 100 - 1;
>> >> >    rtx x = gen_rtx_IF_THEN_ELSE (VOIDmode, cond, label, pc_rtx);
>> >> >    rtx_insn *insn = emit_jump_insn (gen_rtx_SET (pc_rtx, x));
>> >> > -  add_int_reg_note (insn, REG_BR_PROB, very_unlikely);
>> >> > +  add_int_reg_note (insn, REG_BR_PROB, profile_probability::very_unlikely ());
>> >>
>> >> Hmmm isn't this very unlikely to work :) ?
>> >>
>> >> I used this as inspiration to do this for the arm ports but
>> >> add_int_reg_note expects an integer but very_unlikely returns
>> >> profile_probability  ...
>> >
>> > It probably should be converted using to_reg_br_prob_base ?
>>
>> The comments in profile-count.h state that this should go away.
>>
>> We need advice from Honza about the preferred way to transform these idioms.
>
> I plan to change REG_BR_PROB notes to preserve all information from
> profile_probability (this is needed to make RTL expansion splitting work as
> expected), but for now they are still just REG_BR_PROB_BASE fixpoint.
>
> I think the code can stay as it is.  I will add APIs for emitting/interpretting
> br_prob_nodes as followup (after debugging fixing issues with profile updating
> which I can now detect with the new type)
>
> Thanks for looking into this.

Does the computed value of very_unlikely need to change for the new
scale?  Can the profile machinery provide a helper function or macro
instead of the current calculation replicated in many ports?

Thanks, David