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

[Richard Biener <richard.guenther@gmail.com>]

> Am 04.10.2022 um 16:30 schrieb Aldy Hernandez <aldyh@redhat.com>:
> 
> On Tue, Oct 4, 2022 at 3:27 PM Andrew MacLeod <amacleod@redhat.com> wrote:
>> 
>> 
>>> On 10/4/22 08:13, Aldy Hernandez via Gcc-patches wrote:
>>>> 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.
>> 
>> Rangers cache can be a memory bottleneck in pathological cases..
>> Certainly not as bad as it use to be, but I'm sure it can still be
>> problematic.    Its suppose to be a memory efficient representation
>> because of that.  The cache can have an entry for any live ssa-name
>> (which means all of them at some point in the IL) multiplied by a factor
>> involving the number of dominator blocks and outgoing edges ranges are
>> calculated on.   So while SSA_NAME_RANGE_INFO is a linear thing, the
>> cache lies somewhere between a logarithmic and exponential factor based
>> on the CFG size.
> 
> Hmmm, perhaps the ultimate goal here should be to convert the cache to
> use vrange_storage, which uses trailing wide ints for all of the end
> points plus the masks (known_ones included for the next release).
> 
>> 
>> if you are growing the common cases of 1 to 2 endpoints to more than
>> double in size (and most of the time not be needed), that would not be
>> very appealing :-P  If we have any wide-ints, they would need to be a
>> memory efficient version.   The Cache uses an irange_allocator, which is
>> suppose to provide a memory efficient objects.. hence why it trims the
>> number of ranges down to only what is needed.  It seems like a trailing
>> wide-Int might be in order based on that..
>> 
>> Andrew
>> 
>> 
>> PS. which will be more problematic if you eventually introduce a
>> known_ones wide_int.    I thought the mask tracking was/could be
>> something simple like  HOST_WIDE_INT..  then you only tracks masks in
>> types up to the size of a HOST_WIDE_INT.  then storage and masking is
>> all trivial without going thru a wide_int.    Is that not so/possible?
> 
> That's certainly easy and cheaper to do.  The hard part was fixing all
> the places where we weren't keeping the masks up to date, and that's
> done (sans any bugs ;-)).
> 
> Can we get consensus here on only tracking masks for type sizes less
> than HOST_WIDE_INT?  I'd hate to do all the work only to realize we
> need to track 512 bit masks on a 32-bit host cross :-).

64bits are not enough, 128 might be.  But there’s trailing wide int storage so I don’t see the point in restricting ourselves?

> Aldy
>