> -----Original Message-----
> From: Richard Biener <rguent...@suse.de>
> Sent: Thursday, July 13, 2023 12:49 PM
> To: Tamar Christina <tamar.christ...@arm.com>
> Cc: gcc-patches@gcc.gnu.org; nd <n...@arm.com>; j...@ventanamicro.com
> Subject: Re: [PATCH 8/19]middle-end: updated niters analysis to handle
> multiple exits.
>
> On Wed, 28 Jun 2023, Tamar Christina wrote:
>
> > Hi All,
> >
> > For early break vectorization we have to update niters analysis to
> > record and analyze all exits of the loop, and so all conds.
> >
> > The niters of the loop is still determined by the main/natural exit of
> > the loop as this is the O(n) bounds. For now we don't do much with
> > the secondary conds, but their assumptions can be used to generate
> versioning checks later.
> >
> > Bootstrapped Regtested on aarch64-none-linux-gnu and no issues.
> >
> > Ok for master?
>
> I probably confused vec_init_exit_info in the previous patch - that said, I'm
> missing a clear function that determines the natural exit of the original (if-
> converted) scalar loop. As vec_init_exit_info seems to (re-)compute that I'll
> comment on it here.
Ah was wondering if you'd seen it 😊
>
> + /* The main IV is to be determined by the block that's the first
> reachable
> + block from the latch. We cannot rely on the order the loop analysis
> + returns and we don't have any SCEV analysis on the loop. */
> + auto_vec <edge> workset; workset.safe_push (loop_latch_edge (loop));
> + hash_set <edge> visited;
> +
> + while (!workset.is_empty ())
> + {
> + edge e = workset.pop ();
> + if (visited.contains (e))
> + continue;
> +
> + bool found_p = false;
> + for (edge ex : e->src->succs)
> + {
> + if (exits.contains (ex))
> + {
> + found_p = true;
> + e = ex;
> + break;
> + }
> + }
> +
> + if (found_p)
> + {
> + loop->vec_loop_iv = e;
> + for (edge ex : exits)
> + if (e != ex)
> + loop->vec_loop_alt_exits.safe_push (ex);
> + return;
> + }
> + else
> + {
> + for (edge ex : e->src->preds)
> + workset.safe_insert (0, ex);
> + }
> + visited.add (e);
> + }
>
> So this greedily follows edges from the latch and takes the first exit. Why's
> that better than simply choosing the first?
>
> I'd have done
>
> auto_vec<edge> exits = get_loop_exit_edges (loop); for (e : exits)
> {
> if (vect_get_loop_niters (...))
> {
> if no assumptions use that edge, if assumptions continue
> searching, maybe ther's an edge w/o assumptions
> }
> }
> use (first) exit with assumptions
>
> we probably want to know 'may_be_zero' as well and prefer an edge without
> that. So eventually call number_of_iterations_exit_assumptions
> directly and look for the best niter_desc and pass that to
> vect_get_loop_niters
> (or re-do the work).
>
> As said for "copying" the exit to the loop copies use the block mapping.
>
The issue is with the scalar loops, where we have no SCEV data and also no
SSA mapping data (from what I can tell, the map was cleared in ifcvt itself).
So for this to work with SCEV, we'd have to start analyzing the loop coming out
of
LOOP_VINFO_SCALAR_LOOP as well unless I'm missing something?
Regards,
Tamar
