Re: [PATCH 8/9] shrink-wrap: shrink-wrapping for separate concerns

[Bernd Schmidt <bschmidt at redhat dot com>]

On 07/18/2016 06:34 PM, Segher Boessenkool wrote:
> > > +  /* The frequency of executing the prologue for this BB and all BBs
> > > +     dominated by it.  */
> > > +  gcov_type cost;
> >
> > Is this frequency consideration the only thing that attempts to prevent
> > placing prologue insns into loops?
>
> Yes.  The algorithm makes sure the prologues are executed as infrequently
> as possible.  If a block that would get a prologue has the same frequency
> as a predecessor does, and that predecessor always has that first block as
> eventual successor, the prologue is moved to the earlier block (this
> handles the case where both have a frequency of zero, and other cases
> where the range of freq is too limited).

Ugh, that is really scaring me. I'd much prefer a classification of 
valid blocks based on cfg structure alone - I'll need serious convincing 
that the frequency data is reliable enough for what you are trying to do.


Bernd