Re: [PATCH] PR/7344, O(n^2) branch prediction

[Zdenek Dvorak <rakdver at atrey dot karlin dot mff dot cuni dot cz>]

> > > > > I tried to make bootstrap, but it didn't finish due to unrelated issues.
> > > > > Would appreciate if you could run it.
> > > > 
> > > > OK, it's in progress.  Results in a few hours.
> > > > 
> > > > Brad
> > > > 
> > > 
> > > Also, it would be good if someone more familiar with branch prediction
> > > checked the patch, and make sure I'm testing for the right condition...
> > 
> > !   for (e = BASIC_BLOCK (y)->succ, count = 0; e; e = e->succ_next)
> > !     {
> > !       if (++count >= 2)
> > !         break;
> > !       if (e->dest->index >= 0
> > !           && dominated_by_p (post_dominators, e->dest, bb))
> > !         predict_edge_def (e, pred, taken);
> > !     }
> > 
> > I believe there should be "if (count++ >= 2)"; the way it is here it
> > will break too soon.
> 
> No it won't, it'll break if the branch is multiway. Multiway is  > 1, not 
> > 2.
> 
> In fact, the whole loop is pointless, since it will only execute once. You 
> might as well just say
> if (BASIC_BLOCK (y)->succ)
> {
> 	...
> }	

But this is wrong -- we want it to work for ordinary conditional jumps,
i.e. we really want to break only if >2; alternatively
we could test for anycondjump_p.

Zdenek