Re: PING [Patch][Middle-end]Add -fzero-call-used-regs=[skip|used-gpr|all-gpr|used|all]

[Qing Zhao via Gcc-patches]

> On Sep 23, 2020, at 9:40 AM, Richard Sandiford <richard.sandif...@arm.com> 
> wrote:
> 
> Qing Zhao <qing.z...@oracle.com> writes:
>>>>> Dropping them is fine with me FWIW.  That seems like a natural use
>>>>> for the new hook: drop zeroing that isn't actively wrong, but isn't
>>>>> likely to be useful either.
>>>> 
>>>> Okay, I will add a  new hook for this purpose.
>>> 
>>> It doesn't need to be a new hook.  The one I mentioned before
>>> would enough:
>>> 
>>>> The kind of target hook interface I was thinking of was:
>>>> 
>>>> HARD_REG_SET TARGET_EMIT_MOVE_ZEROS (const HARD_REG_SET &regs)
>>>> 
>>>> which:
>>>> 
>>>> - emits zeroing instructions for some target-specific subset of REGS
>>>> 
>>>> - returns the set of registers that were actually cleared
>>> 
>>> Not clearing mm0-7 and k0-7 would come under the first bullet point.
>> This makes sense.
>> 
>> However, how about the second bullet point:
>> 
>> - returns the set of registers that were actually cleared
>> 
>> Should we delete mm0-7 and k0-7 from the return set even though they are not 
>> zeroed by the target?
> 
> Yes, the point of the return value is to tell the caller what the
> hook actually did.  If the hook didn't clear mm0-7 then the returned
> set shouldn't include mm0-7.

Okay.

Qing
> 
> Thanks,
> Richard