Re: [PATCH 1/5] PowerPC: Consolidate NPTL/non versions of vfork

[Adhemerval Zanella <azanella at linux dot vnet dot ibm dot com>]

On 23-05-2014 15:29, Richard Henderson wrote:
> On 05/23/2014 06:26 AM, Adhemerval Zanella wrote:
>> +	/* Load the TCB-cached PID value and negates it. If It it is zero
>> +	   sets it to 0x800000.  And then sets its value again on TCB field.
>> +	   See raise.c for the logic that relies on this value.  */
>> +	lwz	r0,PID(r13)
>> +	cmpwi	cr0,r0,0
>> +	neg	r0,r0
>> +	bne-	cr0,1f
>> +	lis	r0,0x8000
>> +1:	stw	r0,PID(r13)
> I was thinking about this when I did the alpha port:
>
> Why don't we use ~pid instead of -pid?  That way we still get high-bit set
> semantics, but don't have to special-case 0, and so don't need a branch here
> (or a conditional move on other hosts)?
>
> There is a tiny matter of updating all of the architectures all at once though...
>
>
> r~
>
I don't see why not, however I think such change would be better suited for a following
patch.  Also, do you see this code sequence as a hotspot for such optimization?