Re: Reorgnanization of profile count maintenance code, part 1

[Jan Hubicka <hubicka@ucw.cz>]

> On Tue, Jun 06, 2017 at 10:25:29PM +0200, Jan Hubicka wrote:
> > > On Thu, Jun 01, 2017 at 01:35:56PM +0200, Jan Hubicka wrote:
> > > > +      /* FIXME: shrink wrapping violates this sanity check.  */
> > > > +      gcc_checking_assert ((num >= 0
> > > > +			    && (num <= REG_BR_PROB_BASE
> > > > +			        || den <= REG_BR_PROB_BASE)
> > > > +			    && den > 0) || 1);
> > > > +      ret.m_val = RDIV (m_val * num, den);
> > > > +      return ret;
> > > 
> > > Sorry if I missed this...  But where/how does it violate this?
> > 
> > It sums multiple probabilties together and overflows the limit.
> 
> Ah.  Yes, the scale it uses (num/den in shrink-wrap.c:924) isn't a
> probability: it's a just a fraction, but also <= 1.  The calculation cannot
> overflow, not while there are at most 4G incoming edges to a BB.

Problem is that when you multiply count by it, the buffer is considerably
smaller, because count itself can be quite large number...

It is not too bad - I added the sanity check mostly to see if there
is a safe cap on num so I do not need to worry about overflows at all.

Honza
> 
> 
> Segher