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

[James Y Knight via llvm-dev]

On Mon, Nov 30, 2020 at 8:44 PM Duncan P. N. Exon Smith via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

>
>
> On 2020 Nov  27, at 20:45, Chris Lattner via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
> Sorry for falling off the map on this thread:
>
> On Nov 17, 2020, at 1:42 PM, David Blaikie <dblaikie at gmail.com> wrote:
>
> 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.
>
>
> Out of curiosity: Why a single rather than zero?
>
>
> My rationale for this is basically that SmallVector is typically used for
> the case when you want to avoid an out-of-line allocation for a small
> number of elements, this was the reason it was created.  While there is
> some performance benefits of SmallVector<T,0> over std::vector<> they are
> almost trivial.
>
>
> The performance benefits aren't trivial.
>
> std::vector grow operations will refuse to use std::move for some T, a
> pessimization required by its exception guarantees, even if you're building
> with `-fno-exceptions`. We had a massive compile-time problem in 2016
> related to this that I fixed with 3c406c2da52302eb5cced431373f240b9c037841
> by switching to SmallVector<T,0>. You can see the history in r338071 /
> 0f81faed05c3c7c1fbaf6af402411c99d715cf56.
>

That issue, at least, is fixable without switching from std::vector just by
adding noexcept to the appropriate user-defined move constructors.
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