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

[Duncan Exon Smith via llvm-dev]

> On Nov 30, 2020, at 17:57, James Y Knight <jyknight at google.com> wrote:
> 
> 
> 
>> 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.

Sure, once we’ve added noexcept to all types in LLVM/Clang/etc. That’s a pretty long tail though; a lot of work for relatively little gain given that we don’t care about exceptions anyway and we have an optimized vector implementation in tree. 
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