Re: [Qemu-devel] [PATCH v6 4/4] target/mips: Optimize ILVR.<B|H|W|D> MSA instructions

[Mateja Marjanovic <Mateja.Marjanovic@rt-rk.com>]

On 16.4.19. 23:20, Aleksandar Markovic wrote:
>> From: Mateja Marjanovic <Mateja.Marjanovic@rt-rk.com>
>>>> +void helper_msa_ilvr_b(CPUMIPSState *env, uint32_t wd,
>>>> +                       uint32_t ws, uint32_t wt)
>>>> +{
>>>> +    wr_t *pwd = &(env->active_fpu.fpr[wd].wr);
>>>> +    wr_t *pws = &(env->active_fpu.fpr[ws].wr);
>>>> +    wr_t *pwt = &(env->active_fpu.fpr[wt].wr);
>>>> +
>>> Why do we use here env->active_fpu.fpr[wd].wr, while for other instructions in
>>> this patch, we access msa_wr_d<b|h|w|d[] directly?
>> With a pointer to wr_t we have an array of bytes, halfwords, words or
>> doublewords
>> and can read from them and change them like an ordinary array. In other
>> cases
>> we use a variable that is TCGv_i64 and would have to use tcg_gen
>> functions to
>> modify the value of the register. Before my changes in ilvr instruction
>> helpers
>> env->active_fpu.fpr[wd].wr was used, so I just copy-pasted that.
>>
> Your answer touches just surface, and doesn't fully answer my question.
> I would like you to show deeper understanding of the code you are working
> with. You can't just copy/paste without thinking.
>
> Why do majority of MSA helpers use env->active_fpu.fpr[<index>].wr, while
> your code mostly reference the MSA register directly? Is this the same
> thing? If yes, why all MSA code doesn't use registers directly, which
> would certainly be simpler than referencing active_fpu? What is the role
> of "active_fpu"? Can it be changed? Can you analyze the underlying
> reasons for referencing "active_fpu", and can you claim that it is safe
> to circumvent it and reference the MSA registers directly?
I will look into that, and try to analyze it and understand it.
Thanks,
Mateja
> Thanks,
> Aleksandar