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.