Re: Testing the lws_compare_and_swap_2 syscall

[John David Anglin <dave.anglin@bell.net>]

On 2017-11-06 5:45 PM, John David Anglin wrote:
> On 2017-11-06 4:27 PM, Helge Deller wrote:
>>       /* Check the validity of the size pointer */
>> -    subi,>>= 4, %r23, %r0
>> -    b,n    lws_exit_nosys
>> +    cmpib,COND(<<),n 3, %r23, lws_exit_nosys
> I don't believe that we want to use COND here (i.e., we want a 32-bit 
> check).  We might not
> need to trim the upper 32-bits r23.
>
> The reason the code uses nullification is the fast path occurs when 
> the "b,n" is nullified.   So we
> avoid the branch prediction penalty.  I'd have to check whether the 
> fast path with the cmpib instruction
> is the taken branch or not.
For cmpib with "<<" condition, the hint for a backward branch is likely 
taken.  The branch to
lws_exit_nosys is backward.

Dave

-- 
John David Anglin  dave.anglin@bell.net