RFR: 8307795: AArch64: Optimize VectorMask.truecount() on Neon

[Andrew Haley]

On Mon, 15 May 2023 10:04:22 GMT, Chang Peng <duke at openjdk.org> wrote:

> > This looks like it might be removed by loop opts. I think you might need a blackhole somewhere.
> 
> `m` will be updated in every iteration of this loop, so `m` is not a loop-invariants actually. I can see the assembly code of this loop by using JMH perfasm.

Isn't it? Looks to me like all it does is flip `m` each time. Whether or not this code is optimized today isn't relevant.

So it's the same as


        for (int i = 0; i < LENGTH/2; i++) {
            res += m.trueCount();
        }
        m = m.not();
        for (int i = 0; i < LENGTH/2; i++) {
            res += m.trueCount();
        } 


... which is trivially optimizable, no?

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PR Review Comment: https://git.openjdk.org/jdk/pull/13974#discussion_r1193674595