Re: [PATCH 3/3] tree-optimization/114052 - niter analysis from undefined behavior

[Richard Biener <rguenther@suse.de>]

On Mon, 8 Apr 2024, Richard Biener wrote:

> On Fri, 5 Apr 2024, Jan Hubicka wrote:
> 
> > > +	  /* When there's a call that might not return the last iteration
> > > +	     is possibly partial.  This matches what we check in invariant
> > > +	     motion.
> > > +	     ???  For the call argument evaluation it would be still OK.  */
> > > +	  if (!may_have_exited
> > > +	      && is_gimple_call (stmt)
> > > +	      && gimple_has_side_effects (stmt))
> > > +	    may_have_exited = true;
> > 
> > I think you are missing here non-call EH, volatile asms and traps.
> >  We have stmt_may_terminate_function_p which tests there.
> 
> That returns true for all variable array accesses, I think we want
> to catch traps explicitly here.  I'm going to do
> 
>           if (!may_have_exited
>               && (gimple_has_side_effects (stmt)
>                   || stmt_can_throw_external (cfun, stmt)))
>             may_have_exited = true;
> 
> that should cover all but the generic trapping and not use IPA info
> to prove no side-effects.

Hum.  Maybe I'm a bit confused but we seem to "properly" take things
into account via maybe_lower_iteration_bound and are not directly using
the recorded bounds?  The function does a DFS walk though, not
reliably finding exits via calls early enough (it also lacks external
EH).  Oddly enough it seems to handle(?) gcc.dg/tree-ssa/cunroll-9.c
"correctly", but not gcc.dg/tree-ssa/cunroll-10.c which has the
number of iterations wrongly computed.

Maybe we should really record all the bounds properly but merge them
to a loop upper bound at one place?  gcc.dg/tree-ssa/cunroll-11.c
needs to see the g_x[i] bound is enforced in all paths to the latch
for example.

I'm most definitely defering this to GCC 15 now, I wonder if you
have any preferences here (and maybe want to pick this up also
for cleanup - it's mostly your code).

Richard.

> Richard.
> 
> > Honza
> > > +
> > > +	  infer_loop_bounds_from_array (loop, stmt,
> > > +					reliable && !may_have_exited);
> > >  
> > > -	  if (reliable)
> > > +	  if (reliable && !may_have_exited)
> > >              {
> > >                infer_loop_bounds_from_signedness (loop, stmt);
> > >                infer_loop_bounds_from_pointer_arith (loop, stmt);
> > >              }
> > >    	}
> > > -
> > >      }
> > >  }
> > >  
> > > @@ -4832,7 +4855,7 @@ estimate_numbers_of_iterations (class loop *loop)
> > >       diagnose those loops with -Waggressive-loop-optimizations.  */
> > >    number_of_latch_executions (loop);
> > >  
> > > -  basic_block *body = get_loop_body (loop);
> > > +  basic_block *body = get_loop_body_in_rpo (cfun, loop);
> > >    auto_vec<edge> exits = get_loop_exit_edges (loop, body);
> > >    likely_exit = single_likely_exit (loop, exits);
> > >    FOR_EACH_VEC_ELT (exits, i, ex)
> > > -- 
> > > 2.35.3
> > 
> 
> 

-- 
Richard Biener <rguenther@suse.de>
SUSE Software Solutions Germany GmbH,
Frankenstrasse 146, 90461 Nuernberg, Germany;
GF: Ivo Totev, Andrew McDonald, Werner Knoblich; (HRB 36809, AG Nuernberg)