Re: [PATCH] Implement global ranges for all vrange types (SSA_NAME_RANGE_INFO).

[Aldy Hernandez <aldyh@redhat.com>]

On Sat, Jul 9, 2022 at 6:16 PM Jeff Law via Gcc-patches
<gcc-patches@gcc.gnu.org> wrote:
>
>
>
> On 7/6/2022 11:10 AM, Aldy Hernandez via Gcc-patches wrote:
> > Currently SSA_NAME_RANGE_INFO only handles integer ranges, and loses
> > half the precision in the process because its use of legacy
> > value_range's.  This patch rewrites all the SSA_NAME_RANGE_INFO
> > (nonzero bits included) to use the recently contributed
> > vrange_storage.  With it, we'll be able to efficiently save any ranges
> > supported by ranger in GC memory.  Presently this will only be
> > irange's, but shortly we'll add floating ranges and others to the mix.
> >
> > As per the discussion with the trailing_wide_ints adjustments and
> > vrange_storage, we'll be able to save integer ranges with a maximum of
> > 5 sub-ranges.  This could be adjusted later if more sub-ranges are
> > needed (unlikely).
> >
> > Since this is a behavior changing patch, I would like to take a few
> > days for discussion, and commit early next week if all goes well.
> >
> > A few notes.
> >
> > First, we get rid of the SSA_NAME_ANTI_RANGE_P bit in the SSA_NAME
> > since we store full resolution ranges.  Perhaps it could be re-used
> > for something else.
> >
> > The range_info_def struct is gone in favor of an opaque type handled
> > by vrange_storage.  It currently supports irange, but will support
> > frange, prange, etc, in due time.
> >
> >  From the looks of it, set_range_info was an update operation despite
> > its name, as we improved the nonzero bits with each call, even though
> > we clobbered the ranges.  Presumably this was because doing a proper
> > intersect of ranges lost information with the anti-range hack.  We no
> > longer have this limitation so now we formalize both set_range_info
> > and set_nonzero_bits to an update operation.  After all, we should
> > never be losing information, but enhancing it whenever possible.  This
> > means, that if folks' finger-memory is not offended, as a follow-up,
> > I'd like to rename set_nonzero_bits and set_range_info to update_*.
> >
> > I have kept the same global API we had in tree-ssanames.h, with the
> > caveat that all set operations are now update as discussed above.
> >
> > There is a 2% performance penalty for evrp and a 3% penalty for VRP
> > that is coincidentally in line with a previous improvement of the same
> > amount in the vrange abstraction patchset.  Interestingly, this
> > penalty is mostly due to the wide int to tree dance we keep doing with
> > irange and legacy.  In a first draft of this patch where I was
> > streaming trees directly, there was actually a small improvement
> > instead.  I hope to get some of the gain back when we move irange's to
> > wide-ints, though I'm not in a hurry ;-).
> >
> > Tested and benchmarked on x86-64 Linux.  I will also test on ppc64le
> > before the final commit.
> >
> > Comments welcome.
> >
> > gcc/ChangeLog:
> >
> >       * gimple-range.cc (gimple_ranger::export_global_ranges): Remove
> >       verification against legacy value_range.
> >       * tree-core.h (struct range_info_def): Remove.
> >       (struct irange_storage_slot): New.
> >       (struct tree_base): Remove SSA_NAME_ANTI_RANGE_P documentation.
> >       (struct tree_ssa_name): Add vrange_storage support.
> >       * tree-ssanames.cc (range_info_p): New.
> >       (range_info_fits_p): New.
> >       (range_info_alloc): New.
> >       (range_info_free): New.
> >       (range_info_get_range): New.
> >       (range_info_set_range): New.
> >       (set_range_info_raw): Remove.
> >       (set_range_info): Adjust to use vrange_storage.
> >       (set_nonzero_bits): Same.
> >       (get_nonzero_bits): Same.
> >       (duplicate_ssa_name_range_info): Remove overload taking
> >       value_range_kind.
> >       Rewrite tree overload to use vrange_storage.
> >       (duplicate_ssa_name_fn): Adjust to use vrange_storage.
> >       * tree-ssanames.h (struct range_info_def): Remove.
> >       (set_range_info): Adjust prototype to take vrange.
> >       * tree-vrp.cc (vrp_asserts::remove_range_assertions): Call
> >       duplicate_ssa_name_range_info.
> >       * tree.h (SSA_NAME_ANTI_RANGE_P): Remove.
> >       (SSA_NAME_RANGE_TYPE): Remove.
> >       * value-query.cc (get_ssa_name_range_info): Adjust to use
> >       vrange_storage.
> >       (update_global_range): Use int_range_max.
> >       (get_range_global): Remove as_a<irange>.
> I'll be so happy once we don't have to keep doing the conversions
> between the types.
>
> Anti-ranges no more!

Yeah, it took a little longer than the 6 weeks Andrew had estimated
originally :-P.

Note that anti range kinda sorta still exist in two forms:

a) If you use value_range, as it still uses the legacy stuff
underneath.  But any new consumers (evrp, DOM, etc), all pass an
int_range<N> or an int_range_max, so anyone who cares about ranges
should never see an anti range.  Later this cycle value_range will be
typedefed to what is now Value_Range, which is an infinite precision
range that works for all types the ranger supports.  So anti-ranges
here will die a quick death.

b) There are some passes which still use the deprecated
irange::kind().  This method will return VR_ANTI_RANGE if the range
looks like this [-MIN, 123][567,+MAX].  But kind() is just a
convenience function so that passes that have yet to be converted can
still pretend they see anti-ranges.  Underneath a non-legacy irange
has no concept of an anti-range.

Currently, I see the following passes still using the anti-range nonsense:

gimple-array-bounds.cc
gimple-ssa-warn-restrict.cc
ipa-fnsummary.cc
ipa-prop.cc
pointer-query.cc
tree-ssa-strlen.cc

I don't understand the ipa-* stuff, so I never touched it.  OTOH, the
middle end warnings always break when you improve ranges so I left
them alone.

Aldy

>
> I've got no real concerns here.  So unless someone objects, your plan is OK.
>
> jeff
>