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

[James Y Knight via llvm-dev]

On Mon, Nov 30, 2020, 11:30 PM Duncan Exon Smith <dexonsmith at apple.com>
wrote:

>
>
> 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.
>

Why does this need a long tail? We have fancy ast refactoring tooling, and
a single repository with all the code visible, after all. We can use thar
to discover all of the missing noexcepts, and add them, all at once. And
then use a clang tidy to help it remain true.
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