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

On 05/23/2014 11:42 AM, Adhemerval Zanella wrote:
> 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? 

Of course a following patch, and of course it's not a hotspot.
It just seems like an oddity that could be done better.


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