[PATCH] Add iterator_adaptor to iterate over SDNode operand SDNode's

Pete Cooper peter_cooper at apple.com
Fri Jun 26 11:00:37 PDT 2015 

> On Jun 26, 2015, at 10:33 AM, David Blaikie <dblaikie at gmail.com> wrote:
> 
> As to the code in the patch itself:
> 
> Do you need to implement op* and op->, or does the adapter have convevience implementations of one in terms of the other? (be nice if it did, but I could believe/imagine that it might not)
You’re right, they do define one.  I needed my own operator* :

const SDValue &operator*() const { return I->get(); }

but ultimately the iterator_facade_base which we inherit from provides -> in terms of *.
> 
> Is that naming convention consistent with other names we use when making iterator/range pairs? Plurality in the range accessor, singularity in the begin/end? Should we just avoid providing begin/end and only provide the range accessor (people can always use op_values().begin() if they really need it - I don't mind the syntax being a bit heavier for that case to discourage raw iterator usage a little bit)
I just copied the naming from the iterator immediately prior to this one in SelectionDAGNodes.h (so honestly that means I didn’t even take the time to notice about the naming.  Probably should have).  It had op_begin(), op_end() and ops().  I took a look at Value.h for what is one of the most commonly used iterators.  In there we have use_begin(), use_end(), and uses().  So I think my naming is fairly consistent, but I can see your point here.

I like the idea of just removing begin()/end().  I can’t think of a reason to iterate over a subset of the operands to a SDNode, so I’ve removed that.

What do you think?

