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

[David Blaikie via llvm-dev]

I will say I'm not a huge fan of adding even more names for things in
this fairly core/common use case (now we'll have even more vector
names to pick from) - can we use default template arguments so we can
write SmallVector<T> instead of SmallVector<T, N> and would that
address some of the use cases proposed here?

I think someone (JYKnight, perhaps) mentioned in the code review
(always difficult fragmenting the discussion between code review and
RFC, unfortunately - not sure there's a great solution to that - some
way to lock comments on a Phab review might be nice) that there are
cases where you do want a small inline buffer even when you're nested
inside another data structure and/or heap allocated (like tail
allocations).

Got any sense of the total value here? Major savings to be had?

(if SmallVector<T> can do what your proposed SVec<T> does, that leaves
the Vec<T> - could you expound on the benefits of SmallVector<T, 0>
over std::vector<T>? I guess the SmallVectorImpl generic algorithm
opportunities? Though that's rarely needed compared to ArrayRef.)
If SmallVector<T> would suffice then maybe Vec<T> could be
ZeroSmallVector<T>? Not sure.

On Fri, 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".
>
> 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.
>
> A lot of this is boiled out from the discussion in https://groups.google.com/g/llvm-dev/c/q1OyHZy8KVc/m/1l_AasOLBAAJ?pli=1
>
> The goals here are twofold:
>
> 1. convenience: not having to read/write "N", or do an extra edit/recompile cycle if you forgot it
>
> 2. avoiding pathological cases: The choice of N is usually semi-arbitrary in our experience, and if one isn't careful, can result in sizeof(SmallVector) becoming huge, especially in the case of nested SmallVectors. This patch avoids pathological cases in two ways:
>   A. SVec<T>'s heuristic keeps sizeof(SVec<T>) bounded, which prevents pathological size amplifications like in `SmallVector<struct {SmallVector<T, 4> a, b; }, 4>`, where the small sizes effectively multiply together. Of course, users can always write SmallVector<T, N> explicitly to bypass this, but the need for that seems rare.
>   B. SmallVector<T, 0> feels "weird to write" for most folks, even though it is frequently the right choice. Vec<T> mitigates that by "looking natural".
>
> I'm surfacing this as an RFC to get feedback on a couple higher-level points:
> - does everybody agree that SVec<T> and Vec<T> are useful to have?
> - get wider consensus around suggesting these as "defaults" (see my updates to ProgrammersManual.rst in the patch)
> - how much we want to bulk migrate code vs let it organically grow. Replacing SmallVector<T, 0> with Vec<T> should be completely mechanical. Replacing SmallVector<T, N> for general N would be a lot more work.
> - of course: naming. SVector/Vector were floated in the patch as well and seem ok. SmallVec was rejected as it was a prefix of SmallVector (messes up autocomplete).
>
> Looking forward to a world with fewer guessed SmallVector sizes,
>
> -- Sean Silva
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