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

[Andrew MacLeod <amacleod@redhat.com>]

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.

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?




> Aldy
>