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

[Jeff Law <law at redhat dot com>]

On 10/14/2015 09:43 AM, Jan Hubicka wrote:
> > > > > 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.
Yea.  I still  see a variety of trivial jump threads lying around early 
in the pipeline.

The nice thing about the backwards walking stuff in this context is we 
can control how hard it looks for jump threads much better.

The difficult thing is it's not currently prepared to find the implicit 
sets from conditionals.  Re-using the ASSERT_EXPR mechanisms from vrp 
may be the solution.  I haven't tried that yet, but it's in the back of 
my mind for solving that class of problems cleanly.



jeff