Re: [PATCH RFC] mips: add TARGET_ZERO_CALL_USED_REGS hook [PR104817, PR104820]

[Qing Zhao <qing.zhao@oracle.com>]

> On Mar 18, 2022, at 11:09 AM, Richard Sandiford <richard.sandiford@arm.com> wrote:
> 
> Xi Ruoyao <xry111@mengyan1223.wang> writes:
>>> 
>>> If we have to go this way, I think it’s better to make the change you
>>> suggested above, 
>>> and then also update the documentation, both internal documentation on
>>> how to define
>>>  the hook and the user level documentation on what the user might
>>> expect when using 
>>> this option (i.e, it’s possible that the compiler might clear more
>>> registers than the user 
>>> requests on some targets due to the implementation limitation). 
>>> 
>>> I can make this change if we decide to do this.
>> 
>> I'd vote for this.  Richard?
> 
> Fine by me too, although I don't think this should be mentioned
> in the user documentation.  E.g. used-arg means that non-argument,
> non-return registers can have any value on return from the function;
> the compiler doesn't make any guarantees.  If the compiler happens to
> use a temporary register in the implementation of the option, and if
> that temporary register happens to still be zero on return, then
> that's OK.  It's just an internal implementation detail.  The same
> thing could happen for any part of the epilogue.

This makes good sense to me. I agree. 

Okay, will just add an extra argument and update the internal documentation. 

thanks.

Qing
> 
> Thanks,
> Richard