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Re: [Qemu-ppc] [Qemu-devel] [PATCH v3 2/8] target/ppc: rework vmrg{l, h}

From: Richard Henderson
Subject: Re: [Qemu-ppc] [Qemu-devel] [PATCH v3 2/8] target/ppc: rework vmrg{l, h}{b, h, w} instructions to use Vsr* macros
Date: Sun, 27 Jan 2019 10:07:12 -0800
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.4.0

On 1/27/19 9:45 AM, Mark Cave-Ayland wrote:
>> I would expect the i < n/2 loop to be faster, because the assignments are
>> unconditional.  FWIW.
> Do you have any idea as to how much faster? Is it something that would show
> up as significant within the context of QEMU?

I don't have any numbers on that, no.

> As well as eliminating the HI_IDX/LO_IDX constants I do find the updated
> version much easier to read, so I would prefer to keep it if possible.
> What about unrolling the loop into 2 separate ones...

I doubt that would be helpful.

I would think that

#define VMRG_DO(name, access, ofs)
    int i, half = ARRAY_SIZE(r->access(0)) / 2;
    for (i = 0; i < half; i++) {
        result.access(2 * i + 0) = a->access(i + ofs);
        result.access(2 * i + 1) = b->access(i + ofs);

where OFS = 0 for HI and half for LO is best.  I find it quite readable, and it
avoids duplicating code between LO and HI as you're currently doing.


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