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

[Mehdi AMINI via llvm-dev]

On Tue, Dec 1, 2020 at 2:39 PM Mehdi AMINI <joker.eph at gmail.com> wrote:

>
>
> On Tue, Dec 1, 2020 at 2:19 PM Chris Lattner via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
>> 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.
>>
>>
>> Can you spell this out for me?  Why do we need noexcept if we’re building
>> with -fno-exceptions?  What is going on here?
>>
>
> It isn't clear to me that -fno-exceptions can get the same benefit as
> noexcept for code that is written with exceptions in mind (I think
> https://stackoverflow.com/questions/61417534/can-stdvectort-use-ts-move-constructor-if-exceptions-are-disabled
> explains some of it).
>
> While it is possible to write a library that optimizes
> for -fno-exceptions, I am not sure it would be standard-compliant for the
> STL to do so (and it'd be anyway another code path that the one written in
> term of `std::move_if_noexcept`).
>
> Does it make sense or did you see it differently with your question?
>
>
After digging a bit, I found that libcxx was changed last year to optimize
with moving in std::vector when -fno-exceptions is present :
https://reviews.llvm.org/D62228

That means that a copy-only class can't be used in vector anymore in
clang-10 but it was possible in clang-9 (but only with -fno-exceptions):
https://godbolt.org/z/M54bqn

-- 
Mehdi
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