Re: [PATCH] Allow FSM to thread single block cases too

[Jan Hubicka <hubicka at ucw dot cz>]

> >>> I think he asked for trivial forward threads though due to repeated
> >>> tests.
> >>> I hacked FRE to do this (I think), but maybe some trivial cleanup
> >>> opportunities
> >>> are still left here.  Honza?

Well, unthreaded jumps quite confuse profile prediction and create profiles
that we can't fix later. An of course they count in time (and size sometimes)
estimates.

>From cases I commonly see it is the usual lazyness of repeated tests comming
from early inlining/macro expansion and also C++ love to introduce

  if (ptr != NULL)
    ptr2 = &ptr->foo;
  else
    ptr2 = NULL

for instances of multiple inheritance. usually ptr is known to be non-NULL.
And also cases where if is uses to check individual cases without having proper
esles.

Honza
> >>
> >>
> >> This or other related patches in the range r228731:228774 has caused a
> >> quite
> >> big jump in SPEC CPU 2000 binary sizes (notably 176.gcc - so maybe
> >> affecting
> >> bootstrap as well, at -O3).  Are you sure this doesn't re-introduce DOM
> >> effectively peeling all loops once?
> >
> > It's possible.  I've actually got a patch in overnight testing that
> > introduces some of the heuristics to avoid mucking up loops to the FSM bits.
> 
> Like never threading a loop exit test to the loop header (but only to the exit).
> At least if it is the only exit in the loop (but maybe better for all exits).
> 
> Richard.
> 
> > jeff
> >