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

David Blaikie via llvm-dev llvm-dev at lists.llvm.org
Tue Mar 10 14:30:38 PDT 2020

On Tue, Mar 10, 2020 at 8:31 AM Alina Sbirlea <alina.sbirlea at gmail.com>

> 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/ )

So currently we have:

class Instruction;
class BasicBlock;
class 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:


what would be the negative side of adding that, rather than porting the
extra 3 to member functions?

> 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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