__lower_bound improvement for arithmetical types

[Jonathan Wakely]

On Thu, 9 Mar 2023 at 19:58, Александр Шитов via Libstdc++
<libstdc++@gcc.gnu.org> wrote:
>
> I want to propose an improvement to std::__lower_bound for arithmetic types
> with the standard comparators.
>
>
> The main idea is to use linear search on a small number of elements to aid
> the branch predictor and CPU caches, but only when it is not observable by
> the user. In other words, if a standard comparator (std::less,
> std::greater) is used for arithmetic types.
>
>
> In benchmarks I achieved twice the increase in speed for small vectors(16
> elements) and increase for 10-20% in large vectors(1'000, 100'000 elements).
>
>
> Code: https://gist.github.com/ATGsan/8a1fdec92371d5778a65b01321c43604
>
> PR: https://github.com/ATGsan/gcc/pull/1

This is an interesting idea, thanks.

You limit the linear search to a single cacheline, but you don't
ensure that the range to search doesn't cross two cachelines, right?
Maybe it doesn't matter in practice, but I wonder if limiting the
linear search to a single cacheline would be even better.