29 Numerics library [numerics]

29.10 Data-parallel types [simd]

29.10.8 basic_vec non-member operations [simd.nonmembers]

29.10.8.9 Mask permute [simd.permute.mask]

template<simd-vec-type V>
  constexpr V compress(const V& v, const typename V::mask_type& selector);
template<simd-mask-type V>
  constexpr V compress(const V& v, const type_identity_t<V>& selector);

Let:

(1.1)
bit-index(i) be a function which returns the index of the $i^{th}$ element of selector that is true.
(1.2)
select-value(i) be a function which returns v[bit-index(i)] for i in the range [0, reduce_count(selector)) and a valid but unspecified value otherwise.

[Note 1:
Different calls to select-value can return different unspecified values.
— end note]

Returns: A data-parallel object where the

i^{th}

element is initialized to the result of select-value(i) for all i in the range [0, V::size()).

🔗

template<simd-vec-type V>
  constexpr V compress(const V& v, const typename V::mask_type& selector,
                       const typename V::value_type& fill_value);
template<simd-mask-type V>
  constexpr V compress(const V& v, const type_identity_t<V>& selector,
                       const typename V::value_type& fill_value);

Let:

(3.1)
bit-index(i) be a function which returns the index of the $i^{th}$ element of selector that is true.
(3.2)
select-value(i) be a function which returns v[bit-index(i)] for i in the range [0, reduce_count(selector)) and fill_value otherwise.

Returns: A data-parallel object where the

i^{th}

element is initialized to the result of select-value(i) for all i in the range [0, V::size()).

🔗

template<simd-vec-type V>
  constexpr V expand(const V& v, const typename V::mask_type& selector, const V& original = {});
template<simd-mask-type V>
  constexpr V expand(const V& v, const type_identity_t<V>& selector, const V& original = {});

Let:

(5.1)
set-indices be a list of the index positions of true elements in selector, in ascending order.
(5.2)
bit-lookup(b) be a function which returns the index where b appears in set-indices.
(5.3)
select-value(i) be a function which returns v[bit-lookup(i)] if selector[i] is true, otherwise returns original[i].

Returns: A data-parallel object where the

i^{th}

element is initialized to the result of select-value(i) for all i in the range [0, V::size()).