Re: [PATCH V2] Use subscalar mode to move struct block for parameter

[Jiufu Guo <guojiufu@linux.ibm.com>]

Hi,

Jeff Law <jeffreyalaw@gmail.com> writes:

> On 11/28/22 20:53, Jiufu Guo wrote:
>
>>>
>>> Right, but the number of registers is target dependent, so I don't see
>>> how using "8" or any number of that matter is correct here.
>> I understand.  And even for the same struct type, using how many
>> registers to pass a parameter, it also dependends on the size of the
>> parameter and how many leading parameters there is.
>> So, as you said, "8" or any numbers are not always accurate.
>>
>> Because, the enhancement in this patch is just make "block move" to be
>> more friendly for follow optiomizations(cse/dse/dce...) by moving
>> through sub-mode.  So, I just selected one arbitrary number which may
>> not too large and also tolerable.
>> I also through to query the max number registers from targets for
>> param/ret passing, but it may not very accurate neither.
>>
>> Any sugguestions are welcome! and thanks!
> OK, so it's just a magic number and (in theory) any number should
> still generate correct code -- the number merely places limits on when
> we'll consider performing this optimization.
Right.
>
> It may be overkill, but you might consider making it a PARAM that can
> be adjusted.

Yes. I also feel the magic number is not perfect. I'm thinking of a way
to mark if a param is stored to stack from the register when setup, and
then reference the marker during this optimization. This would be more
accurate and able to avoid the magic number.

>
>
>>
>> For this patch, only simple stuffs are handled like "D.xxx = param_1",
>> where the source and dest of the assignment are all in memory which is
>> the DECL_RTL(of D.xx/param_xx) in MEM_P/BLK.
>> And to avoid complicate, this patch only handle the case where
>> "(size % mode_size) == 0".
>>
>> If any misunderstandings, please point out, thanks.
>> And thanks for comments!
> How values are justified varies on the PA depending on whether the
> parameter is passed in registers or in memory.  Though thinking more
> about things, I don't think you're changing how the parameter is
> passed. Just how it's subsequently pulled out of memory.
Right!

Thanks a lot for your comments!

BR,
Jeff (Jiufu)
>
> jeff