Re: replacing the backwards threader and more

[Jeff Law <jeffreyalaw@gmail.com>]

On 6/9/2021 9:34 AM, Aldy Hernandez wrote:
>
>
> On 6/9/21 2:09 PM, Richard Biener wrote:
>> On Wed, Jun 9, 2021 at 1:50 PM Aldy Hernandez via Gcc 
>> <gcc@gcc.gnu.org> wrote:
>>>
>>> Hi Jeff.  Hi folks.
>>>
>>> What started as a foray into severing the old (forward) threader's
>>> dependency on evrp, turned into a rewrite of the backwards threader
>>> code.  I'd like to discuss the possibility of replacing the current
>>> backwards threader with a new one that gets far more threads and can
>>> potentially subsume all threaders in the future.
>>>
>>> I won't include code here, as it will just detract from the high level
>>> discussion.  But if it helps, I could post what I have, which just 
>>> needs
>>> some cleanups and porting to the latest trunk changes Andrew has made.
>>>
>>> Currently the backwards threader works by traversing DEF chains through
>>> PHIs leading to possible paths that start in a constant.  When such a
>>> path is found, it is checked to see if it is profitable, and if so, the
>>> constant path is threaded.  The current implementation is rather 
>>> limited
>>> since backwards paths must end in a constant.  For example, the
>>> backwards threader can't get any of the tests in
>>> gcc.dg/tree-ssa/ssa-thread-14.c:
>>>
>>>     if (a && b)
>>>       foo ();
>>>     if (!b && c)
>>>       bar ();
>>>
>>> etc.
>>>
>>> After my refactoring patches to the threading code, it is now possible
>>> to drop in an alternate implementation that shares the profitability
>>> code (is this path profitable?), the jump registry, and the actual jump
>>> threading code.  I have leveraged this to write a ranger-based threader
>>> that gets every single thread the current code gets, plus 90-130% more.
>>>
>>> Here are the details from the branch, which should be very similar to
>>> trunk.  I'm presenting the branch numbers because they contain Andrew's
>>> upcoming relational query which significantly juices up the results.
>>>
>>> New threader:
>>>            ethread:65043    (+3.06%)
>>>            dom:32450      (-13.3%)
>>>            backwards threader:72482   (+89.6%)
>>>            vrp:40532      (-30.7%)
>>>     Total threaded:  210507 (+6.70%)
>>>
>>> This means that the new code gets 89.6% more jump threading
>>> opportunities than the code I want to replace.  In doing so, it reduces
>>> the amount of DOM threading opportunities by 13.3% and by 30.7% from 
>>> the
>>> VRP jump threader.  The total  improvement across the jump threading
>>> opportunities in the compiler is 6.70%.
>>>
>>> However, these are pessimistic numbers...
>>>
>>> I have noticed that some of the threading opportunities that DOM and 
>>> VRP
>>> now get are not because they're smarter, but because they're picking up
>>> opportunities that the new code exposes.  I experimented with 
>>> running an
>>> iterative threader, and then seeing what VRP and DOM could actually 
>>> get.
>>>    This is too expensive to do in real life, but it at least shows what
>>> the effect of the new code is on DOM/VRP's abilities:
>>>
>>>     Iterative threader:
>>>       ethread:65043    (+3.06%)
>>>       dom:31170    (-16.7%)
>>>           thread:86717    (+127%)
>>>           vrp:33851    (-42.2%)
>>>     Total threaded:  216781 (+9.90%)
>>>
>>> This means that the new code not only gets 127% more cases, but it
>>> reduces the DOM and VRP opportunities considerably (16.7% and 42.2%
>>> respectively).   The end result is that we have the possibility of
>>> getting almost 10% more jump threading opportunities in the entire
>>> compilation run.
>>
>> Yeah, DOM once was iterating ...
>>
>> You probably have noticed that we have very man (way too many)
>> 'thread' passes, often in close succession with each other or
>> DOM or VRP.  So in the above numbers I wonder if you can break
>> down the numbers individually for the actual passes (in their order)?
>
> Sure, I can do that.  Let me whip up the old branch and gather some info.
I'll save you some time.  The jump threaders in VRP are doing the least 
amount of lifting and the ones we want to kill first.  IIRC the one from 
vrp2 is doing nearly nothing at this point.


Jeff