Re: [PATCH] Make loops_list support an optional loop_p root

["Kewen.Lin" <linkw@linux.ibm.com>]

on 2021/7/24 上午12:26, Martin Sebor wrote:
> On 7/23/21 2:41 AM, Kewen.Lin wrote:
>> on 2021/7/22 下午8:56, Richard Biener wrote:
>>> On Tue, Jul 20, 2021 at 4:37
>>> PM Kewen.Lin <linkw@linux.ibm.com> wrote:
>>>>
>>>> Hi,
>>>>
>>>> This v2 has addressed some review comments/suggestions:
>>>>
>>>>    - Use "!=" instead of "<" in function operator!= (const Iter &rhs)
>>>>    - Add new CTOR loops_list (struct loops *loops, unsigned flags)
>>>>      to support loop hierarchy tree rather than just a function,
>>>>      and adjust to use loops* accordingly.
>>>
>>> I actually meant struct loop *, not struct loops * ;)  At the point
>>> we pondered to make loop invariant motion work on single
>>> loop nests we gave up not only but also because it iterates
>>> over the loop nest but all the iterators only ever can process
>>> all loops, not say, all loops inside a specific 'loop' (and
>>> including that 'loop' if LI_INCLUDE_ROOT).  So the
>>> CTOR would take the 'root' of the loop tree as argument.
>>>
>>> I see that doesn't trivially fit how loops_list works, at least
>>> not for LI_ONLY_INNERMOST.  But I guess FROM_INNERMOST
>>> could be adjusted to do ONLY_INNERMOST as well?
>>>
>>
>>
>> Thanks for the clarification!  I just realized that the previous
>> version with struct loops* is problematic, all traversal is
>> still bounded with outer_loop == NULL.  I think what you expect
>> is to respect the given loop_p root boundary.  Since we just
>> record the loops' nums, I think we still need the function* fn?
>> So I add one optional argument loop_p root and update the
>> visiting codes accordingly.  Before this change, the previous
>> visiting uses the outer_loop == NULL as the termination condition,
>> it perfectly includes the root itself, but with this given root,
>> we have to use it as the termination condition to avoid to iterate
>> onto its possible existing next.
>>
>> For LI_ONLY_INNERMOST, I was thinking whether we can use the
>> code like:
>>
>>      struct loops *fn_loops = loops_for_fn (fn)->larray;
>>      for (i = 0; vec_safe_iterate (fn_loops, i, &aloop); i++)
>>          if (aloop != NULL
>>              && aloop->inner == NULL
>>              && flow_loop_nested_p (tree_root, aloop))
>>               this->to_visit.quick_push (aloop->num);
>>
>> it has the stable bound, but if the given root only has several
>> child loops, it can be much worse if there are many loops in fn.
>> It seems impossible to predict the given root loop hierarchy size,
>> maybe we can still use the original linear searching for the case
>> loops_for_fn (fn) == root?  But since this visiting seems not so
>> performance critical, I chose to share the code originally used
>> for FROM_INNERMOST, hope it can have better readability and
>> maintainability.
> 
> I might be mixing up the two patches (they both seem to touch
> the same functions), but in this one the loops_list ctor looks
> like a sizeable function with at least one loop.  Since the ctor
> is used in the initialization of each of the many range-for loops,
> that could result in inlining of a lot of these calls and so quite
> a bit code bloat.  Unless this is necessary for efficiency  (not
> my area) I would recommend to consider defining the loops_list
> ctor out-of-line in some .c or .cc file.
> 

Yeah, they touch the same functions.  Good point on the code bloat,
I'm not sure the historical reason here, it needs Richi's input.  :)

> (Also, if you agree with the rationale, I'd replace loop_p with
> loop * in the new code.)
> 

Oh, thanks for the reminder, will update it.  

BR,
Kewen

> Thanks
> Martin
> 
>>
>> Bootstrapped and regtested on powerpc64le-linux-gnu P9,
>> x86_64-redhat-linux and aarch64-linux-gnu, also
>> bootstrapped on ppc64le P9 with bootstrap-O3 config.
>>
>> Does the attached patch meet what you expect?
>>
>> BR,
>> Kewen
>> -----
>> gcc/ChangeLog:
>>
>>     * cfgloop.h (loops_list::loops_list): Add one optional argument root
>>     and adjust accordingly.
>>
>