Re: add AVX2 support to simd.h

On Tue, Mar 19, 2024 at 04:53:04PM +0700, John Naylor wrote:
> On Tue, Mar 19, 2024 at 10:16 AM Nathan Bossart
> <nathandbossart@gmail.com> wrote:
>> 0002 does the opposite of this.  That is, after we've completed as many
>> blocks as possible, we move the iterator variable back to "end -
>> block_size" and do one final iteration to cover all the remaining elements.
> 
> Sounds similar in principle, but it looks really complicated. I don't
> think the additional loops and branches are a good way to go, either
> for readability or for branch prediction. My sketch has one branch for
> which loop to do, and then performs only one loop. Let's do the
> simplest thing that could work. (I think we might need a helper
> function to do the block, but the rest should be easy)

I tried to trim some of the branches, and came up with the attached patch.
I don't think this is exactly what you were suggesting, but I think it's
relatively close.  My testing showed decent benefits from using 2 vectors
when there aren't enough elements for 4, so I've tried to keep that part
intact.  This changes pg_lfind32() to something like:

    if not many elements
        process one by one

    while enough elements for 4 registers remain
        process with 4 registers

    if no elements remain
        return false

    if more than 2-registers-worth of elements remain
        do one iteration with 2 registers

    do another iteration on last 2-registers-worth of elements

-- 
Nathan Bossart
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