dmix optimization

[Giuliano Zannetti - ART S.p.A. <giuliano.zannetti@artgroup-spa.com>]

Hi,

I'm trying to optimize the dmix because I'm working with a big number of channels (up to 16) and in this case the dmix has not a negligible impact on performance.

I'm working with ALSA 1.1.9. I gave my first look to the generic_mix_areas_16_native function (https://github.com/alsa-project/alsa-lib/blob/v1.1.9/src/pcm/pcm_dmix_generic.c#L130).

I would ask you if I can avoid to check, for each loop iteration, if the current dst sample is not 0.

    for (;;) {
        sample = *src;
        if (! *dst) {
            *sum = sample;
            *dst = *src;
        } else {
            sample += *sum;
            *sum = sample;
            if (sample > 0x7fff)
                sample = 0x7fff;
            else if (sample < -0x8000)
                sample = -0x8000;
            *dst = sample;
        }
        if (!--size)
            return;
        src = (signed short *) ((char *)src + src_step);
        dst = (signed short *) ((char *)dst + dst_step);
        sum = (signed int *)   ((char *)sum + sum_step);
    }

Could it be possible check for the first sample of the period only, as reported in the code below? My assumption is that if dst[0] is 0 also dst[1] ... dst[period-1] will be 0, and I don't need to check every time. This is already an optimization, but it could be also a starting point for other optimization based on my HW. But, first of all, I would ask to you if my assumption is right.

    if (! *dst) {
        for (;;) {
            sample = *src;
            *sum = sample;
            *dst = *src;

            if (!--size)
                return;

            src = (signed short *) ((char *)src + src_step);
            dst = (signed short *) ((char *)dst + dst_step);
            sum = (signed int *)   ((char *)sum + sum_step);
        }

    } else {
        for (;;) {
            sample = *src;
            sample += *sum;
            *sum = sample;

            if (sample > 0x7fff)
                sample = 0x7fff;
            else if (sample < -0x8000)
                sample = -0x8000;
            *dst = sample;

            if (!--size)
                return;

            src = (signed short *) ((char *)src + src_step);
            dst = (signed short *) ((char *)dst + dst_step);
            sum = (signed int *)   ((char *)sum + sum_step);
        }
    }

Thank you!

Best Regards,
Giuliano