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

Alina Sbirlea via llvm-dev llvm-dev at lists.llvm.org
Tue Mar 10 08:30:55 PDT 2020

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

Does this clarify your question or did I misunderstand your suggestion?


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.
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.


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
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