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### 2985. `std::reverse` should be permitted to be vectorized

Section: 25.7.10 [alg.reverse] Status: LEWG Submitter: Billy O'Neal III Opened: 2017-06-24 Last modified: 2018-04-03

Priority: Not Prioritized

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Discussion:

The fine folks on our backend team suggested that we special case `std::reverse` of 1/2/4/8 to take advantage of vector units. Unfortunately, at present `std::reverse` says it does `N/2 iter_swap`s, which doesn't permit our vector implementation even if the iterator inputs are pointers to trivially copyable `T`s.

The vectorized version for pointers to `short`s is ~8x faster on Skylake than the serial version, and about 7x faster for `unsigned long long`s; and users don't actually care whether or not we call `swap` here.

[2017-07 Toronto Monday issue prioritization]

Status to LEWG; this is similar to 2973

[2018-04-02, Billy comments]

This issue should be resolved by P0551, because it prohibits user specialization of `std::swap` and `std::iter_swap`, which means the proposed vectorization optimization for pointers-to-trivially-copyable is now implementable without changes to `reverse`'s specification (We can detect if the user has provided an alternate `swap` in their own namespace, but not if they explicitly specialized `swap` or `iter_swap`).

Proposed resolution:

This wording is relative to N4659.

1. Edit 25.7.10 [alg.reverse] as indicated:

   >     template<class BidirectionalIterator>
   >       void reverse(BidirectionalIterator first, BidirectionalIterator last);
   >     template<class ExecutionPolicy, class BidirectionalIterator>
   >       void reverse(ExecutionPolicy&& exec,
   >                    BidirectionalIterator first, BidirectionalIterator last);

   > > -1- Requires: `*first` shall be swappable (16.4.4.3 [swappable.requirements]).

   > > -2- Effects: For each non-negative integer `i < (last - first) / 2`, applies `iter_swap` to all pairs of iterators `first + i, (last - i) - 1`. If `is_trivially_copyable_v<typename iterator_traits<BidirectionalIterator>::value_type>` is `true`, an implementation may permute the elements by making temporary copies, rather than by calling `iter_swap`. [Note: this allows the implementation to be vectorized. — end note]