Cheers,
Pete


> 
> On Fri, Jun 26, 2015 at 10:29 AM, David Blaikie <dblaikie at gmail.com <mailto:dblaikie at gmail.com>> wrote:
> 
> 
> On Fri, Jun 26, 2015 at 10:18 AM, Pete Cooper <peter_cooper at apple.com <mailto:peter_cooper at apple.com>> wrote:
> 
>> On Jun 25, 2015, at 5:49 PM, David Blaikie <dblaikie at gmail.com <mailto:dblaikie at gmail.com>> wrote:
>> 
>> 
>> 
>> On Thu, Jun 25, 2015 at 5:36 PM, Pete Cooper <peter_cooper at apple.com <mailto:peter_cooper at apple.com>> wrote:
>> 
>>> On Jun 25, 2015, at 5:21 PM, David Blaikie <dblaikie at gmail.com <mailto:dblaikie at gmail.com>> wrote:
>>> 
>>> 
>>> 
>>> On Thu, Jun 25, 2015 at 5:11 PM, Pete Cooper <peter_cooper at apple.com <mailto:peter_cooper at apple.com>> wrote:
>>> 
>>> On Jun 25, 2015, at 2:40 PM, David Blaikie <dblaikie at gmail.com <mailto:dblaikie at gmail.com>> wrote:
>>> 
>>> 
>>> 
>>> On Thu, Jun 25, 2015 at 2:30 PM, Pete Cooper <peter_cooper at apple.com <mailto:peter_cooper at apple.com>> wrote:
>>> Hi David
>>> 
>>> Currently the only SDNode iterator over operands does so with SDUse*.  Users frequently then call getNode() on the operand.
>>> 
>>> This patch adds an iterator to SDNode which returns the SDNode of the operand.  This allows more patterns to be converted to foreach.  It is based on value_op_iterator which I found in User.h.
>>> 
>>> For now i’ve only used it in a single place, but I found a bunch more in DAGCombiner for example which should be applicable.  I would convert those in a later commit assuming you are ok with this solution.
>>> 
>>> This immediately raises red flags:
>>> 
>>>   SDNode *operator*() const { return I->getNode(); }  SDNode *operator->() const { return operator*(); }
>>> 
>>> op* should return a T& and op-> should return T*
>>> I’d forgotten about that.
>>>> 
>>>> If these SDNode*s can never be null, then perhaps this should be:
>>> I wasn’t actually sure if they could be.  My initial reaction was that null operands wouldn’t make sense, but it turns out we never checked.  So here’s a patch which does actually ensure that the SDNode's referenced as operands are never null.  It passes make check.  I can put it on another email for review if you prefer I don’t add it here.
>>> 
>>> It might be worth a separate thread, or at least a drive-by by someone who deals with this part of the code. I don't really understand the necessity/merits/drawbacks of the 'SDUse::reset()" member function you've introduced.
>> No problem.  Thanks for taking a look.  I’ve just sent out an email to llvm-commits and asked Hal for review as I know he’s done lots of SD work.
>>>  
>>> 
>>> 
>>>> 
>>>>   SDNode &operator*
>>>>   SDNode *operator->
>>>> 
>>>> ? (because I assume you don't have an SDNode* lvalue to return a reference to) I assume the adapter helper can implement one of those in terms of the other so you only have to implement one of them? I forget how the adapter utility works.
>>> I think it makes sense to do this.  This will unfortunately be one of the few SDNode & in the entire codebase though, which makes it stand out.  SDNode really does seem to always be a pointer.  I’ll fix up the patch to do this soon.
>>> 
>>> Yeah, there are a few types (read: Lots) like this in LLVM. I personally don't mind being more referential in spite of that, but I can understand others might feel less comfortable with that.
>>> 
>>> If you wanted to preserve the pointer-ness, you'd have op* return SDNode * (this would be a bit incorrect, it should really be SDNode * const &, and you can do that by having an SDNode* member in the iterator that you init and return a ref to... technically that's the more correct option - I'm not entirely sure where that matters) and no op->.
>> I like this solution.  I tried SDNode * const & earlier but of course getNode() is a temporary so this doesn’t work.  I’ll get a patch together which makes this change.  Technically I guess that means we don’t need the nonnull SDNode patch, but I don’t see any harm in it anyway.
>> 
>> Personally I'd go the reference route - the more references the clearer the documentation of nullness contracts (& tools like ubsan null reference checking can catch bugs sooner/closer to the point of the problem rather than later on). But I realize that's a bit jarring, stylistically, so I certainly wouldn't insist on it.
> I tried making the op* return an SDNode& then propagated that through to the dyn_cast in AArch64ISelLowering.cpp.  Unfortunately after that things got messy.
> 
> I could have done this
> 
> for (const SDNode &Elt : N->op_values()) {
>     if (const ConstantSDNode *C = dyn_cast<ConstantSDNode>(&Elt)) {
> 
> but it seemed a shame to have to add the & in the dyn_cast.  So I tried to actually make a version of dyn_cast on references to non-pointers, and return an optional if the cast fails.  Unfortunately that wasn’t much better.
> 
> What went wrong there?
> 
> (actually a while back I proposed having dyn_cast be able to accept references and return pointers (Optional<T> and T* are basically the same - though Optional<T> is nice documentation for "this really can be null" rather than pointers where that's a bit more vague) - Chandler didn't particularly like the idea so I have up... *shrug*)
>  
> But it did lead me to an interesting piece of code in Casting.h.  We have a struct called simplify_type which can be used to automatically do things like convert to other classes.  This is implemented to go from SDValue to SDNode* which is really what I want.
> 
> Ah, it's already implemented? Good-o then.
>  
> So, i’ve totally changed the iterator now to return SDValue& and SDValue*.  Thats really what the SDUse’s used as operands were storing anyway.
> 
> Sounds plausible - and I'll neglect to pass judgment on the merits or issues of the existing simplify_type from SDValue -> SDNode*. Again, don't know enough of the context there to even have a clue if that's a nice thing to do or not. I do think, generally, that simplify_type gets a bit overused/abused. But if it's already doing this one.. *shrug*
>  
>  Then the dyn_cast can be left unchanged.
> 
> What do you think?
> 
> Cheers,
> Pete
> 
> 
> 
>> 
>> Mightn't hurt to think a bit about the tradeoff of using temporary storage or just cheating and returning a pointer by value from op*... I mean it'll probably work for the range-for. I'm not sure where it'd really break - if the value_type of the iterator is "const SDNode" then it might be hard-to-impossible to for any algorithm to really have a problem with this.
>> 
>>  
>> 
>> Thanks,
>> Pete
>>>>  
>>>> 
>>>> BTW, been trying to work out if there would ever be a good solution for an iterator combined with isa<> or dyn_cast<>.  If you look at the code this patch touches in AArch64ISelLowering, it is immediately followed by a dyn_cast.  I’d really like to find a clean way to fold that it to the foreach loop, i.e.,
>>>> 
>>>> for (auto *C : dyn_cast<ConstantSDNode>(N->op_nodes()))
>>>> 
>>>> just a thought, but thats unrelated to this patch for now.
>>>> 
>>>> Yep, though probably more in the form of a filtered range, I suspect:
>>>> 
>>>>   for (auto &C : filtered_transform(N->op_nodes(), [](SDUse *U) { return U->getNode(); }))
>>>> 
>>>> It'd be a bit tricky to deal with the value type of this range's iterators - chances are the predicate should return an Optional<T&> (Hmm, don't think our Optional template supports ref parameters yet anyway) or T* (not sure if we could generalize it so it could cope with Optional<T>, maybe - so we could support generators where the values are not already/permanently in-memory) and then the value_type is T.
>>> Interesting.  I hadn’t though to use Optional.  I might try to implement something like this if i get time.
>>> 
>>> Cheers,
>>> Pete
>>>>  
>>>> 
>>>> Cheers,
>>>> Pete
> 
> 
> 
> 

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