Re: [COMMITTED] Convert nonzero mask in irange to wide_int.

[Aldy Hernandez <aldyh@redhat.com>]

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On Tue, Oct 4, 2022, 13:28 Aldy Hernandez <aldyh@redhat.com> wrote:

> On Tue, Oct 4, 2022 at 9:55 AM Richard Biener
> <richard.guenther@gmail.com> wrote:
> >
> >
> > Am 04.10.2022 um 09:36 schrieb Aldy Hernandez via Gcc-patches <
> gcc-patches@gcc.gnu.org>:
> > >
> > > The reason the nonzero mask was kept in a tree was basically inertia,
> > > as everything in irange is a tree.  However, there's no need to keep
> > > it in a tree, as the conversions to and from wide ints are very
> > > annoying.  That, plus special casing NULL masks to be -1 is prone
> > > to error.
> > >
> > > I have not only rewritten all the uses to assume a wide int, but
> > > have corrected a few places where we weren't propagating the masks, or
> > > rather pessimizing them to -1.  This will become more important in
> > > upcoming patches where we make better use of the masks.
> > >
> > > Performance testing shows a trivial improvement in VRP, as things like
> > > irange::contains_p() are tied to a tree.  Ughh, can't wait for trees in
> > > iranges to go away.
> >
> > You want trailing wide int storage though.  A wide_int is quite large.
>
> Absolutely, this is only for short term storage.  Any time we need
> long term storage, say global ranges in SSA_NAME_RANGE_INFO, we go
> through vrange_storage which will stream things in a more memory
> efficient manner.  For irange, vrange_storage will stream all the
> sub-ranges, including the nonzero bitmask which is the first entry in
> such storage, as trailing_wide_ints.
>
> See irange_storage_slot to see how it lives in GC memory.
>

That being said, the ranger's internal cache uses iranges, albeit with a
squished down number of subranges (the minimum amount to represent the
range).  So each cache entry will now be bigger by the difference between
one tree and one wide int.

I wonder if we should change the cache to use vrange_storage. If not now,
then when we convert all the subranges to wide ints.

Of course, the memory pressure of the cache is not nearly as problematic as
SSA_NAME_RANGE_INFO. The cache only stores names it cares about.

Aldy