[PATCH] libstdc++: Use canonical loop form in std::reduce

[Marc Glisse]

On Fri, 31 Jan 2025, Abhishek Kaushik wrote:

> The current while loop in std::reduce and related functions is hard to
> vectorize because the loop control variable is hard to detect in icx.
>
> `while ((__last - __first) >= 4)`
>
> Changing the loop header to a for loop following the OpenMP canonical
> form allows easy vectorization, resulting in improved performance.
>
> `for (; __first <= __last - 4; __first += 4)`

Is that always legal? If the sequence has size 1, is __last - 4 well 
defined?

-- 
Marc Glisse