Re: [PATCH] xfrm: branchless addr4_match() on 64-bit

[Alexey Dobriyan <adobriyan@gmail.com>]

On Fri, Mar 24, 2017 at 05:16:44PM +0000, David Laight wrote:
> From: Alexey Dobriyan
> > Sent: 23 March 2017 23:33
> > Current addr4_match() code has special test for /0 prefixes because of
> > standard required undefined behaviour. However, it is possible to omit
> > it on 64-bit because shifting can be done in a 64-bit register and then
> > truncated to the expected value (which is 0 mask).
> > 
> > Implicit truncation by htonl() fits nicely into R32-within-R64 model
> > on x86-64.
> ...
> >  static inline bool addr4_match(__be32 a1, __be32 a2, u8 prefixlen)
> >  {
> > +#ifdef CONFIG_64BIT
> > +	/* L in UL is not a typo. */
> > +	return !((a1 ^ a2) & htonl(~0UL << (32 - prefixlen)));
> > +#else
> >  	/* C99 6.5.7 (3): u32 << 32 is undefined behaviour */
> >  	if (prefixlen == 0)
> >  		return true;
> >  	return !((a1 ^ a2) & htonl(0xFFFFFFFFu << (32 - prefixlen)));
> > +#endif
> 
> Can't this just be written:
> 
>   	if (sizeof (long) == 4 && prefixlen == 0)

Indeed.

>   		return true;
> 	return !((a1 ^ a2) & htonl(0xFFFFFFFFUL << (32 - prefixlen)));

0xFFFFFFFFUL is really long movabs, ~0UL is better.