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

[Andrew MacLeod <amacleod@redhat.com>]

On 10/4/22 11:14, Aldy Hernandez wrote:
> On Tue, Oct 4, 2022 at 4:34 PM Richard Biener
> <richard.guenther@gmail.com> wrote:
>>
>>
>>> 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?
> Fair enough.  Perhaps we should bite the bullet and convert the cache
> to vrange_storage which is all set up for streaming irange's with
> trailing_wide_ints.  No changes should be necessary for irange, since
> we never have more than 3-4 live at any one time.  It's the cache that
> needs twiddling.
>
Wouldnt it be irange_allocator that needs twiddling?  It purpose in life 
is to allocate iranges for memory storage...  the cache is just a 
client, as is rangers global cache, etc...  that was the intention of 
irange_allocator to isolate clients from having to worry about memory 
storage issues?

Or is that problematic?


Andrew