Re: [PATCH] rs6000 branch probability changes

[Jan Hubicka <hubicka@ucw.cz>]

> >> > * 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.

Honza
> 
> Thanks, David