RE: [PATCH]middle-end: add two debug counters for early-break vectorization debugging

[Tamar Christina <Tamar.Christina@arm.com>]

> -----Original Message-----
> From: Richard Biener <rguenther@suse.de>
> Sent: Thursday, February 8, 2024 2:16 PM
> To: Tamar Christina <Tamar.Christina@arm.com>
> Cc: gcc-patches@gcc.gnu.org; nd <nd@arm.com>; jlaw@ventanamicro.com
> Subject: Re: [PATCH]middle-end: add two debug counters for early-break
> vectorization debugging
> 
> On Thu, 8 Feb 2024, Tamar Christina wrote:
> 
> > Hi All,
> >
> > This adds two new debug counter to aid in debugging early break code.
> >
> > - vect_force_last_exit: when reached will always force the final loop exit.
> > - vect_skip_exit: when reached will skip selecting the current candidate exit
> > 		  as the loop exit.
> >
> > The first counter essentially allows you to turn off the PEELED case and the
> > second counter to pick a different exit, which may mean you pick no exit at
> > all.
> >
> > Bootstrapped Regtested on aarch64-none-linux-gnu and no issues.
> >
> > Ok for master?
> >
> > Thanks,
> > Tamar
> >
> > gcc/ChangeLog:
> >
> > 	* dbgcnt.def (vect_force_last_exit, vect_skip_exit): New.
> > 	* tree-vect-loop.cc (vec_init_loop_exit_info): Use them.
> >
> > --- inline copy of patch --
> > diff --git a/gcc/dbgcnt.def b/gcc/dbgcnt.def
> > index
> ed9f062eac2c28c52df76b39d4312dd9fde1c800..8f7bebf93fceabdf6ae86c2df5
> 91eae4848b8a5c 100644
> > --- a/gcc/dbgcnt.def
> > +++ b/gcc/dbgcnt.def
> > @@ -213,5 +213,7 @@ DEBUG_COUNTER (stv_conversion)
> >  DEBUG_COUNTER (tail_call)
> >  DEBUG_COUNTER (tree_sra)
> >  DEBUG_COUNTER (treepre_insert)
> > +DEBUG_COUNTER (vect_force_last_exit)
> >  DEBUG_COUNTER (vect_loop)
> > +DEBUG_COUNTER (vect_skip_exit)
> >  DEBUG_COUNTER (vect_slp)
> > diff --git a/gcc/tree-vect-loop.cc b/gcc/tree-vect-loop.cc
> > index
> eed2268e9bae7e7ad36d13da03e0b54eab26ef6f..854e9d78bc71721e6559a6bc
> 5dff78c813603a78 100644
> > --- a/gcc/tree-vect-loop.cc
> > +++ b/gcc/tree-vect-loop.cc
> > @@ -58,6 +58,7 @@ along with GCC; see the file COPYING3.  If not see
> >  #include "tree-eh.h"
> >  #include "case-cfn-macros.h"
> >  #include "langhooks.h"
> > +#include "dbgcnt.h"
> >
> >  /* Loop Vectorization Pass.
> >
> > @@ -977,6 +978,20 @@ vec_init_loop_exit_info (class loop *loop)
> >    if (exits.length () == 1)
> >      return exits[0];
> >
> > +  /* Check to see if we've been asked to force the last exit.  */
> > +  if (!dbg_cnt (vect_force_last_exit))
> > +    {
> > +      basic_block bb = ip_normal_pos (loop);
> > +      if (!bb)
> > +	return NULL;
> > +
> > +      edge exit = EDGE_SUCC (bb, 0);
> > +      if (exit->dest == loop->latch)
> > +	return EDGE_SUCC (bb, 1);
> > +
> > +      return exit;
> 
> Err, that's quite odd.  Why not just below do
> 
> > +    }
> > +
> >    /* If we have multiple exits we only support counting IV at the moment.
> >       Analyze all exits and return the last one we can analyze.  */
> >    class tree_niter_desc niter_desc;
> > @@ -998,6 +1013,7 @@ vec_init_loop_exit_info (class loop *loop)
> >  		   && exit->src == single_pred (loop->latch)
> >  		   && (integer_nonzerop (may_be_zero)
> >  		       || COMPARISON_CLASS_P (may_be_zero))))
> > +	      && dbg_cnt (vect_skip_exit)
> 
>   && (dbg_cnt (vect_force_last_exit)
>       || exit->src == single_pred (loop->latch))
> 
> (also computed above already)?  It's also oddly named, it's more like
> vect_allow_peeled_exit or so.

Because this isn't deterministic. If a loop has n exits the above always forces
you to pick the final one regardless of n, rather than just skip consideration of an exit.

And in that case is there a point in analyzing all the exits just to throw away the information?

Doing in inside the consideration check would only skip one exit unless I'm misunderstanding.

> 
> It's also seemingly redundant with vect_skip_exit, no?
> 
> Note the counter gets incremented even if we'd not consider the exit
> because we have a later candidate already.
> 
> I fear it's going to be quite random even with the debug counter.

It is, I think the first counter is more useful. But in general the reason I kept the second counter
which kinda does what was suggested in the RFC I sent before was that it should in theory at
least allow us to test forcing of a PEELED case. Since we generally prefer the non-PEELED case
if possible.

At least that was the intention.

Thanks,
Tamar

> 
> Can you see whether it really helps you?
> 
> >  	      && (!candidate
> >  		  || dominated_by_p (CDI_DOMINATORS, exit->src,
> >  				     candidate->src)))
> >
> >
> >
> >
> >
> 
> --
> 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)