Re: [PATCH] phiopt: Adjust instead of reset phires range

[Aldy Hernandez <aldyh@redhat.com>]

On Thu, Dec 22, 2022 at 1:54 PM Jakub Jelinek <jakub@redhat.com> wrote:
>
> On Thu, Dec 22, 2022 at 01:09:21PM +0100, Aldy Hernandez wrote:
> >  INTEGER_CST singleton and
> > > union that into the SSA_NAMEs range and then do set_range_info
> > > with the altered range I guess.
> > >
> >
> > Note that set_range_info is an intersect operation. It should really be
> > called update_range_info. Up to now, there were no users that wanted to
> > clobber old ranges completely.
>
> Thanks.
> That would be then (I've committed the previous patch, also for reasons of
> backporting) following incremental patch.
>
> For the just committed testcase, it does the right thing,
> # RANGE [irange] int [-INF, -1][1, +INF]
> # iftmp.2_9 = PHI <iftmp.3_10(4), 8(5)>
> is before the range (using -fdump-tree-all-alias) and r below is
> [irange] int [-INF, -1][1, +INF],
> unioned with carg of 0 into VARYING.
> If I try attached testcase though (which just uses signed char d instead of
> int d to give more interesting range info), then I see:
> # RANGE [irange] int [-128, -1][1, 127]
> # iftmp.2_10 = PHI <iftmp.3_11(4), 8(5)>
> but strangely r I get from range_of_expr is
> [irange] int [-128, 127]
> rather than the expected [irange] int [-128, -1][1, 127].
> Sure, it is later unioned with 0, so it doesn't change anything, but I
> wonder what is the difference.  Note, this is before actually replacing
> the phi arg 8(5) with iftmp.3_11(5).
> At that point bb4 is:
> <bb 4> [local count: 966367640]:
> # RANGE [irange] int [-128, 127]
> # iftmp.3_11 = PHI <iftmp.3_15(3), 0(2)>
> if (iftmp.3_11 != 0)
>   goto <bb 6>; [56.25%]
> else
>   goto <bb 5>; [43.75%]
> and bb 5 is empty forwarder, so [-128, -1][1, 127] is actually correct.
> Either iftmp.3_11 is non-zero, then iftmp.2_10 is that value and its range, or
> it is zero and then iftmp.2_10 is 8, so [-128, -1][1, 127] U [8, 8], but
> more importantly SSA_NAME_RANGE_INFO should be at least according to what
> is printed be without 0.
>
> 2022-12-22  Jakub Jelinek  <jakub@redhat.com>
>             Aldy Hernandez  <aldyh@redhat.com>
>
>         * tree-ssa-phiopt.cc (value_replacement): Instead of resetting
>         phires range info, union it with oarg.
>
> --- gcc/tree-ssa-phiopt.cc.jj   2022-12-22 12:52:36.588469821 +0100
> +++ gcc/tree-ssa-phiopt.cc      2022-12-22 13:11:51.145060050 +0100
> @@ -1492,11 +1492,25 @@ value_replacement (basic_block cond_bb, basic_block middle_bb,
>                     break;
>                   }
>               if (equal_p)
> -               /* After the optimization PHI result can have value
> -                  which it couldn't have previously.
> -                  We could instead of resetting it union the range
> -                  info with oarg.  */
> -               reset_flow_sensitive_info (gimple_phi_result (phi));
> +               {
> +                 tree phires = gimple_phi_result (phi);
> +                 if (SSA_NAME_RANGE_INFO (phires))
> +                   {
> +                     /* After the optimization PHI result can have value
> +                        which it couldn't have previously.  */
> +                     value_range r;

I haven't looked at your problem above, but have you tried using
int_range_max (or even int_range<2>) instead of value_range above?

value_range is deprecated and uses the legacy anti-range business,
which has a really hard time representing complex ranges, as well as
union/intersecting them.

> +                     if (get_global_range_query ()->range_of_expr (r, phires,
> +                                                                   phi))
> +                       {
> +                         int_range<2> tmp (carg, carg);
> +                         r.union_ (tmp);

Here you are taking the legacy value_range and unioning into it.
That's bound to lose precision.

Ideally you should use int_range_max for intermediate calculations.
Then set_range_info() will take care of squishing things down into
whatever we allow into a global range (I think it's a 6-sub range
object ??).

Note, that if "r" can contain non integer/pointers (i.e. floats), you
should use:

  // Range of <TYPE>.
  Value_Range r (<TYPE>);

The goal is for Value_Range to become value_range, and for it to be
used for anything not explicitly an integer/pointer.  Thus the camel
case for this release.

Aldy

> +                         reset_flow_sensitive_info (phires);
> +                         set_range_info (phires, r);
> +                       }
> +                     else
> +                       reset_flow_sensitive_info (phires);
> +                   }
> +               }
>               if (equal_p && MAY_HAVE_DEBUG_BIND_STMTS)
>                 {
>                   imm_use_iterator imm_iter;
>
>
>         Jakub