[llvm-dev] RFC: [SmallVector] Adding SVec<T> and Vec<T> convenience wrappers.

Wed Nov 25 15:51:52 PST 2020

On Wed, Nov 25, 2020 at 3:47 PM Sean Silva via llvm-dev
<llvm-dev at lists.llvm.org> wrote:
>
> Ping.
>
> It sounds like we left off with consensus around just making SmallVector<T> have a default N.
>
> We were converging to a default N policy of:
> - at least 1 inlined element

I'm still not sure why "at least 1" is an important goal. I'd be
curious to hear from Chris/others why that's especially valuable.

> - sizeof(SmallVector<T>) <= 64 when it doesn't contradict the "at least 1 inlined element".
>
> Any objections to moving forward with that?
>
> -- Sean Silva
>
> On Tue, Nov 17, 2020 at 5:29 PM Sean Silva <chisophugis at gmail.com> wrote:
>>
>>
>>
>> On Tue, Nov 17, 2020 at 2:16 PM Duncan P. N. Exon Smith <dexonsmith at apple.com> wrote:
>>>
>>>
>>>
>>> On 2020 Nov  17, at 13:40, Sean Silva <chisophugis at gmail.com> wrote:
>>>
>>>
>>>
>>> On Tue, Nov 17, 2020 at 7:26 AM Chris Lattner <clattner at nondot.org> wrote:
>>>>
>>>> On Nov 13, 2020, at 2:06 PM, Sean Silva via llvm-dev <llvm-dev at lists.llvm.org> wrote:
>>>>
>>>> We've pretty happy now with a patch that adds two wrappers around SmallVector that make it 1) more convenient to use and 2) will tend to mitigate misuse of SmallVector. We think it's ready for wider discussion: https://reviews.llvm.org/D90884
>>>>
>>>> SVec<T> is a convenience alias for SmallVector<T, N> with N chosen automatically to keep its size under 64 Bytes (that heuristic is easy to change though). The recommendation in the patch is to use this "on the stack, where a "small" number of elements are expected".
>>>>
>>>>
>>>> Hey Sean,
>>>>
>>>> I agree with other comments that his approach unnecessarily fragments the API surface area for a core class.  You’re doing two things here: 1) adding a new name for an existing thing, and 2) adding a default.
>>>>
>>>> My major concern and objection is about #1, since the codebase will get to be a mishmash of the two.  I’d rather keep the world consistent here and have an opinionated “one way to do it”.
>>>
>>>
>>> Thanks Chris. That was my original proposal when I first drafted the patch, and I'm happy to just add the default.
>>>
>>>>
>>>>
>>>> Thoughts/suggestions:
>>>>  - Adding the default seems very reasonable to me, and I think that 64 bytes is a good default.  I think you should change the behavior so that SmallVector<LargeThing> defaults to a single inline element instead of zero though.  Perhaps generate a static_assert when it is crazy large.
>>>
>>>
>>> That would work for me.
>>>
>>> Mehdi, would SmallVector<T>, defaulting to a minimum single inline element with static_assert(sizeof(T) < 512) address your concerns?
>>>
>>>
>>> I'd be a bit concerned about this for two reasons:
>>>
>>> - Adding a large inlined element by-default may reinforce one of the problems with SmallVector, that people accidentally put big vectors in places that shouldn't have them. If we're going to make accidentally large inlined vectors more ergonomic, I think we ought to make 0-element versions more ergonomic too.
>>
>>
>> I would prefer 0, but I'm ok with 1 because SmallVector<SmallVector<T>> avoids the "exponential" effect of SmallVector<SmallVector<T, 4>, 2> == 8 inlined T's (+ headers and stuff).
>>
>> As an experiment, I added in a static_assert(sizeof(T) < 512) into SmallVector. Almost all the errors were due to nested SmallVector<SmallVector<T, 4 or 8>, 4 or 8> and such. The only case I could see in the error log that was *not* due to nested SmallVector's was this legitimately enormous struct in llvm-objcopy: https://github.com/llvm/llvm-project/blob/67e0f791c93a23d0a523f3f05082c020f7c9109f/llvm/tools/llvm-objcopy/CopyConfig.h#L149 (which, btw, is used only in a SmallVector<T, 1>)
>>
>> So I feel pretty confident that 1 inline element from SmallVector<LargeType> is probably less or equal to what users would choose. Thus, I still think 1 inline element minimum is net positive if Chris won't budge on lowering that to 0.
>>
>> Chris, thoughts?
>>
>>
>>>
>>>
>>> - The `static_assert` is going to fail differently on different platforms, since `T`'s size will tend to be platform-dependent for large `T`. I'm not sure it'll be predictable enough.
>>
>>
>> Agreed.
>>
>>>
>>>
>>> -- Sean Silva
>>>
>>>>
>>>>
>>>>  - The name SmallVector has always been confusing, InlinedVector is more principled, but is even more verbose for such a common type.  If you think it is worth shrinkifying the name SmallVector, then I think the best thing to do is *rename* SmallVector (perhaps to IVector?) everywhere in the codebase.  I don’t think that introducing a redundant name is a good thing (except to smooth the transition).
>>>
>>>
>>> I like the idea of a minor rename, although a bit different:
>>> - SmallVectorBase => VectorBase
>>> - SmallVectorImpl => VectorImpl (we could keep both around for a transition period)
>>> - (etc.)
>>> - SmallVector => Vector, but give `Vector` a default small size of 0
>>> - Add SmallVector as a wrapper around Vector that has a nice default for the small size:
>>> ```
>>> template <class T>
>>> using SmallVector = Vector<T, DefaultInlinedElementSize<T>>;
>>> ```
>>
>>
>> I'm generally +1 on making SmallVector<T, 0> be easier to use, but it seems that the discussion of default N is already complex enough, so for now let's focus only on the default N situation.
>>
>> -- Sean Silva
>>
>>>
>>>>  - If people are concerned about the default being bad, then you could choose to make “SmallVector<T, -1>” be the thing that autosizes.  I tend to agree with your viewpoint that the N chosen is arbitrary in almost all cases anyway though, so I wouldn’t go this way.
>>>>
>>>>
>>>> Vec<T> is a convenience alias for SmallVector<T, 0>. It lets us get the (little-known?) benefits of SmallVector even when it has no inline elements (see https://llvm.org/docs/ProgrammersManual.html#llvm-adt-smallvector-h). The recommendation in the patch is to use this when the SmallVector is on the heap.
>>>>
>>>>
>>>> These benefits are tiny, I really don’t think it is worth introducing a new name for this.  I think this is better handled with a change to CodingStandards (saying “don’t use std::vector”) and the programmers manual.
>>>>
>>>> -Chris
>>>>
>>>
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