[llvm-dev] RFC: Making a common successor/predecessor interface

Thu Mar 12 10:52:46 PDT 2020

Think we got this all figured out with: https://reviews.llvm.org/D76034 -
but if there's more issues here (or just general C++-y design questions,
etc) more than happy to discuss them here/other threads/whenever :)

On Tue, Mar 10, 2020 at 4:42 PM Alina Sbirlea <alina.sbirlea at gmail.com>
wrote:

> On Tue, Mar 10, 2020 at 2:30 PM David Blaikie <dblaikie at gmail.com> wrote:
>
>>
>>
>> On Tue, Mar 10, 2020 at 8:31 AM Alina Sbirlea <alina.sbirlea at gmail.com>
>> wrote:
>>
>>> Hi Dave,
>>>
>>> It may be possible to do this with the current API, but what I was
>>> looking for is a common API for existing block types. For example there is
>>> no succ_begin for Machine BasicBlock.
>>>
>>
>>> I'm looking to make the CFGSuccessors and CFGPredecessors classes in
>>> CFGDiff.h templated, and this needs a common API for all types
>>> instantiations.
>>>
>>> Does this clarify your question or did I misunderstand your suggestion?
>>>
>>
>> Possibly some misunderstanding - sounds like Nicolai had the same
>> suggestion phrased more clearly.
>>
>>
>>> Nicolai,
>>>
>>> Yes, I considered declaring the "global" succ_begin for the other block
>>> types, but it seems like a more complex change (probably to declare the
>>> type as Dave described, and these need adding for 3 more types vs 2 now)
>>> and these wouldn't be used anywhere else.
>>>
>>
>> What do you mean by "adding 3 more types vs 2 now" - where the iterator
>> types are written is pretty separate from this API change (even if the
>> iterator types remain non-members - the succ_begin non-member -> member
>> functions with a member type could be achieved by using a member typedef
>> rather than a member /type/ )
>>
>
> I meant adding the global APIs for
> MachineBasicBlock, VPBlockBase, clang::CFGBlock (3), vs adding member APIs
> in BasicBlock and VPBlockBase (2).
> This is only because the use case I am looking at involves the
> DominatorTree and there are no instantiations around other types (like you
> mentioned: Interval).
> Looking closer at the comments for Interval, I see that the global APIs
> were added to mirror the BasicBlock ones, and they're essentially wrappers
> over iterators expressed as class members.
>
>
>> So currently we have:
>>
>> class Instruction;
>> class BasicBlock;
>> class Interval;
>>
>> succ_begin(Instruction*)
>> succ_begin(BasicBlock*)
>> succ_begin(Interval*)
>> (& some kind of Region thing in RegionIterator.h)
>>
>> and you'd like to generalize this over more types, MachineBasicBlocks,
>> VPBlockBase and clang::CFGBlock?
>>
>> So the suggestion would be to add:
>>
>> succ_begin(MachineBasicBlock*)
>> succ_begin(VPBlockBase*)
>> succ_begin(CFGBlock*)
>>
>> what would be the negative side of adding that, rather than porting the
>> extra 3 to member functions?
>>
>
> I don't think there is any negative, tbh. I simply found it clearer to
> reason about these as class member methods, and, looking at the existing
> cases, the "global" API approach looked to me to be the outlier.
> I also found somewhat confusing that, for Instructions, there are defined
> successor iterators via the global APIs but not predecessors, and these are
> in a separate file (CFG.h) so it's not as obvious this is the case. If
> these were class iterators, this would stand out.
>
> Thinking more, I think your suggestion to add the global ones is easier to
> implement as as quick solution:
> succ_begin(MachineBasicBlock*)
> succ_begin(VPBlockBase*)
> succ_begin(CFGBlock*)
> just like you suggested, as these are just wrappers over the class member
> methods (like those defined for Interval).
> If folks feel this is a cleaner/better approach, I'm happy to work towards
> that.
>
> The member methods still seem cleaner to me, but I realize I'll have to
> sign up for changing the whole code-base to use that if going that route
> :-).
> Again, as long as the end result is consistent, I'm flexible to go either
> way.
>
> Best,
> Alina
>
>
>>
>>> AFAICT, there is no issue with replacing the current "global" iterators
>>> with class specific ones, they are already used as such. But perhaps I
>>> don't have the full picture.
>>>  In the first patch I put up, the iterators added inside BasicBlock can
>>> co-exist with the global ones, so the switch can be done incrementally.
>>>
>>>
>>> Thanks,
>>> Alina
>>>
>>> On Mon, Mar 9, 2020, 4:16 PM David Blaikie <dblaikie at gmail.com> wrote:
>>>
>>>>
>>>>
>>>> On Mon, Mar 9, 2020 at 3:57 PM Alina Sbirlea via llvm-dev <
>>>> llvm-dev at lists.llvm.org> wrote:
>>>>
>>>>> Hi,
>>>>>
>>>>> As part of an ongoing work to extend the GraphDiff (this models a CFG
>>>>> view), I came across the need to have a common interface for accessing
>>>>> successors/predecessors in various IR units, such that a type such as
>>>>> `typename NodeT::succ_iterator` could be used in templated code.
>>>>>
>>>>
>>>> I /think/ this can be achieved with the existing API by using
>>>> "decltype(succ_begin(std::declval<NodeT>()))" instead of the typename
>>>> you've got as an example (it looks like succ_begin is the extension point -
>>>> but the problem you're having is naming its return type? decltype would be
>>>> one option) - you could make a trait wrapper around that or the like if you
>>>> need this type name in a bunch of disparate places (where they can't share
>>>> a typedef).
>>>>
>>>> Would that suffice? are there other aspects of your use case that don't
>>>> line up well with the existing non-member/overload API?
>>>>
>>>> - Dave
>>>>
>>>>
>>>>> In particular, the need arose for BasicBlocks, MachineBasicBlocks,
>>>>> VPBlockBase and clang::CFGBlock.
>>>>>
>>>>> The least invasive change seemed to be to use the interface already
>>>>> being used in MachineBasicBlock and clang::CFGBlock, and:
>>>>> (1) update BasicBlock to use this instead of the "global"
>>>>> `succ_iterator` in IR/CFG.h
>>>>> (2) add the same interfaces in VPBlockBase as simple wrappers over
>>>>> existing Successors/Predecessors vectors.
>>>>>
>>>>> I've been working on a few patches to make this happen, but I'd like
>>>>> the community's thoughts on this before deep-diving into code reviews.
>>>>>
>>>>> For some concrete view of what the changes look like, I uploaded two
>>>>> preliminary patches:
>>>>> (1) part 1: D75881 <https://reviews.llvm.org/D75881>: Introducing
>>>>> class specific iterators
>>>>> (2) D75882 <https://reviews.llvm.org/D75882>
>>>>> (1) part 2: pending: Cleaning up existing usages; example replacement:
>>>>> `succ_begin(BB)` with `BB->succ_begin()`.
>>>>> (1) part3/4: pending: Add class specific iterators to `Instruction`
>>>>> and clean up existing usages just as for `BasicBlock`.
>>>>>
>>>>> I split the above (1) just to clarify what interfaces are added versus
>>>>> the NFC cleanups that follow. But it could be done just as well in a single
>>>>> patch.
>>>>>
>>>>> I welcome comments on this, and if there's something I missed
>>>>> explaining please let me know.
>>>>>
>>>>> Thank you,
>>>>> Alina
>>>>>
>>>>>
>>>>>
>>>>>
>>>>> _______________________________________________
>>>>> LLVM Developers mailing list
>>>>> llvm-dev at lists.llvm.org
>>>>> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>>>>
>>>>
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