On Mon, 21 Apr 2008 22:13:06 +1000, "Paul Mackerras" <[EMAIL PROTECTED]>
said:
> Alexander van Heukelum writes:
> > Powerpc would pick up an optimized version via this chain: generic fls64
> > ->
> > powerpc __fls --> __ilog2 --> asm (PPC_CNTLZL "%0,%1" : "=r" (lz) : "r"
> > (x)).
>
> Why wouldn't powerpc continue to use the fls64 that I have in there
> now?
In Linus' tree that would be the generic one that uses (the 32-bit)
fls():
static inline int fls64(__u64 x)
{
__u32 h = x >> 32;
if (h)
return fls(h) + 32;
return fls(x);
}
> > However, the generic version of fls64 first tests the argument for zero.
> > From
> > your code I derive that the count-leading-zeroes instruction for
> > argument zero
> > is defined as cntlzl(0) == BITS_PER_LONG.
>
> That is correct. If the argument is 0 then all of the zero bits are
> leading zeroes. :)
So... for 64-bit powerpc it makes sense to have its own implementation
and ignore the (improved) generic one and for 32-bit powerpc the generic
implementation of fls64 is fine. The current situation in linux-next
seems
optimal to me.
Greetings,
Alexander
> Regards,
> Paul.
--
Alexander van Heukelum
[EMAIL PROTECTED]
--
http://www.fastmail.fm - I mean, what is it about a decent email service?
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