[PATCH]middle-end Add an RPO pass after successful vectorization

[Tamar Christina]

> -----Original Message-----
> From: Richard Biener <rguenther@suse.de>
> Sent: Tuesday, November 2, 2021 2:24 PM
> To: Tamar Christina <Tamar.Christina@arm.com>
> Cc: gcc-patches@gcc.gnu.org; nd <nd@arm.com>
> Subject: Re: [PATCH]middle-end Add an RPO pass after successful
> vectorization
> 
> On Tue, 2 Nov 2021, Tamar Christina wrote:
> 
> > Hi All,
> >
> > Following my current SVE predicate optimization series a problem has
> > presented itself in that the way vector masks are generated for masked
> > operations relies on CSE to share masks efficiently.
> >
> > The issue however is that masking is done using the & operand and & is
> > associative and so reassoc decides to reassociate the masked operations.
> 
> But it does this for the purpose of canonicalization and thus CSE.

Yes, but it turns something like

(a & b) & mask into a & (b & mask).

When (a & b) is used somewhere else you now lose the CSE.  So it's actually hurting
In this case.

> 
> > This makes CSE then unable to CSE an unmasked and a masked operation
> > leading to duplicate operations being performed.
> >
> > To counter this we want to add an RPO pass over the vectorized loop
> > body when vectorization succeeds.  This makes it then no longer
> > reliant on the RTL level CSE.
> >
> > I have not added a testcase for this as it requires the changes in my
> > patch series, however the entire series relies on this patch to work
> > so all the tests there cover it.
> >
> > Bootstrapped Regtested on aarch64-none-linux-gnu, x86_64-linux-gnu and
> > no issues.
> >
> > Ok for master?
> 
> You are running VN over _all_ loop bodies rather only those vectorized.
> We loop over vectorized loops earlier for optimizing masked store sequences.
> I suppose you could hook in there.  I'll also notice that we have
> pass_pre_slp_scalar_cleanup which eventually runs plus we have a late FRE.
> So I don't understand why it doesn't work to CSE later.
> 

Atm, say you have the conditions a > b, and a > b & a > c

We generate

mask1 = (a > b) & loop_mask
mask2 = (a > b & a > c) & loop_mask

with the intention that mask1 can be re-used in mask2.

Reassoc changes this to mask2 = a > b & (a > c & loop_mask)

Which has now unmasked (a > b) in mask2, which leaves us unable to combine
the mask1 and mask2.  It doesn't generate incorrect code, just inefficient.

>   for (i = 1; i < number_of_loops (cfun); i++)
>     {
>       loop_vec_info loop_vinfo;
>       bool has_mask_store;
> 
>       loop = get_loop (cfun, i);
>       if (!loop || !loop->aux)
>         continue;
>       loop_vinfo = (loop_vec_info) loop->aux;
>       has_mask_store = LOOP_VINFO_HAS_MASK_STORE (loop_vinfo);
>       delete loop_vinfo;
>       if (has_mask_store
>           && targetm.vectorize.empty_mask_is_expensive (IFN_MASK_STORE))
>         optimize_mask_stores (loop);
>       loop->aux = NULL;
>     }
> 

Ah thanks, I'll make the changes.

Thanks,
Tamar

> 
> > Thanks,
> > Tamar
> >
> > gcc/ChangeLog:
> >
> > 	* tree-vectorizer.c (vectorize_loops): Do local CSE through RPVN
> upon
> > 	successful vectorization.
> >
> > --- inline copy of patch --
> > diff --git a/gcc/tree-vectorizer.c b/gcc/tree-vectorizer.c index
> >
> 4712dc6e7f907637774482a71036a0bd381c2bd2..1e370d60fb19b03c3b6bce45c
> 660
> > af4b6d32dc51 100644
> > --- a/gcc/tree-vectorizer.c
> > +++ b/gcc/tree-vectorizer.c
> > @@ -81,7 +81,7 @@ along with GCC; see the file COPYING3.  If not see
> > #include "gimple-pretty-print.h"
> >  #include "opt-problem.h"
> >  #include "internal-fn.h"
> > -
> > +#include "tree-ssa-sccvn.h"
> >
> >  /* Loop or bb location, with hotness information.  */
> > dump_user_location_t vect_location; @@ -1323,6 +1323,27 @@
> > vectorize_loops (void)
> >  	 ???  Also while we try hard to update loop-closed SSA form we fail
> >  	 to properly do this in some corner-cases (see PR56286).  */
> >        rewrite_into_loop_closed_ssa (NULL,
> > TODO_update_ssa_only_virtuals);
> > +
> > +      for (i = 1; i < number_of_loops (cfun); i++)
> > +	{
> > +	  loop = get_loop (cfun, i);
> > +	  if (!loop || !single_exit (loop))
> > +	    continue;
> > +
> > +	  bitmap exit_bbs;
> > +	  /* Perform local CSE, this esp. helps because we emit code for
> > +	     predicates that need to be shared for optimal predicate usage.
> > +	     However reassoc will re-order them and prevent CSE from working
> > +	     as it should.  CSE only the loop body, not the entry.  */
> > +	  exit_bbs = BITMAP_ALLOC (NULL);
> > +	  bitmap_set_bit (exit_bbs, single_exit (loop)->dest->index);
> > +	  bitmap_set_bit (exit_bbs, loop->latch->index);
> > +
> > +	  do_rpo_vn (cfun, loop_preheader_edge (loop), exit_bbs);
> > +
> > +	  BITMAP_FREE (exit_bbs);
> > +	}
> > +
> >        return TODO_cleanup_cfg;
> >      }
> >
> >
> >
> >
> 
> --
> Richard Biener <rguenther@suse.de>
> SUSE Software Solutions Germany GmbH, Maxfeldstrasse 5, 90409
> Nuernberg, Germany; GF: Ivo Totev; HRB 36809 (AG Nuernberg)