[libc++] Vectorize all the algorithms

[philnik777]

There are quite a few algorithms which can be vectorized, or one should at least look into doing so. This issue lists all the algorithms which I think can be vectorized (and a few that shouldn't be with explanations).

To be vectorized:

• find (already done partially by forwarding to {w,}memchr)
• find_end
• find_first_of (possibly not worth it, since clang does it automatically in some cases)
• adjacent_find
• mismatch - [libc++] Vectorize mismatch #73255
• search (not checked whether it actually makes sense)
• search_n
• remove (requires efficient compressstore)
• remove_copy (requires efficient compressstore)
• unique (might require efficient compressstore)
• unique_copy (might require efficient compresstore)
• is_sorted_until
• includes
• min_element
• max_element
• minmax (range overload): Should first be checked whether it can be rewritten to allow auto-vectorization - [libc++] Optimize ranges::minmax #87335
• minmax_element - Vectorize minmax_element. #112397
• lexicographical_compare: Should use mismatch for the actual vectorization, but only forward to it if it's known to vectorize
• lexicographical_compare_three_way: same as lexicographical_compare
• replace_if autovectorized since [LV] Support generating masks for switch terminators. #99808

Should be checked whether they can be vectorized:

• set_union
• set_intersection
• set_difference
• set_symmetric_difference
• merge
• rotate is using memmove when possible, but may make sense to optimize for small ranges

Shouldn't be vectorized:

• contains: forwards to find, which should be vectorized instead
• count: is auto-vectorized
• equal: Already forwards to memcmp for trivially equality comparable types and doesn't seem worth it for floats
• fold (and friends): is auto-vectorized
• copy (and friends): can be forwarded to memmove
• swap_ranges: is auto-vectorized
• replace (and friends): is auto-vectorized
• fill (and friends): is auto-vectorized
• reverse_copy: is auto-vectorized
• rotate_copy: is forwarded to two memmove calls if possible
• shift_left/shift_right: is forwarded to memmove if possible
• is_sorted: forwards to is_sorted_until, which should be vectorized instead
• min, max (range overload): is auto-vectorized
• reverse: auto-vectorized since [SCEV] Support addrec in right hand side in howManyLessThans #92560

[mrdaybird]

@philnik777 std::reverse auto-vectorized by #92560

[philnik777]

@mrdaybird thanks, that's great! I've updated the list.

[sjoerdmeijer]

FYI: vectorization of find_first_of for AArch64 is implemented in #101976