Re: [PATCH] x86 rwsem optimization extreme

[H. Peter Anvin]

On 02/17/2010 02:10 PM, Linus Torvalds wrote:
> 
> The cost of 'adc' may happen to be identical in this case, but I suspect 
> you didn't test on UP, where the 'lock' prefix goes away. An unlocked 
> 'add' tends to be faster than an unlocked 'adc'.
> 
> (It's possible that some micro-architectures don't care, since it's a 
> memory op, and they can see that 'C' is set. But it's a fragile assumption 
> that it would always be ok).
> 

FWIW, I don't know of any microarchitecture where adc is slower than
add, *as long as* the setup time for the CF flag is already used up.
However, as I already commented, I don't think this is worth it.  This
inline appears to only be instantiated once, and as such, it takes a
whopping six bytes across the entire kernel.

	-hpa

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/