[Bug rtl-optimization/78200] [7 Regression] 429.mcf of cpu2006 regresses in GCC trunk for avx2 target.

[venkataramanan.kumar at amd dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78200

--- Comment #19 from Venkataramanan <venkataramanan.kumar at amd dot com> ---
(In reply to rguenther@suse.de from comment #18)
> On Tue, 15 Nov 2016, venkataramanan.kumar at amd dot com wrote:
> 
> > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78200
> > 
> > --- Comment #17 from Venkataramanan <venkataramanan.kumar at amd dot com> ---
> > Looking at the check 
> >                red_cost < 0 && arc->ident == AT_LOWER)
> >             || (red_cost > 0 && arc->ident == AT_UPPER
> > 
> > The order if-combine created seem to be the best.
> > 
> > if (red_cost_86 < 0)
> >     goto <bb 17>;
> >   else
> >     goto <bb 18>;
> > 
> >   <bb 17>:
> >   if (_23 == 1)
> >     goto <bb 19>;
> >   else
> >     goto <bb 20>;
> > 
> >   <bb 18>:
> >   _340 = _23 == 2;
> >   _341 = red_cost_86 > 0;
> >   _338 = _340 & _341;
> >   if (_338 != 0)
> >     goto <bb 19>;
> >   else
> >     goto <bb 20>;
> > 
> >   <bb 19>:
> >   basket_size.5_30 = basket_size;
> >   _31 = basket_size.5_30 + 1;
> >   basket_size = _31;
> >   _32 = perm[_31];
> >   _32->a = arc_47;
> >   _32->cost = red_cost_86;
> >   _33 = ABS_EXPR <red_cost_86>;
> >   _32->abs_cost = _33;
> > 
> > If red_cost < 0  is false then checking for arc->ident == AT_UPPER first. This
> > is better, since we know red_cost >0 will always be true.
> 
> red_cost can be zero.  The "bad" order is best (just slower for some
> still unknown reason).

Could be the case 

red_cost is always > zero but arc->ident is not AT_UPPER,

checking like this (arc->ident == AT_UPPER && red_cost > 0)  is faster.

The static predication also says so red_cost > 0 is 88% true.

  if (red_cost_86 < 0)
    goto <bb 17>;
  else
    goto <bb 18>;
;;    succ:       17 [36.0%]  (TRUE_VALUE,EXECUTABLE)
;;                18 [64.0%]  (FALSE_VALUE,EXECUTABLE)

;;   basic block 17, loop depth 2, count 0, freq 684, maybe hot
;;    prev block 16, next block 18, flags: (NEW, REACHABLE, VISITED)
;;    pred:       16 [36.0%]  (TRUE_VALUE,EXECUTABLE)
  if (_23 == 1)
    goto <bb 20>;
  else
    goto <bb 21>;
;;    succ:       20 [34.0%]  (TRUE_VALUE,EXECUTABLE)
;;                21 [66.0%]  (FALSE_VALUE,EXECUTABLE)

;;   basic block 18, loop depth 2, count 0, freq 1217, maybe hot
;;    prev block 17, next block 19, flags: (NEW, REACHABLE, VISITED)
;;    pred:       16 [64.0%]  (FALSE_VALUE,EXECUTABLE)
  if (red_cost_86 > 0)
    goto <bb 19>;
  else
    goto <bb 21>;
;;    succ:       19 [87.7%]  (TRUE_VALUE,EXECUTABLE) <== ~88% 
;;                21 [12.3%]  (FALSE_VALUE,EXECUTABLE)

;;   basic block 19, loop depth 2, count 0, freq 1067, maybe hot
;;    prev block 18, next block 20, flags: (NEW, REACHABLE, VISITED)
;;    pred:       18 [87.7%]  (TRUE_VALUE,EXECUTABLE)
  if (_23 == 2)
    goto <bb 20>;
  else
    goto <bb 21>;
;;    succ:       20 [34.0%]  (TRUE_VALUE,EXECUTABLE)
;;                21 [66.0%]  (FALSE_VALUE,EXECUTABLE)