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

[Jeff Law <jeffreyalaw@gmail.com>]

On 7/9/2022 1:31 PM, Aldy Hernandez wrote:
> 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.
I wouldn't worry much about the IPA stuff.  But we should make an effort 
to kill the ANTI ranges in other places (then we'll bug the IPA guys to 
fix their code :-).

I'm not sure what's best to tackle first.  array-bounds and 
warn-restrict are probably smaller, though you have to work through the 
problems that pop up when they're given better range data.

pointer-query.cc is probably the easiest place to start.  It looks 
fairly well documented in terms of what it's doing and why. 
tree-ssa-strlen is probably a good second choice, mostly because I think 
we're less likely to run into "we gave it better range data and got 
more/new diagnostics" problem.

jeff